Steel Designer''''s Manual Part 2 doc

Plain composite beams and shallow metal deckSimple beams with precast slabs Slim floor and deep metal deck Beam span m Fig.. Composite steel floor trussesUse of composite steel floor tru

Fig 2.7 Integration of services: (a) separated (traditional); (b) integrated (shallow floor

‘Slimdek ® ’ system); (c) integrated (long span ‘primary’ beam – stub girder); (d) grated (long span ‘secondary’ beams)

inte-Fig 2.8 Tapered beams and services

01 deflection

Ms2 moment of

resistance

of connection-I- T

4-02<01-H h-

Recently, shallow floor systems have been developed for spans up to about 9 mwhich allow integration of services within the slab depth Structural systems rangefrom conventional fabricated beams using precast units to proprietary systems usingnew asymmetric rolled beams and deep metal decking These approaches can formthe basis of energy-saving sustainable solutions

Semi-continuous braced frames can provide an economic balance between theprimary benefits associated with simple or continuous design alternatives Thedegree of continuity between the beams and columns can be chosen so that complexstiffening to the column is not required Methods of analysis have been developedfor non-composite construction to permit calculation by hand It is possible toachieve reduced beam depths and reduced beam weights

Fig 2.9 Floor depth: (a) simple; (b) semi-continuous; (c) continuous

External wall construction

The external skin of a multi-storey building is supported off the structural frame

In most high quality commercial buildings the cost of external cladding systemsgreatly exceeds the cost of the structure This influences the design and construc-tion of the structural system in a number of ways:

• The perimeter structure must provide a satisfactory platform to support thecladding system and be sufficiently rigid to limit deflections of the external wall

• A reduction to the floor zone may significantly reduce the area and hence cost

of cladding

• Fixings to the structure should facilitate rapid erection of cladding panels

• A reduction in the weight of cladding at the expense of cladding cost will notnecessarily lead to a lower overall construction cost

Lateral stiffness

Steel buildings must have sufficient lateral stiffness and strength to resist wind andother lateral loads In tall buildings the means of providing sufficient lateral stiff-ness forms the dominant design consideration This is not the case for low- tomedium-rise buildings

Most multi-storey buildings are designed on the basis that wind and/or notionalhorizontal forces acting on the external cladding are transmitted to the floors, whichform horizontal diaphragms transferring the lateral load to rigid elements and then

to the ground These rigid elements are usually either braced-bay frames, jointed frames, reinforced concrete or steel–concrete–steel sandwich shear walls.Low-rise unbraced frames up to about six storeys may be designed using the sim-plified wind-moment method In this design procedure, the frame is made staticallydeterminate by treating the connections as pinned under vertical load and rigidunder horizontal loads This approach can be used on both composite and non-composite frames, albeit with strict limitations on frame geometry, loading patternsand member classification

rigid-British Standard BS 5950 sets a limit on lateral deflection of columns as height/300but height/600 may be a more reasonable figure for buildings where the externalenvelope consists of sensitive or brittle materials such as stone facings

Accidental loading

A series of incidents in the 1960s culminating in the partial collapse of a built tower block at Ronan Point in 1968 led to a fundamental reappraisal of theapproach to structural stability in building

system-Traditional load-bearing masonry buildings have many in-built elements ing inherent stability which are lacking in modern steel-framed buildings Modernstructures can be refined to a degree where they can resist the horizontal and

provid-vertical design loadings with the required factor of safety but may lack the ability

to cope with the unexpected

It is this concern with the safety of the occupants and the need to limit the extent

of any damage in the event of unforeseen or accidental loadings that has led to theconcept of robustness in building design Any element in the structure that supports

a major part of the building either must be designed for blast loading or must becapable of being supported by an alternative load path In addition, suitable tiesshould be incorporated in the horizontal direction in the floors and in the verticaldirection through the columns The designer should be aware of the consequences

of the sudden removal of key elements of the structure and ensure that such anevent does not lead to the progressive collapse of the building or a substantial part

of it In practice, most modern steel structures can be shown to be adequate withoutany modification

