Steel Designer''''s Manual Part 12 pptx

For bearings carrying both horizontal and vertical loads it is common that the design of the bearing requires a minimum vertical load to be present to ensure satisfactory performance und

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FOUNDATION EXAMPLE 1

Silwood Park, Ascot, Berks SL5 7QN

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FOUNDATION EXAMPLE 4

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BS 5950: Part 1 GWO Problem

Design a built-up base for the valley stanchion of a double bay

crane shed that is shown belon The stanchion comprises twin

406 ¥ 178 UB.

Taking moments about the tensile bolt, with n, the trial neutral axis

as 0.4 m depth, and taking

Subject Chapter ref.

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FOUNDATION EXAMPLE 1

Silwood Park, Ascot, Berks SL5 7QN

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FOUNDATION EXAMPLE 4

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BS 5950: Part 1 GWO

Taking n as 0.2 m, C becomes 598 kN and f c is 4.98 N/mm 2

T is then: 598 + 350 - 828 = 120 kN

To check, take moments about C

The relative stiffness of the base plate and channels will determine

the point of application of the compressive force As an alternative

therefore assume the lever arm to be equal to the bolt centres and

the centre of compression at the bolt line with appropriate

stiffen-ing added at this point.

This is the minimum value of n for concrete strength of 20 N/mm 2

Design of channels & gusset

M = 669 ¥ 300/10 3 = 200.7 kNm

Use 2/229 ¥ 89 ¥ 32.76 RSCs, M cx = 95 kNm

These are satisfactory by inspection since the gussets and base plate

acting compositely would also make a contribution.

The internal stiffener and base plate would similarly be designed

as a composite member taking the maximum outstand given in

2

3 B

Worked examples 841

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FOUNDATION EXAMPLE 1

Silwood Park, Ascot, Berks SL5 7QN

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FOUNDATION EXAMPLE 4

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BS 5950: Part 1 GWO The base plate panel between the stiffeners should be checked using

the Pounder expressions given in Chapter 30 as follows – the panel

is shown below.

Base plate

K me , the Pounder expression for moment, in the centre of the long

edge, when all four edges are encastre, is given below:

w me , the ultimate load intensity is given by:

The plate thickness of 16 mm is therefore satisfactory.

.

All structures move to some extent Movements may be permanent and irreversible

or short-term and possibly reversible The effects can be significant in terms of thebehaviour of the structure, its performance during its lifetime, and the continuedintegrity of the materials from which it is built

Movements can arise from a variety of sources:

(1) environmental: thermal, humidity, wind-induced

(2) material properties: creep, shrinkage

(3) loading: axial and flexural strains, impact, braking, traction, centrifugal forces.(4) external sources: tilt, settlement, subsidence, seismic loads

(5) use of the building: heating, cold storage

(6) others: requirements for moving or lifting bridges, allowances for jacking cedures, during or after construction

pro-In general it is necessary to consider the behaviour of the structure at each point

in terms of its possible movement in each of three principal directions, together withany associated rotations The movements of a structure are not in themselves detri-mental; the problems arise where movements are restrained, either by the way inwhich the structure is connected to the ground, or by surrounding elements such ascladdings, adjacent buildings, or other fixed or more rigid items If provision is notmade for such movements and associated forces it is possible that they will lead to,

or contribute towards, deterioration in one or more elements Deterioration in thiscontext can range from, for example, cracking or disturbance of the finishes on abuilding to buckling or failure of primary structural elements due to large forcesdeveloped through inadvertent restraint

Note that for bridges with total lengths of up to 60 m, it is possible to dispensewith bearings and expansion joints through use of abutments and piers which aredesigned to be integral with the bridge deck Further guidance on this topic can befound in Reference 1

Bearings 843

28.1.2 Design philosophies

In catering for movement of a structure, one of three methods can be adopted:(1) Design the structure to withstand all the forces developed by restraint of move-ment This is possible with smaller structures (small-span bridges) or structureswhich are comparatively flexible (portal frames, in the plane of the frame).The method will avoid joints but may require the use of additional material inconstruction

(2) Subdivide the structure into smaller structurally stable units, each of which thenbecomes essentially a structure in its own right, able to move independently ofthe surrounding units This principle is ideal for controlling those factors such

as thermal movement which are related to the size of the overall structure Inmany cases, the need for bearings as discrete elements can be eliminated Thedisadvantage lies in the need to provide joints between the various units of thestructure capable of accommodating all the anticipated relative movementsbetween the units, while at the same time fulfilling all the other requirements,i.e visual, practical, etc It is, however, generally possible to achieve a balance

by subdividing the structure so that the movements at the joints between unitsare kept relatively small, permitting the joints to be simple and economical(possibly at the expense of larger numbers of joints)

(3) Subdivide the structure into fewer but larger sections, and make provision for

a smaller number of joints, each with larger movement capacity, and thus sibly more complex than those that would be used at (2) Examples are to befound in bridges where use of the least number of road deck joints is prefer-able both in terms of riding quality, and also in the minimization of long-termmaintenance requirements

pos-The need to restrict strains on elements and thus to protect finishes will lead tothe adoption of the second of the above methods for design of building structures.Bridges, for reasons cited above, are more frequently designed adopting the thirdmethod

28.2 Bearings

28.2.1 Criteria for design and selection

28.2.1.1 Form of the unit

Choice of form depends on several criteria:

(1) Physical size limitations The space available in the structure for the bearing.

As bearings are subject to more wear than other parts of the structure they

may have a shorter life and consequently this space should include allowancefor access, inspection, maintenance and possible replacement.