Cost considerations

The time taken to realize a steel building from concept to completion is generallyless than that for a reinforced concrete alternative This reduces time-related build-ing costs, enables the building to be used earlier, and produces an earlier return onthe capital invested

To gain full benefit from the ‘factory’ process and particularly the advantages

of speed of construction, prefabrication, accuracy and lightness, the cladding andfinishes of the building must have similar attributes The use of heavy, slow and insitu finishing materials is not compatible with the lightweight, prefabricated and fastconstruction of a steel framework

The cost of steel frameworks is governed to a great extent by the degree of plicity and repetition embodied in the frame components and connections This alsoapplies to the other elements which complete the building

sim-The criterion for the choice of an economic structural system will not ily be to use the minimum weight of structural steel Material costs represent only30–40% of the total cost of structural steelwork.The remaining 60–70% is accountedfor in the design, detailing, fabrication, erection and protection Hence a choicewhich needs a larger steel section to avoid, say, plate stiffeners around holes orallows greater standardization will reduce fabrication costs and may result in themost economic overall system

necessar-Because a steel framework is made up of prefabricated components produced in

a factory, repetition of dimensions, shapes and details will streamline the turing process and is a major factor in economic design (Fig 2.10)

manufac-Fabrication

The choice of structural form and method of connection detailing have a significantimpact on the cost and speed of fabrication and erection Simple braced frameworkswith bolted connections are considered the most economic and the fastest to buildfor low- to medium-rise buildings

Economy is generally linked to the use of standard rolled sections but, with theadvent of automated cutting and welding equipment, special fabricated sections arebecoming economic if there is sufficient repetition.

The development of efficient, automated, cold-sawing techniques and punchingand drilling machines has led to the fabrication of building frameworks with boltedassemblies Welded connections involve a greater amount of handling in the fabri-cation shop, with consequent increases in labour and cost

Site-welded connections require special access, weather protection, inspectionand temporary erection supports By comparison, on-site bolted connections enablethe components to be erected rapidly and simply into the frame and require nofurther handling

The total weight of steel used in continuous frames is less than in continuous or simple frames, but the connections for continuous frames are morecomplex and costly to fabricate and erect On balance, the cost of a continuous framestructure is greater, but there may be other considerations which offset this cost differential For example, in general the overall structural depth of continuousframes is less This may reduce the height of the building or improve the distribution

semi-of building services, both semi-of which could reduce the overall cost semi-of the building.Corrosion protection to internal building elements is an expensive and time-consuming activity Experience has shown that it is unnecessary for most internallocations and consequently only steelwork in risk areas should require any protec-tion Factory-applied coatings of intumescent fire protection can be cost-effectiveand time-saving by removing the operation from the critical path

Construction

A period of around 8–12 weeks is usual between placing a steel order and the arrival

of the first steel components on site Site preparation and foundation constructiongenerally take a similar or longer period (see Fig 2.11) Hence, by progressing fab-rication in parallel with site preparation, significant on-site construction time may

be saved, as commencement of shop fabrication is equivalent to start-on-site for an

in situ concrete-framed building By manufacturing the frame in a factory, the risks

Fig 2.10 Structural costs: (a) economic and (b) uneconomic layouts

of delay caused by bad weather or insufficient or inadequate construction resources

in the locality of the site are significantly reduced

Structural steel frameworks should generally be capable of being erected withouttemporary propping or scaffolding, although temporary bracing will be required,especially for welded frames This applies particularly to the construction of the con-crete slab, which should be self-supporting at all stages of erection Permanent metal

or precast concrete shutters should be used to support the in situ concrete

In order to allow a rapid start to construction, the structural steelwork frameshould commence at foundation level, and preference should be given to singlefoundations for each column rather than raft or shared foundations (Fig 2.12).Speed of erection is directly linked to the number of crane hours available Toreduce the number of lifts required on site, the number of elements forming theframework should be minimized within the lifting capacity of the craneage provided

on site for other building components For similar-sized buildings, the one with thelonger spans and fewer elements will be the fastest to erect However, as has beenmentioned earlier, longer spans require deeper, heavier elements, which willincrease the cost of raw materials and pose a greater obstruction to the distribution

of building services, thereby requiring the element to be perforated or shaped andhence increasing the cost of fabrication

Columns are generally erected in multi-storey lengths: two is common and three

is not unusual The limitation on longer lengths is related more to erection than torestrictions on transportation, although for some urban locations length is a majorconsideration for accessibility

To provide rapid access to the framework the staircases should follow the tion of the frame This is generally achieved by using prefabricated stairs which aredetailed as part of the steel frame

erec-The speed of installation of the following building elements is hastened if theirconnection and fixing details are considered at the same time as the structural steelframe design In this way the details can be either incorporated in the framework

or separated from it, whichever is the most effective overall: it is generally moreefficient to separate the fixings and utilize the high inherent accuracy of the frame

Fig 2.12 Columns on large diameter bored piles

to use simple post-fixed details, provided these do not require staging or ing to give access.

scaffold-Finally, on-site painting extends the construction period and provides potentialcompatibility problems with following applied fire protection systems Paintingshould therefore only be specified when absolutely necessary

2.3 Anatomy of structure

In simple terms, the vertical load-carrying structure of a multi-storey building comprises a system of vertical column elements interconnected by horizontal beamelements which support floor-element assemblies The resistance to lateral loads isprovided by diagonal bracing elements, or wall elements, introduced into the verti-cal rectangular panels bounded by the columns and beams to form vertical trusses,

or walls Alternatively, lateral resistance may be provided by developing a ous or semi-continuous frame action between the beams and columns The flexibil-ity of connections should be taken into account in the analysis All structures shouldhave sufficient sway stiffness, so that the vertical loads acting with lateral displace-ments of the structure do not induce excessive secondary forces or moments in themembers or connections A building frame may be classed as ‘non-sway’ if the swaydeformation is sufficiently small for the resulting secondary forces and moments

continu-to be negligible In all other cases the building frame should be classed as sensitive’ The stiffening effect of cladding and infill wall panels may be taken intoaccount by using the method of partial sway bracing The floor-element assembliesprovide the resistance to lateral loads in the horizontal plane

‘sway-In summary, the components of a building structure are columns, beams, floorsand bracing systems (Fig 2.13)

These are generally standard, universal column, hot-rolled sections They provide acompact, efficient section for normal building storey heights Also, because of thesection shape, they give unobstructed access for beam connections to either theflange or web For a given overall width and depth of section, there is a range ofweights which enable the overall dimensions of structural components to be nomi-nally maintained for a range of loading intensities

Where the loading requirements exceed the capacity of standard sections, tional plates may be welded to the section to form plated columns, or fabricatedcolumns may be formed by welding plates together to form a plate-column (Fig 2.14).The use of circular or rectangular tubular elements marginally improves the load-carrying efficiency of components as a result of their higher stiffness-to-weight ratio.Concrete filling significantly improves the axial load-carrying capacity and fireresistance

Fig 2.13 Conventional steel frame components

Fig 2.14 Types of column: (a) plated (by addition of plates to U.C section); (b) universal;

(c) tubular; (d) fabricated plate

2.3.2 Beams

Structural steel floor systems consist of prefabricated standard components, andcolumns should be laid out on a repetitive grid which establishes a standard struc-tural bay For most multi-storey buildings, functional requirements will determinethe column grid which will dictate spans where the limiting criterion will be stiff-ness rather than strength (Fig 2.15)

Steel components are uni-directional and consequently orthogonal structuralcolumn and beam grids have been found to be the most efficient The most efficientfloor plan is rectangular, not square, in which main, or ‘primary’, beams span theshorter distance between columns and closely-spaced ‘secondary’ floor beams spanthe longer distance between main beams The spacing of the floor beams is con-trolled by the spanning capability of the concrete floor construction (Fig 2.16)

Fig 2.15 Typical floor layout

Plain composite beams and shallow metal deck

Simple beams with precast slabs

Slim floor and deep metal deck

Beam span (m)