(2) Bearing pressure The allowable bearing pressure on the materials above and

below the bearing will dictate the minimum size of the top and bottom faces

of the bearing unit

(3) Loading The magnitude of the design load to be withstood by the bearing in

each of the three principal directions will govern the form and type of thebearing For each direction the maximum and minimum load should be considered at ultimate limit state, serviceability limit state or working loaddepending on the requirements of the design In each case co-existent load andmovement effects should be considered, together with a check for the exis-tence of any load combinations which would act so as to separate the compo-nents of the bearing (e.g uplift) For bearings carrying both horizontal and

vertical loads it is common that the design of the bearing requires a minimum

vertical load to be present to ensure satisfactory performance under tal loads

horizon-(4) Rotations The magnitude of the maximum anticipated rotations in the three

principal directions should be considered For certain types of bearing (e.g.elastomeric bearings) there exists an interaction between maximum load-carrying capacity and rotation/translation capacity, so that it may be necessary

to consider co-existent effects under loading (3) and movement (5)

(5) Movements Provision for maximum calculated movements can affect the size

of the moving parts of the bearing and thus the overall size of the unit As withrotations, the design of certain types of bearing is sensitive to the interaction

of movement and loading requirements

(6) Stiffness (vertical, rotational or translational) Certain structures may be

sensi-tive to the deformation which occurs within the bearing during its support ofthe loads The various types of bearing have different stiffness characteristics

so that an appropriate form can be selected

(7) Dynamic considerations Any particularly onerous dynamic loadings on the

structure will have to be considered Certain types of bearings (e.g elastomericbearings) have damping characteristics which may be desirable in particularinstances, such as vibration of footbridges or machine foundations

(8) Connections to structure The form of connection of the bearing to the

struc-ture requires careful consideration of the materials involved and the need for installation, maintenance and replacement of the bearing In addition,bearings are frequently at a position in the structure where different forms ofconstruction meet, perhaps constructed by different contractors In this case,

it is necessary to ensure that surrounding construction is properly detailed sothat design requirements for load transfer are achieved

(9) Use of proprietary bearings Many types of bearings are commercially

avail-able These range from items which are available ‘off the shelf’ to more cialized units which may be designed and proven, but which are only produced

spe-to order It is often appropriate for bearings spe-to be individually designed spe-tomeet a particular need in situations where proprietary types may not be suit-

Bearings 845

able In these instances the engineer has the option of designing the units usingavailable literature (see references to Chapter 28) and perhaps incorporatingstandard bearings from a manufacturer as components of a completed assem-bly or alternatively engaging a recognized manufacturer to design and producethe item as a special bearing For straightforward applications such as may berequired on a short single-span bridge, it may be worthwhile investigating therelative costs of a simple fabricated bearing compared with the equivalent pro-prietary unit Bearings (particularly ‘special’ bearings) can prove to be a largeitem of expenditure in a structure and an estimate of the costs involved should

be made early in the design stage

(10) Summary of design requirements Before selecting a particular bearing it is

suggested that a summary of all relevant parameters is prepared This can then

be used if necessary for submission to the bearing manufacturers for nation and recommendations as to particular bearing types A typical format

28.2.1.2 Materials

Generally materials fall into three groups:

(1) those able to withstand high localized contact pressures e.g steel

(2) those able to withstand lower contact pressures but having a low coefficient offriction; these slide easily in a direction perpendicular to the direction of thepressure and thus accommodate translational movement, e.g polytetrafluoro-ethylene (PTFE)

(3) those able to withstand contact pressure and also to accommodate translational

or rotational movements by deformation of the material (e.g elastomers).Certain of these materials may be confined within a steel cylinder in order toincrease their compressive resistance

(a) Mild or high-yield steel

The coefficient of friction of steel on steel is of the order of 0.3 to 0.5, unless tinuously lubricated; in order to provide for movement alternative arrangementsare usually necessary Traditionally this has been through the use of single or multiple rollers or knuckles Rollers will permit translation in one direction and,

con-if a single roller is used, rotation about an axis perpendicular to that direction.Knuckles permit rotation about one axis only Rotation in two directions may beachieved using spherical-shaped bearing surfaces

The allowable pressures between surfaces for steel on steel contact depend uponthe radii of the two surfaces and the hardness and ultimate tensile strength of the

note! (i) if pure ptfe used unlubricated, use

2 x coefficient of friction shown

(ii) if filled ptfe used, use 4 x values

5 10 15 20 25 30bearing stress (N/mm2)

0.08 coefficient 0.06

of friction

0.04 0.02

cases As load-carrying requirements increase, the use of steels with greater ness is dictated This can be achieved by use of high-grade alloy steels of various

values of coefficients of friction of between 0.01 and 0.05 for steel roller bearings

(b) Stainless steel

Stainless steel is frequently used in strip or plate form to provide a smooth path forsliding surfaces It is important to utilize a material for the sliding surface which willnot deteriorate and adversely affect the coefficient of friction assumed for design

of the structure A typical arrangement is a polished austenitic stainless steel surfacesliding against dimpled PTFE

(c) Polytetrafluoroethylene (PTFE)

PTFE has good chemical resistance and very low coefficients of static and dynamicfriction Unfortunately, pure PTFE has a low compressive strength, high thermalexpansion and very low thermal conductivity As a consequence it is frequently used

in conjunction with ‘filler’ materials which improve these detrimental effects withoutsignificantly affecting the coefficient of friction

The coefficient of friction varies with the bearing stress acting upon it BS 5400:

pure PTFE sliding on stainless steel

Lubrication of the pure PTFE is commonly achieved by means of silicone greaseconfined in dimples which are rolled on to the surface of the material References

2 and 3 give further guidance on the restrictions on shape, thickness and ment on the PTFE and stainless steel components

contain-In preliminary design and assessment of forces on structures using PTFE sliding

Fig 28.1 Coefficient of friction for continuously lubricated pure PTFE

For particular applications, such as bearing guides, phosphor bronze may be used,

BS 5400: Section 9.1 suggests a coefficient of friction of 0.35 for phosphor bronzesliding on steel or cast iron

(e) Elastomers

An elastomer is either a natural rubber or a man-made material which has like characteristics Elastomers are used frequently in bearings; they either consti-tute the bulk of the bearing itself or act as a medium for permitting rotation to takeplace (see sections 28.2.2.2 and 28.2.2.3(7))

rubber-Elastomers are principally characterized by their hardness, which is measured inseveral ways, the most common of which is the international rubber hardness(IRHD) This ranges on a scale from very soft at 0 to very hard at 100 Those elas-tomers used in bearings which are to comply with BS 5400: Part 9 have hardnesses

in the range 45 IRHD to 75 IRHD

The tensile capacity of most elastomers is considerable.As an illustration BS 5400:Part 9 specifies a minimum tensile elongation at failure of between 300% and 450%depending on IRHD

When considering the behaviour of a block of elastomer under vertical pression it is assumed that the material is securely bonded to top and bottom loadingplates In this case (which is representative of most bearing situations) the verticalbehaviour is related to the material’s ability to bulge on the four non-loaded faces

com-and is expressed in terms of the shape factor for the block, which is the ratio of the

loaded area to the force free surface area (see Fig 28.2)

for a rectanglefor a circle

Fig 28.2 Elastomeric bearing dimensions for (a) a rectangular block, (b) a circular block

stress dissipated energy

An important property of elastomers is related to the fact that the strain in thematerial tends to lag behind the stress which causes it As a consequence, some ofthe energy input during deformation is dissipated within the bearing as heat Atypical plot of stress versus strain for an elastomer is shown in Fig 28.3 The energylost as heat in one loading cycle is represented by the area of the loop

This effect, known as hysteresis, has two implications:

(1) it can result in a build-up of heat in the bearing under dynamic loading conditions,

(2) if appropriately sized it can be used to act as a form of damping device to thestructure

The sensitivity of the elastomer to dynamic loading depends upon both the quency of the applied stress and the temperature, as elastomers exhibit hardening

fre-at low temperfre-atures

Elastomers are prone to creep and are sensitive to attack by atmospheric oxygenand ozone, petrol and radiation from nuclear sources They are not suitable for oper-ation in temperatures above 120°C

(f) Concrete

The concept of the use of a small, highly contained block of concrete as a hinge hasbeen employed in the form of the Freysinnet hinge or the Mesnager hinge Details

of these are given in Reference 6

Fig 28.3 Energy loss in elastomer

mechan-In the following sections the principal types of bearing are briefly described.

28.2.2.2 Elastomeric bearings

Elastomeric bearings rely for their operation on the interaction between verticalload, rotation and translation As a consequence, design of most elastomeric bear-ings must be carefully checked Large proprietary ranges are available, and althoughload tables of the various capacities are published by manufacturers, it is prudent

to ask the supplier to confirm that the selected bearing is suitable for the ment conditions under which it will be used The basis of design of these bearings

load/move-is related to controlling strains and stresses in the elastomer and any reinforcingmaterial and ensuring that the bearing does not deform excessively, become un-stable, lift off, or slip under the anticipated design effects Further guidance on

gives a detailed discussion of the properties of elastomers

Bearing types are:

(1) Rubber pad or strip bearings As the name implies, these bearings consist simply

of a block or strip of elastomer They have the advantage of being inexpensiveand simple although their load-carrying and movement capability is limited

(2) Fabric-reinforced bearings In order to increase the capabilities of the simple

pad bearing, use is made of fabric (e.g compressed cotton duck) to reinforcethe elastomer Movement in these bearings is usually provided by use of a PTFEsurface bonded to the top of the block and sliding against a stainless steel plateattached to the underside of the superstructure In this manner, the elastomer

is used to provide rotational capability only, rather than rotation and movement

as in the case of other elastomeric bearings

(3) Elastomeric-laminated bearings This type of bearing consists of a block of

elas-tomeric material reinforced with steel plates to which the elastomer is also

alter-‘fixed’ for translation by means of a steel dowel passing through the bearinglayers (Fig 28.4(a)) or ‘free’ bearings which permit translation and rotation bydeformation of the bearing (see Fig 28.4(b)) This type of bearing is capable ofcarrying quite substantial loadings and movements and has the benefit of beingcheaper than mechanical bearings.

28.2.2.3 Mechanical bearings

(1) Roller (Fig 28.5(a)) The earlier and more traditional forms of bearing

com-prised single or multiple steel rollers sandwiched between upper and lower steelplates Single rollers will allow for longitudinal movement and rotation aboutthe axis of the roller, while at the same time carrying comparatively high vertical loads, hut will not permit transverse rotation or movement Bearings ofvery large capacity have been produced by use of special alloy steels to formthe contact surfaces Note that bearings which utilize multiple rollers will notallow rotation about an axis parallel to the axis of the rollers Rollers are some-times used enclosed in an oil bath or grease box to exclude deleterious matter.Other forms of bearing have, to a large extent, supplanted the use of rollers forthe most common applications

(2) Rocker (Fig 28.5(b)) Rocker bearings will not permit translational movement.

The bearings may be cylindrical or spherical on one surface with the othersurface flat or curved In the cylindrical form there is no provision for trans-verse rotation, which may have consequences for design of the structures aboveand below the unit Rocker bearings usually incorporate a pin or shear keybetween the two surfaces to maintain relative position

Fig 28.4 Two types of elastomeric bearing: (a) fixed, (b) free

(3) Knuckle bearings (Fig 28.5(c)) These are similar to rocker bearings.

(4) Leaf bearings (Fig 28.5(d)) These are formed of leaves of steel with a common

pin They will carry large vertical loads and permit large rotations about the axis

Fig 28.5 Mechanical bearings: (a) single/multiple roller, (b) cylindrical/spherical rocker, (c)

cylindrical knuckle, (d) knuckle leaf, (e) swing link, (f) spherical – with sliding top plate, (g) cylindrical PTFE bearings combined to form ‘anticlastic’ bearing, (h) ‘pot’ bearing which can have sliding top plate similar to (f)

of the pin but not transversely They have the benefit that they can be designed

to resist uplift It should be noted, however, that they are unlikely to be thing other than produced to order and that there may be other means of con-trolling comparatively small uplifts (e.g ‘pot’ type bearing with separate verticalrestraints) Leaf bearings have been used in suspension bridges to form theswing link bearings which are necessary to cater for large movements and uplifts (Fig 28.5(e))

any-(5) Spherical (PTFE, circular) (Fig 28.5(f)) These comprise a spherical lower

surface which is lined with PTFE and a matched upper spherical surface of aluminium or stainless steel This arrangement allows considerable rotationcapacity in all directions Horizontal translation is frequently achieved usinganother (flat) sliding surface above the upper part of the bearing

An important consideration with spherical bearings is that in order to withstand any horizontal loads it is necessary to have a minimum co-existentvertical load to prevent instability

Spherical bearings are capable of carrying high vertical loads and also permithigher rotations than many other types