Fig 2.16 Beam and shallow deck layout: (a) inefficient; (b) efficient

Having decided on the structural grid, the designer must choose an economicstructural system to satisfy all the design constraints The choice of system and itsdepth depends on the span of the floor (Fig 2.17) The minimum depth is fixed bypractical considerations such as fitting practical connections As the span increases,the depth will be determined by the bending strength of the member and, for longerspans, by the stiffness necessary to prevent excessive deflection under imposed load

or excessive sensitivity to induced vibrations (Fig 2.18) For spans up to 9 m, shallowbeams with precast floors or deep composite metal deck floors can be used to

Fig 2.17 Span ranges for different composite floor systems

of castellated beams fabricated from standard sections, cellular sections or plates.Above 15 m, composite steel trusses may be economic As the span increases, thedepth and weight of the structural floor increase, and above 15 m spans depth pre-dominates because of the need to achieve adequate stiffness.

Castellated and cellular beam sections

Castellated beams (Fig 2.19(a)) have been used for many years to increase thebending capacity of the beam section and to provide limited openings for services.These openings are rarely of sufficient size for ducts to penetrate without sig-nificant modification, which increases fabrication cost The cellular concept is adevelopment of castellated beams that provides circular openings and greater shearcapacity Since their introduction in 1990, they have proved to be increasinglypopular for longer span solutions where services and structure have to be integrated.Bespoke openings for services can be cut in the webs of universal beams and fabricated plate girders

Fig 2.18 Structural criteria governing choice

—(a) castellated beam (b) cellular beam

(c) beam with web openings

Fabricated plate girders

Conventional universal beams span a maximum of about 15 m Recent advances inautomatic and semi-automatic fabrication techniques have allowed the economicproduction of plate girders for longer span floors Particularly if a non-symmetricplate girder is used, it is possible to achieve economic construction well in excess of

15 m (Fig 2.20) Such plate girders can readily accommodate large openings formajor services If these are in regions of high stress, single-sided web stiffening may

be used Away from regions of high stress, stiffening is usually not required.The use of intumescent paints, applied offsite, is becoming increasingly popular.One major fabricator is now offering an integrated design and fabrication servicefor customized plate girders which can achieve a fire resistance of 2 hours whenapplied as a single layer in an off-site, factory-controlled process

Taper beams

Taper beams (Fig 2.20) are similar to fabricated plate girders except that their depthvaries from a maximum in mid-span to a minimum at supports, thus achieving ahighly efficient structural configuration For simply-supported composite taperbeams in buildings the integration of the services can be accommodated by locat-ing the main ducts close to the columns Alternative taper beam configurations can

be used to optimize the integration of the building services

Fig 2.19 Beams with web openings for service penetrations

Composite steel floor trusses

Use of composite steel floor trusses as primary beams in the structural floor systempermits much longer spans than would be possible with conventional universalbeams.The use of steel trusses for flooring systems is common for multi-storey build-ings in North America but seldom is used in Britain Although they are consider-ably lighter than the equivalent universal beam section the cost of fabrication is very

Fig 2.20 Fabricated plate girders and taper beams

to accept service ducts, and if a larger opening is required a Vierendeel panel can

be incorporated at the centre of the span Because a greater depth is required forfloor trusses, the integration of the services is always within the structural zone (Fig.2.21)

Stub girder construction

Stub girders were developed in North America in the 1970s as an alternative form

of construction for intermediate range spans of between 10 and 14 m They have notbeen used significantly in the UK Figure 2.22 shows a typical stub girder with abottom chord consisting of a compact universal column section which supports thesecondary beams at approximately 3-metre centres Between the secondary beams

a steel stub is welded on to the bottom chord to provide additional continuity and

to support the floor slab The whole system acts as a composite Vierendeel truss Adisadvantage of stub girders is that the construction needs to be propped while theconcrete is poured and develops strength Arguably, a deep universal beam withlarge openings provides a more cost-effective alternative to the stub girder because

of the latter’s high fabrication content

Fig 2.21 Composite truss

• precast concrete slabs acting non-compositely with the floor beams:

• in situ concrete slab, with conventional removable shuttering, acting compositelywith the floor beams;

• in situ concrete slab cast on thin precast concrete slabs to form a composite slab,which in turn acts compositely with the floor beams