(6) Cylindrical (PTFE) ‘anticlastic’ bearings (Fig 28.5(g)) These are similar in

concept to rocker bearings but instead of using (for example) steel on steelbearing surfaces they have enlarged bearing areas which are coated with PTFE

on one surface and stainless steel or aluminium on the other This produces

a bearing with high rotation capabilities about an axis as well as high carrying capacity One unit can be combined with another similar arrangement

load-to provide rotation about an axis at right angles load-to the first and also with asliding plate arrangement to provide translational capability

(7) Disc or ‘pot’ (Fig 28.5(h)) These are often of similar proportions to spherical

hearings but instead of a sliding spherical surface being used to provide tion capability, a disc of elastomeric material is used, confined in a cylindricalpot Loading is applied to the surface of the disc via a closely fitting steel piston.Under these conditions, the confined elastomer is in a near fluid state, andpermits rotation in all directions without significant resistance Sliding isachieved by means of a PTFE/sliding surface above the piston, in a similarmanner to spherical bearings Disc bearings are popular for many applications,

rota-as they tend to be cheaper than spherical bearings but can carry higher loadings than elastomeric-laminated bearings of comparable plan area They have rotation capabilities intermediate between spherical and laminated bearings

(8) Fabricated Fabricated bearings have become less popular largely through the

availability of a wide range of proprietary units They are used for footbridgesand temporary works applications There is, however, no reason why properlydesigned fabricated bearings should not be used to support a structure, partic-ularly, say, for a fixed bearing where there is no requirement for sliding surfaces.Guidance on design of bearings is given in References 2–9

(9) Special Special bearings will always be required for particular locations.

Perhaps the most common demands are for:

Bearings 853

(a) bearings which will resist horizontal loads only in order to restrain the ture in the horizontal plane, but without providing any vertical support (seesection 28.2.4.2(3))

struc-(b) bearings which will withstand uplift under certain loading conditions.Uplift bearings can be special versions of normal proprietary bearing types, orcan use a proprietary bearing set in a subframe which controls the tendency to upliftwithin prescribed limits adopted in consultation with the bearing manufacturer

28.2.3 Use of bearings

28.2.3.1 General

The parameters which dictate the form of the bearing as a unit are discussed insection 28.2.1 It is also necessary to consider the action of the bearing in the broaderconcept of the behaviour of the two elements of structure which the unit connects

(2) square or cylindrical dowels, tapped to receive the bearing fixing bolts,

(3) direct bolting of the bearing to the structure

In all forms it is desirable to allow for tolerances in the processes of installationand possible need for replacement of the bearing The system shown in Fig 28.6allows for support of the bearing during fine adjustment, but requires large jacking

Fig 28.6 Bearing fixing

capability (possibly more than a continuous structure could accommodate) toremove it.

In bearings subjected to dynamic loadings such as machine foundations, it is essary to ensure that the fixings are vibration-proof

nec-28.2.3.3 Effect on the structure

The elements of the structure above and below the bearing are affected by the type

of bearing, which can be classified as:

(1) fixed – not permitting movement in any horizontal direction,

(2) guided – movement, constrained by guides of some form, to be in one

horizon-tal direction only,

(3) free – movement permitted in all horizontal directions,

(4) elastomeric, which may be laminated or not These bearings can be ‘fixed’ by

means of steel dowels passing through them but are more often used ‘free’ inall directions and their capability to generate forces when shearing takes place

is utilized to withstand horizontal loadings If the whole structure is supported

on such bearings it effectively ‘floats’, with all horizontal loads shared by allbearings (See also section 28.2.4.2.)

If the bearings are fixed or guided, the neighbouring structure must be designedfor the forces arising from the restraints Even when the bearing is free in a particu-lar direction and movement is permitted, some forces are developed – either fromfriction effects at the movement interfaces of a sliding mechanical bearing, or fromshearing deformation in the case of an elastomeric bearing (see Fig 28.7(a))

In addition to forces developed laterally, the effects of the eccentricities produced

by the movement must be allowed for, and also the rotation capability of the bearing

in the transverse direction (see Figs 28.7(b), (c) and (d)) It is possible to controlthe extent of the additional eccentricity effects on a steel superstructure by use of

a sliding bearing inverted which transfers the eccentricity to the substructure, where

it may be more easily accommodated In this case however care should be taken toprotect the sliding surfaces against falling dust, debris, etc by use of a flexible skirtenclosure

28.2.3.4 Installation

Bearings must be correctly installed into the structure The procedure will dependupon the form of the structure above and below the bearing, and the type of bearing,but in general care should be taken not to load the bearing significantly beforebedding materials between the bearing and the structure have fully cured, or to load

centre of pressure does not apparently

change but note reduction in effective

area — affects compressive stress of

bearing; additional forces F are generated

(a)

design superstructure

for eccentricity i.

design for eccentricity

(b)

position after movement

no transverse rotation capacity

1

Iifailure of bearing, or structure, or both

Factory-assembled bearings are usually provided with transit straps to preventinadvertent dismantling of the unit When a significant irreversible movement isanticipated at a bearing, due to shrinkage or prestressing for example, an allowancefor this movement may be pre-set in the factory To allow for departure of the actualstructure temperature, when the bearings are set, from the mean temperatureassumed in design, it may be necessary to make alterations of the relative positions

when this may be most conveniently carried out – when the bridge temperature isapproximately equivalent to the air shade temperature This can be taken as:

Fig 28.7 Effect of bearing on structure: (a) elastomeric, (b) roller, (c) sliding, (d) need for

transverse rotation capacity

(1) concrete bridges: 0900 BST ± 1 hour each day,

(2) steel bridges: at about 0400 to 0600 BST each day during the summer, and atany time on ‘average’ days during the winter

Further guidance on bridge temperatures is given in Reference 10

Frequently bearings are incorporated into the structure using a bedding layerabove and below the unit, typically of 25 mm thickness, which allows some toler-ance in fixing of the bearing and will also permit final adjustment of the levels ofthe structure above during construction The form of the bedding may be ‘dry pack’,trowelable or pourable material Epoxy resin, sand/cement, sand/epoxy, orsand/polyester compounds are commonly used The same material may also be usedfor filling the spaces around fixing devices once final positioning has been carriedout

28.2.4 Assemblies of bearings

28.2.4.1 General

The selection and use of bearings of various types has been discussed in terms ofthe individual units The behaviour of the structure or substructure as a whole willnow be considered, and the use of the four principal forms of bearing to controlmovement illustrated

28.2.4.2 Structures straight in plan

As an example, the movement of a typical bridge deck will be considered in thehorizontal plane, although the principles involved can equally be applied in otherdirections

The four forms of bearing commonly available are given in section 28.2.3.3.Consider the bridge deck shown in plan in Fig 28.8

The deck vertical loading arises from dead and live loads, from which maximumand minimum values of bearing loads can be derived at each position Longitudi-nal loading on the deck will arise from wind loads, braking and traction of vehicles,and also from the manner in which the chosen restraint system accommodates

Fig 28.8 Straight bridge deck

or by shearing of an elastomer block a horizontal force (due to friction or shearrespectively) will be generated, and the bearing system should be arranged so thatwherever possible these forces cancel one another out, and so minimize the net horizontal force to be resisted by the substructure.

(1) In Figure 28.9(a) all the bearings are elastomeric with no fixed bearings The

horizontal loads in both directions are shared between all bearings and thestructure ‘floats’

Thus all substructures will be loaded when horizontal loads or expansion/contraction occur This system is economic, but is limited by the maximum capabilities of the bearings in rotation, load, and movement

(2) In Fig 28.9(b) all the bearings are mechanical (typically spherical or pot ings) Line ‘C’ provides fixity in the transverse direction Line 1 provides fixity

bear-in the longitudbear-inal direction All longitudbear-inal forces from external sources aretaken at abutment 1, together with longitudinal forces arising from friction at

Fig 28.9 Typical bearing layouts for straight bridge decks: (a) all elastomeric, (b) all

mechanical (i), (c) all mechanical (ii)

bearings on piers 2 to 5 At each pier transversely lateral loads are taken by ‘C’line bearings Friction forces due to transverse expansion, etc will tend to cancelone another out

(3) The arrangement in Fig 28.9(c) is better for longitudinal effects than that in Fig.28.9(b) as friction forces in this direction tend to cancel one another out It hasthe disadvantage that external loads are transmitted to an intermediate pierrather than an abutment The forces due to movement of the deck are mini-mized in both horizontal directions Occasionally, the line of fixed or guidedbearings with both horizontal and vertical capability such as at ‘C’ may bereplaced by two lines, one with bearings with vertical capability only, and onewith bearings with horizontal capability only

28.2.4.3 Structures curved in plan

If the structure shown in Fig 28.9(b) is curved in plan, then any expansion or traction movements longitudinally are accompanied by lateral movements also Thiseffect can be controlled in two ways:

con-(1) set the bearing guides to permit radial expansion from a fixed point on the structure,

(2) set the bearing guides tangential to the plan curvature, and so constrain thestructure to follow this line when it moves (see Fig 28.10)

In radially-guided structures the accuracy of setting out and alignment becomesmore critical as the distance from the fixed point increases In tangentially-guidedstructures, the structure is constrained to move along a particular path, and the horizontal forces developed in so doing must be taken into account in the design ofthe structure and supports

It should be noted that frequently bearings which are nominally ‘guided’ are manufactured with a gap tolerance at the guides The actual value of this toleranceshould be checked with the manufacturer of the particular bearing, but a value of0.5 mm is typical This tolerance can have a significant effect on the permissible accu-

Fig 28.10 Curved bridge deck: (a) radially-guided, (b) tangentially-guided

Bearings 859

racy of setting out of radially-guided structures, and the magnitude of the forces

28.2.4.4 Structures with fixed bearings and flexible supports

An alternative to the use of systems of guided bearings is to provide fixed bearings

at more than one (possibly all) supports In this case the supporting structures (e.g.bridge piers) have to be designed to flex and accommodate the necessary move-ments They also have to cater for the forces developed by these movements in addi-tion to any other design loading effects This arrangement may be appropriate when

it is required to share horizontal load effects over several supports, but it should benoted that replacement of the bearings may be more difficult owing to horizontalloads which may be locked into the bearing/support arrangement

28.2.4.5 Other considerations

(1) Wedging action It is possible to utilize a form of ‘wedging action’ to resist

horizontal loadings by setting two (usually elastomeric) bearings on planesinclined to one another as shown in Fig 28.11 Equally, it is also possible todevelop the action inadvertently by errors in bearing setting out, and thus

(2) Shock transmission units (STUs) Although not strictly bearings, these units can

be utilized in conjunction with bearings to distribute certain components ofloading to other parts of the structure The units typically consist of a cylinderfilled with putty-like material which is acted on by a piston with a hole in itthrough which the putty can flow Slow, steadily applied forces such as thermalexpansion forces will cause the putty to flow from one side of the piston to theother, and allow dissipation of the force through movement Rapidly appliedforces such as seismic loads, braking loads, or wind gusts are too fast to allowthe flow to occur, and the unit therefore effectively transmits this ‘shock’ loadingwithout significant movement A description of the use of these devices is given

in Reference 9

Fig 28.11 Wedging action

28.3 Joints

28.3.1 General

The form of joints in a structure will vary to suit particular requirements at eachposition The basic parameters to be considered in derivation of a joint detail arediscussed below, although they are not all appropriate to every situation Despitethe fact that significant differences exist in the final application, many of the factorsinvolved in joint design are common to both buildings and bridges Joint detailingand construction is considerably facilitated by the many forms of proprietarysealants, gaskets, and fillers that are now commercially available for use as compo-nents, as well as complete prefabricated units which may be used in particular appli-cations The manufacturers of these products will generally be able to supplytechnical information on their products, and also to give guidance as to the suit-ability of items for use in particular applications

28.3.2 Basic criteria

28.3.2.1 Form of the structure

The form of the structure, and the location and orientation of the joint within thestructure, will dictate to a large extent the arrangement of the detail The basic categories of joint are:

(1) Wall joints These may be vertical (e.g expansion joint in a building or a bridge

substructure) or horizontal (e.g joint between preformed cladding units on abuilding façade)

(2) Floor/roof joints Examples are expansion joints in a building, or road deck

joints in a bridge

(3) Internal/external joints This type of joint needs to be weather-proofed.