The most widely used construction internationally is profiled shallow metal decking.Composite action with the steel beam is normally provided by shear connectorswelded through the metal decking on to the beam flange Shallow floor systemsusing deep metal decking are gaining popularity in the UK although precast con-crete systems are still used extensively Composite action enables the floor slab towork with the beam, enhancing its strength and reducing deflection (Fig 2.24).Because composite action works by allowing the slab to act as the compressionflange of the combined steel and concrete system, the advantage is greatest whenthe beam is sagging Consequently composite floor systems are usually designed assimply supported

Fig 2.22 Stub girder

precast

—L concreteshutter welded shear stud

metal deck

,— asymetric slimfior'

J beam (ASB)

I—

•1 k—deep

metal deck

Shallow metal deck floor construction

Experience has shown that the most efficient floor arrangements are those usingshallow metal decking spanning about 3–4.5 m between floor beams For these spansthe metal decking does not normally require propping during concreting and the

Fig 2.23 Floor construction: (a) precast (non-composite); (b) in situ (composite); (c) in

situ/precast (composite); (d) in situ/shallow metal decking (composite); (e) floor – in situ/deep metal decking (composite); (f) Slimdek ® – in situ/deep metal decking (composite)

Slim-Fig 2.24 Metal deck floor slabs

Some of the advantages of composite shallow metal deck floor construction are:

• Steel decking acts as permanent shuttering, which can eliminate the need for slabreinforcement and, due to its high stiffness and strength, propping of the con-struction while the wet concrete develops strength

• Composite action reduces the overall depth of structure

• It provides up to 2 hours fire resistance without additional fire protection and 4hours with added thickness or extra surface protection

• It is a light, adaptable system that can be easily manhandled on site, cut toawkward shapes and drilled or cut out for additional service requirements

• Lightweight construction reduces frame loadings and foundation costs

• It allows simple, rapid construction techniques

Figure 2.26 illustrates alternative arrangements of primary and secondary beams for

a deck span of 3 m

Fig 2.25 Metal deck profiles: (a) shallow deck (50–100 mm); (b) deep deck (150–250 mm)

11111:1 Jooo

(a)

(b)

Deep metal deck, shallow floor construction

Deep metal decks are normally used to create shallow floors e.g Slimflor® andSlimdek®construction The deep metal deck extends the span capability up to 9 m;however, the deck and/or support beams may require propping during concreting

An additional tensile reinforcement bar is provided within the ribs of the deepdecking to improve the load carrying capacity and fire resistance of the floor slab.Although there are several variants internationally, Slimflor®construction in the UKcomprises a universal column with a plate welded to the underside supporting thedeep metal decking Shear studs are shop-welded to the top flange of the beam toform a connection between the steel beam and concrete slab Slimdek® construc-tion is a technologically advanced solution comprising a rolled asymmetric beam(ASB) which supports the deep metal decking directly Shear connection is achievedthrough the bond developed between the steel beam and concrete encasure Thesefeatures reduce material and fabrication content Partial concrete encasement of thesteel beam provides up to 1 hour inherent fire resistance

The range of deep metal deck profiles (Fig 2.25(b)) is more limited than forshallow decks, and those available carry similar attributes and advantages Someadditional advantages of Slimflor®and Slimdek®construction are:

• The shallow composite slab achieves excellent load capacity, diaphragm actionand robustness

• There are fewer frame components to erect, saving construction time

• The shallow floor construction allows more floors for a given building height orreduces the cost of cladding and vertical services, lift shafts, etc

• It provides up to 1 hour inherent fire resistance and up to 2 hours using passivefire protection to the beam soffit only

• Large openings for vertical services can be formed in the floor slab without theneed for secondary framing

Fig 2.26 Alternative framing systems for floors: (a) long span secondary beams; (b) long

span primary beams

RHSFB edge beam

5 to 9 m

tie member

web-openings in the beam

• ASBs achieve composite action without the need for shear studs

• Rectangular hollow section edge beams provide good torsional resistance andmaintain the shallow floor depth