28.3.2.2 Material to be joined, and method of fixing

The material either side of the joint may be steel or aluminium cladding, concrete,brickwork, blockwork, or various forms of surfacing The detail of the joint will varyconsiderably with the properties of the material and the method of fixing to be used

It is important to note that this may affect the stage of construction at which thejoint is formed: e.g PVC waterstops will need to be positioned before concreting ofthe walls on either side of them takes place Where it is anticipated that the jointmay need repair or replacement during the life of the structure (e.g expansion joints

on heavily trafficked bridges) the fixings of the joint should allow for easy removaland reinstatement

(a) seal filler

I, 4

drain to bottom

membrane (PVC, thin metal, etc.)

28.3.2.3 Weather-resistance

It is important to consider the degree of weather-resistance required for a joint Inthis respect, joints can be classified (in a somewhat over-simplified form) into threetypes (see Fig 28.12(a), (b) and (c)):

(a) ‘Closed’ joint, with a filler material and exterior sealant,

(b) ‘Closed’ joint, with a compressible gasket and exterior sealant,

(c) ‘Open’ joint, with a flexible membrane seal, and arrangements to drain water, etc from the inside surfaces of the joint which are ‘open’ to the weather.Where appropriate, arrangements should be made at joints in buildings for con-tinuity or sealing of insulation and vapour barriers, etc to prevent formation of condensation, or loss of heat

rain-In structures where there is likely to be water in contact with the structural envelope, e.g structures buried in ground which has a high water table, or water-retaining structures, flexible waterbars are usually incorporated at construction andmovement joints These have the effect of interrupting the path along which anywater present has to travel

Figure 28.13(a), (b) and (c) are typical of wall details in reinforced concrete orbrickwork construction A typical joint detail for steel cladding in a building is

Fig 28.12 Weather-resistance of joints: (a) closed, with filler, (b) closed, with gasket,

(c) open, with membrane

metal insulation

cladding board

(exterior) PVC/metal

(exterior) compressible

it

1/supporting flexible joint

angle to board

(possible) compressible

shown in Fig 28.13(d), and Fig 28.13(e) shows a detail suitable for a building roofjoint, or small bridge deck movement joint

28.3.2.4 Direction of movement required

The direction of movement required at a joint will affect the form of the joint Alllikely movements should be evaluated, as restraint of unanticipated movements mayresult in failure of the joint, or development of large restraint forces in the joint andthe adjacent structure

Typical broad classifications of joint by movement requirements are given in Fig.28.14(a), (b), (c) and (d)

In order to illustrate the nature of additional movements which can occur in astructure, two particular cases relating to a bridge superstructure are considered (seeFig 28.15(a) and (b))

Fig 28.13 Typical joint details: (a) concrete walls, (b) concrete columns, (c) brick walls,

(d) cladding, (e) roof or bridge deck

4

PIFE or building paper

In Fig 28.15(a), the bridge deck is set to allow radial expansion from a fixed point

It will be observed that the expansion joint will require both longitudinal and lateral

movement capability

Figure 28.15(b) represents a cross-section throughout the end of the last span of

a bridge In (i) the distance from the actual point of rotation to the joint is small,and little vertical movement of the joint is necessary when flexure of the beamcauses rotation In (ii) the effect on the joint of a large overhang is demonstrated:

a much larger vertical movement is induced Further discussion of these and otherrelated effects is given in References 4–7

28.3.2.5 Magnitude of movement required

The magnitude of movement to be allowed for has a significant effect on the type

of joint Joints can be broadly classified into three groups on this basis:

(1) 0–25 mm movement: Common for buildings, where there is a tendency towards

the use of larger numbers of joints, each with a small movement The restricted

Fig 28.14 Typical joint details allowing for moderate movements

discon-(2) 25–300 mm movement: Joints capable of movements of this extent are more

common in bridge work, and are usually prefabricated units, although a simpleplate arrangement can be used in some cases (Fig 28.16(a)) Two basic types ofprefabricated units are common Figure 28.16(b) shows the first type, which uses

an elastomeric material in its construction to accommodate the movement.Figure 28.16(c) shows the second type, which is suited to the larger movementranges This type uses an arrangement of mechanical components supported onstub beams to form a moving joint Detailing of the structure must allow for thespace required by these units

(3) >300 mm movements: Joints for movements of this magnitude are unusual and

will be tailored to particular requirements, e.g expansion joints for suspensionbridge decks

28.3.2.6 Required performance of the joint surface

The nature of effects acting on the surface of the joint should be considered Inbuilding structures, this will frequently be limited to environmental effects on

Fig 28.15 Radial and rotational movement (a) Radial expansion from a fixed point,

(b) end rotation

steel plate 200—250 mm 'flap' (typ)

surfacing steel support

Joints on bridge decks are subject to heavy localized effects from wheel loads andcorrosive effects from de-icing salts and spilt chemicals As a result, the practical-ities of maintenance and perhaps eventual replacement of such joints should be con-sidered The resistance to skidding of the surface of such joints is also important

28.3.2.7 Load generation at joints

Frequently the loads generated by compression or extension of movement jointsare insignificant in terms of overall structural behaviour In some cases, however,the loads developed may be large enough to affect the design of other elements of

Fig 28.16 Joints allowing for larger movements: (a) (b) 200–250 mm, (c) 300–500 mm

the structure A particular example is a large expansion joint of the elastomeric type

in a bridge, where horizontal forces developed in moving the joint should be sidered in design or selection of the bearing which is to form the fixed point of thedeck

con-28.4 Bearings and joints – other considerations

In design and detailing of bearings and joints, it should be remembered that thepositions where they exist may be positions of concentrated load application and/orsignificant discontinuity in the structure Care should be taken in design to estab-lish and preclude all likely means of deterioration and failure This should extend

to adequate supervision to ensure correct installation, as many problems have beenattributed to substandard workmanship applied to an otherwise competent design.The need for maintenance of all joints should be assessed, and adequateallowance made where necessary for inspection, servicing, and facility of replacement

References to Chapter 28

1 Biddle A.R., Iles D.C & Yandzio E (1997) Integral Steel Bridges: Design

Guidance The Steel Construction Institute, Ascot, Berks.

2 British Standards Institution (1983) Steel, concrete and composite bridges Part 9: Section 9.1: Code of practice for design of bridge bearings BS 5400, BSI,

London

3 Kaushke W & Baigent M (1986) Improvements in the Long Term Durability of

Bearings in Bridges ACI Congress, San Antonio, USA, Sept.

4 Baigent M The Design and Application of Structural Bearings in Bridges.

Glacier Metal Co Ltd

5 Long J (1974) Bearings in Structural Engineering Newnes-Butterworths,

London

6 Lee D.J (1971) The Theory and Practice of Bearings and Expansion Joints in

Bridges Cement and Concrete Association.

7 Nicol T & Baigent M The Importance of Accurate Installation of Structural

Bearings and Expansion Joints Glacier Metal Co Ltd.

8 Wallace A.A.C (1988) Design: Bearings and Deck Joints ECCS/BCSA

International Symposium on Steel Bridges, Feb

9 Pritchard B & Hayward A.C.G (1988) Upgrading of the viaducts for the

Docklands Light Railway Symposium on Repair and Maintenance of Bridges,

June Construction Marketing Ltd

10 Emerson M Bridge Temperatures and Movements in the British Isles Transport and Road Research Laboratory Report LR 228 (See also TRRL reports LR

382 (W Black); LR 491 & LR 532 (M Taylor); LR 696, LR 744, LR 748, LR 765

(M Emerson et al.))

Chapter 29

Steel piles

by TONY BIDDLE and ED YANDZIO

This chapter is intended to serve as an introduction to the subject of steel piling and

a source of reference for further reading where detail design is required It is dividedinto two main parts, i.e 29.1 for bearing piles and 29.2 for sheet piles

29.1 Bearing piles

29.1.1 Uses

Bearing piles are used mainly to support vertical loads for which the main designrequirements are to restrict average settlement to a reasonable amount, eliminatedifferential settlement, and achieve an adequate factor of safety or load factoragainst overload

Steel piling offers many advantages compared with other types including:

driving as compared with boring

(DPA) on each pile

The ability to be transported and handled without concern for overstressing due toself-weight is an important consideration where piles need to be lifted, pitched anddriven in long lengths, e.g maritime structures Steel piles can withstand high drivingstresses, thereby enabling them to be installed without significant damage throughdifficult strata such as brownfield sites and subsurface layers of boulders or weath-ered rock Energy-absorbing structures such as jetties and dolphins can be particu-larly well constructed in steel by virtue of its ability to offer large elastic deformation

and bending strength without affecting its durability On projects such as river orestuary crossings, steel piles offer clear advantages, as the soils are typically granu-lar and waterlogged and unsuitable for satisfactory pile boring New applicationssuch as supports and abutments for integral bridges offer more economic con-struction methods and the property of compliance under lateral loading that reducesbending stresses and therefore section requirements.

29.1.2 Types of pile

Steel bearing piles are available in three basic profiles: universal column H-sections,tubular sections and box piles In addition steel sheet piles, High Modulus Piles and Combi-piles can be used to support vertical load as well as a soil retaining function

29.1.2.1 Universal bearing piles

Universal bearing piles are H-sections produced on a universal hot rolling mill –hence their name They are essentially the same as universal column sections, exceptthat they have uniform thickness throughout the section See Fig 29.1 Universalcolumn H-piles are made in two basic qualities of steel, grade S275 (yield =

sizes are presented in the Appendices

Steel H-piles are very efficient in providing a large surface area for generatingshaft friction resistance In any given foundation plan area, a greater number of

steel H-piles can be used in a group with a standard spacing of 2B (or 2 ¥ dia.)

868 Steel piles

Fig 29.1 Universal bearing pile

than concrete piles and either the load supported can be greater or the size of thegroup made smaller The stiffness of H-piles is different about each orthogonal axis,allowing designers to select the orientation necessary to achieve the most efficientdesign.

H-piles are used principally for terrestrial structures where the full length of thepile is embedded, e.g foundations for bridges and buildings, and therefore buckling

is not a problem because there is good soil lateral support They are used very tively to transfer bearing loads into buried bedrock and to get around buriedobstructions It is sometimes advisable to use special cast steel pile shoes tostrengthen the tip and prevent damage or buckling under hard driving conditionsinto bedrock where very high end bearing pressure can be achieved

effec-They are generally not the most suitable for conditions where long lengths of thepile shaft protrude above ground level and are unsupported by surrounding soil orthrough water, because buckling failure about the minor axis can occur at relativelylow axial loads In these cases consider the use of tubular or box piles that havemore stiffness

29.1.2.2 Tubular piles

Tubular piles were first used as foundations for offshore oil platforms in the oil fields

of Lake Maracaibo in Venezuela in the 1920s Initially, spare oil pipe was used out

of convenience but, as the supporting structures became more sophisticated, thecold rolling of piles in structural plate to project-specific diameters and wall thick-nesses became more common

Purpose-rolled tubular piles can be used, but high quality steel line pipe is fectly suitable for piling Tubular piles are available in a large range of diametersand wall thicknesses Typical sections used for piling purposes are produced as line

can be hot- or cold-rolled The cold-rolling process produces consistently higheryield strengths than those of hot-rolled steel products and this can have significantbenefits for highly loaded bearing pile and structural column-pile applications, andcan also permit harder driving

Steel tubular piles have a high stiffness and are therefore also suitable for siteswhere it is necessary to transfer bearing loads into buried bedrock They are par-ticularly suitable for marine structures, especially where sited in deep water, e.g.berthing jetties for deep draught vessels Increasing use is being made of steel tubu-lars in composite columns for buildings and bridges where the tube is first driven

as a pile before filling the upstand above ground level with a reinforced concretecore for added strength and rigidity This permits significant savings in cost due tofaster construction More applications will become possible as the results of researchinto the effects of pile driving, dynamic load testing, corrosion, and new coatings areanalysed and reported to design engineers

29.1.2.3 Box sheet piling

Hot-rolled steel sheet piling is more extensively used nowadays in permanent aswell as temporary retaining walls Applications are not only above-ground earth-retaining structures but also composite basement wall construction, urban railwayand road cuttings, bridge abutments, river bridge pier caissons, as well as the tradi-tional river and coastal protection works Economic design is enabled by betterknowledge of soil–structure interaction and highly developed design and analysisprogrammes