• Floor slabs can be used for fabric energy storage forming part of an mentally sustainable building solution

environ-Figure 2.27 illustrates a typical beam layout at the building perimeter

Precast floor systems

Universal beams supporting precast prestressed floor units (Fig 2.28) have someadvantages over other forms of construction Although of heavier construction than comparable composite metal deck floors, this system offers the followingadvantages

difficulty

• No propping is required

• Shallow floor construction can be obtained by supporting precast floor units onshelf angles or on wide plates attached to the bottom flanges of universal columnsacting as beams (Slimfloor)

• Fast construction because no time is needed for curing and the development ofconcrete strength

Fig 2.27 Slimdek ® floor arrangement

On the other hand the disadvantages are:

• Composite and diaphragm action is not readily achieved without a structuralfloor screed

• Heavy floor units are difficult to erect in many locations and require the use of

a tower crane, which may have implications for the construction programme

2.3.4 Bracings

Three structural systems are used to resist lateral loads: continuous or wind-momentframes, reinforced concrete walls and braced-bay frames (Fig 2.29) Combinations

of these systems may also be used

Fig 2.28 Precast concrete floors

U U U

The advantage of a continuous frame is:

• Provides total internal adaptability with no bracings between columns or walls

to obstruct circulation

Fig 2.29 Bracing structures: (a) continuous frame; (b) reinforced concrete wall; (c) braced

bay frames

additional bracing/rigid frame required

However, the disadvantages are:

• Increased fabrication for complex framing connections

• Increased site connection work, particularly if connections are welded

• Columns are larger to resist bending moments

• Generally, less stiff than other bracing systems

Wind-moment frames are limited in application

Fig 2.30 Core locations: (a) efficient; (b) inefficient

The advantages of shear walls are:

• The beam-to-column connections throughout the frame are simple, easily cated and rapidly erected

fabri-• Shear walls tend to be thinner than other bracing systems and hence save space

in congested areas such as service and lift cores

• They are very rigid and highly effective

• They act as fire compartment walls

The disadvantages are:

• The construction of walls, particularly in low- and medium-rise buildings, is slowand less accurate than steelwork

• The walls are difficult to modify if alterations to the building are required in thefuture

• They are a separate form of construction, which is likely to delay the contractprogramme

• It is difficult to provide connections between steel and concrete to transfer thelarge forces generated

Recent developments in steel–concrete–steel composite sandwich construction (Bi-steel®) largely eliminate these disadvantages and allow pre-fabricated and fullyassembled lift shafts to be erected simultaneously with the main steel framing.Steel–concrete–steel construction can also be used for blast-resistant walls andfloors

Braced-bay frames

Braced-bays are positioned in similar locations to reinforced concrete walls, so theyhave minimal impact upon the planning of the building They act as vertical trusseswhich resist the wind loads by cantilever action

The bracing members can be arranged in a variety of forms designed to carrysolely tension or alternatively tension and compression When designed to taketension only, the bracing is made up of crossed diagonals Depending on the winddirection, one diagonal will take all the tension while the other remains inactive.Tensile bracing is smaller than the equivalent strut and is usually made up of flat-plate, channel or angle sections When designed to resist compression, the bracingsbecome struts and the most common arrangement is the ‘K’ brace

The advantages of braced-bay frames are:

• All beam-to-column connections are simple

• The braced bays are concentrated in location on plan

• The bracing configurations may be adjusted to suit planning requirements tric bracing)

(eccen-0 0

• The system is adjustable if building modifications are required in the future

• Bracing can be arranged to accommodate doors and openings for services

• Bracing members can be concealed in partition walls

• They provide an efficient bracing system

A disadvantage is:

• Diagonal members with fire proofing can take up considerable space

Fig 2.31 Connections: (a) simple; (b) continuous; (c) semi-continuous

(1) Simple connections transmit negligible bending moment across the joint: theconnection is detailed to allow the beam end to rotate The beam behaves as asimply supported beam.