Box piles are particularly suitable for marine structures, such as jetties and dolphins, where part of the pile shaft is exposed above seabed level and the pilefunctions as a free-standing column or is connected at the head in clusters ofcolumns

Box piles are formed by welding two or more sheet pile sections together BothLarssen and Frodingham steel sheet piles (see section 29.2.3) can be used They can

be introduced into a line of sheet piling at any point where local heavy loads are to

be applied, for instance beneath bridge beams, or used separately They are clutchedtogether with adjacent sheet piles and can be positioned in a sheet pile abutment

so that its appearance is unaffected

Larssen box piles are formed by welding together two sheet pile sections withcontinuous welds, and Frodingham plated box piles are formed by continuouslywelding a plate to a pair of interlocked and intermittently welded sheet piles (seeFig 29.2)

Special box piles can be formed using other combinations of sheet piles Furtherinformation can be obtained from steel manufacturers

870 Steel piles

Fig 29.2 Box piles

Frodingham section steel sheet piling

U nicersal beam

29.1.2.4 High modulus piles

Buttressing a sheet pile wall with deep universal beams placed in ‘soldier’ fashion

creates a high modulus pile wall as shown in Fig 29.3 This section provides

addi-tional moment capacity and bearing capability The UBs are welded to the back of

a pair of Frodingham sheet piles and all three are driven together clutched to thenext panel

Tubular piles are available in a wide variety of sizes, and both Larssen and Frodingham sheet piles are suitable as infill piles Appropriate clutches are welded

to the walls of the tubes, and these provide the designer with simple corner detailsfor many different wall angles It is apparent that the clutch locking bar strips fromU-section sheet piles (or Larssen sections) are more stable to weld onto the tubu-lars than those from Frodingham section sheet piles

allow-a discipline where the allow-allowallow-able stress allow-approallow-ach allow-and terms such allow-as the ‘allow-allowallow-ablebearing pressure’, ‘permissible steel stress’, and ‘allowable pile capacity’ are widelyaccepted and understood Nevertheless, the LSD approach is being progressivelyadopted in the British Standards as they are revised, and is the basis for all the

29.1.3.2 Design standards

The common design standard used for the design of bearing piles is the offshoreindustry’s recommended practice for steel tubular piles, based on US and UK NorthSea experience, which is contained in the American Petroleum Institute Code RP

the National Application Document (NAD), was published in July 1995 It presents

a more rigorous treatment of limit state design (LSD) than any of the British dards relating to foundations so far Allowable stress design (ASD) is still, however,

Further information can be obtained from the SCI publication Steel Bearing Piles

Guide.11

872 Steel piles

horizontal and vertical foundation loads

hor-is larger, it hor-is better reshor-isted by the inclusion of raking piles which provide greatlyimproved stiffness in the horizontal plane Any induced uplift forces resulting fromthe use of raking piles can be resisted by the weight of the superstructure or bytension in the remaining associated piles in a group (see Fig 29.5)

29.1.3.4 Axial capacity and load transfer

A pile subjected to a load parallel to its longitudinal axis will support that load partly

by shear generated over its shaft length, due to the soil–pile skin friction or sion, and partly by normal stresses generated at the base or tip of the pile, due toend bearing resistance of the soil (see Fig 29.6)

Rc= Rs+ Rb= qsAs+ qbAb

Fig 29.5 Raking pile foundation

layer down the length of the pile

The relative magnitudes of the ultimate shaft friction and ultimate end bearingresistances depend on the geometry of the pile and the soil profile Where a pile isembedded in a relatively soft layer of soil, but bears on a firmer stratum beneath,

the pile is referred to as an end bearing pile It derives most of its capacity from the

on which to found the pile, the pile is known as a friction pile In cohesive soils the

capacity is more evenly divided between shaft friction and base resistance capacity.Numerous computer programs are available commercially to calculate the verti-cal capacity of piles, but the calculations are very simple One of these is PILE, which

29.1.3.5 Mobilization of shaft friction and base resistance

The equation presented above considers the ultimate limit state condition onlywhere the pile has been allowed to move sufficiently to allow both the ultimate shaft

874 Steel piles

P

Skin friction resistance

End bearing resistance

Fig 29.6 Wall friction and end bearing resistance against vertical loads

friction and the ultimate base resistance capacities to be developed Commonly, load

transfer curves are produced, which are plots of load resistance versus vertical

move-ment of the pile head for displacemove-ments ranging from zero to the ultimate limit or

to a permissible maximum value (e.g 40 mm) These plots include mobilizedsoil–pile shear transfer versus local pile movement and mobilized base resistanceversus tip movement

Axial load transfer curves for clays and sands have been obtained from full-scaleresearch static pile load tests These pile tests monitor both the load and strain at anumber of positions along the pile Tests that have been performed on clays and finegranular soils show the general behaviour (see Fig 29.7)

It is found that, at any position along the pile, full shaft friction resistance is notdeveloped until the pile has moved axially (relative displacement between pile andsoil) to a magnitude in the range 7–10 mm Once this movement is reached nofurther additional wall friction resistance develops, and apart from any small reduc-tion in peak value of wall friction (due possibly to strain softening), the curve tends

to a nearly constant resistance This constant resistance value can be assumed to bethe ultimate unit shaft friction of the soil at any level

The behaviour of a pile in clay in base resistance is also shown in Fig 29.7 It isseen from the base resistance load versus pile tip deflection (equal to pile headmovement) curve that a greater pile tip deflection is required to achieve the ulti-mate base resistance capacity than is required to develop the shaft friction capac-ity, i.e in excess of 40 mm

The mobilization of the total axial resistance with increasing displacement for apile is obtained from the summation of the mobilized shaft friction resistance andthe mobilized base resistance (see Fig 29.7)

Fig 29.7 Axial load transfer curves for soils

29.1.3.6 Vertical settlement and serviceability

x is the material factor to take into account uncertainty of soil parametersdetermined on site or in the laboratory

The unit shaft friction qs for clays can be estimated in terms of the undrained shear

strength of the soil and is given by the relationship

where a is a dimensionless factor

verified an average value of 0.5 for design for most over-consolidated clays

876 Steel piles