(2) Continuous connections are designed to transmit shear force and bendingmoment across the joint The connection should have sufficient stiffness ormoment capacity as appropriate to justify analysis by either elastic or plasticanalysis Beam end moments are transmitted into the column itself and anybeam framing into the column on the opposite side

(3) Semi-continuous connections are designed to transmit the shear force and aproportion of the bending moment across the joint The principle of these connections is to provide a partial restraint to beam end-rotation without introducing complicated fabrication to the joint However, the design of suchjoints is complex, and so simple design procedures based upon experimental evidence have been developed for wider application The advantages of semi-continuous design are lighter beams without the corresponding increase incolumn size and joint complexity that would be the case with fully continuousconnections

References to Chapter 2

1 Hart F., Henn W., Sontag H & Godfrey G.B (Ed.) (1985) Multi-Storey Buildings

in Steel, 2nd edn Collins, London.

2 National Economic Development Office and Economic Development

Commit-tee for Constructional SCommit-teelwork (1985) Efficiency in the Construction of SCommit-teel

Framed Multi-Storey Buildings NEDO, Sept.

3 Owens G (1987) Trends and Developments in the Use of Structural Steel for

Multi-Storey Buildings Steel Construction Institute, Ascot, Berks.

4 McGuire W (1968) Steel Structures Prentice-Hall.

5 Zunz G.J & Glover M.J (1986) Advances in Tall Buildings Council on Tall

Build-ings and Urban Habitat Van Nostrand Reinhold

Further reading for Chapter 2

Brett P & Rushton J (1990) Parallel beam approach – a design guide The Steel

Con-struction Institute, Ascot, Berks

Couchman G.H (1997) Design of semi-continuous braced frames The Steel

Con-struction Institute, Ascot, Berks

Hensman J.S & Way A.G.J (2000) Wind-moment design of unbraced composite

frames The Steel Construction Institute, Ascot, Berks.

Lawson R.M & Rackham J.W (1989) Design of haunched composite beams in

build-ings The Steel Construction Institute, Ascot, Berks.

Lawson R.M & McConnel R (1993) Design of stub girders The Steel Construction

Institute, Ascot, Berks

Lawson R.M., Mullett D.L & Rackham J.W (1997) Design of asymmetric Slimflor

beams using deep composite decking The Steel Construction Institute, Ascot,

Berks

Lawson et al (2002) Design of FABSEC Beams in Non-Composite Applications

(Including Fire) The Steel Construction Institute, Ascot, Berks.

Mullett D.L (1992) Slim floor design and construction The Steel Construction

Insti-tute, Ascot, Berks

Mullett D.L (1998) Composite Floor Systems Blackwell Science.

Mullett D.L & Lawson R.M (1999) Design of Slimflor fabricated beams using deep

composite decking The Steel Construction Institute, Ascot, Berks.

Narayanan R., Roberts T.M & Naji F.J (1994) Design guide for steel–concrete–steel

sandwich construction – Volume 1: General principles and rules for basic elements.

The Steel Construction Institute, Ascot, Berks

Owens G.W (1989) Design of fabricated composite beams in buildings The Steel

Construction Institute, Ascot, Berks

Salter P.R., Couchman G.H & Anderson D (1999) Wind-moment design of low rise

frames The Steel Construction Institute, Ascot, Berks.

Skidmore, Owings & Merrill (1992) Design of composite trusses The Steel

Con-struction Institute, Ascot, Berks

Ward J.K (1990) Design of composite and non-composite cellular beams The Steel

Construction Institute, Ascot, Berks

Yandzio E & Gough M (1999) Protection of buildings against explosions The Steel

Construction Institute, Ascot, Berks

A worked example follows which is relevant to Chapter 2

Vertical bracing

Typical arrangementof

stair tower

MULTI-STOREY DESIGN EXAMPLE

BS 5950: Part 1: 2000 Building geometry

Typical Floor Plan Building use: Office building with basement car parking and high-level plant room Imposed loading for office floors exceeds minimum statutory loading given in BS 6399-1: 1996 at client’s request This design example illustrates the design of elements

in the braced towers provided in four corners of the building to achieve lateral ity The floor plate is generally 130 mm lightweight aggregate concrete on metal decking that acts compositely with the decking and floor beams and is assumed to provide diaphragm action Fire protection is achieved with a sprayed intumescent coating Alter- natively, fire protection could be removed from a number of beams by adopting the fire safe design approach outlined in Chapter 34.

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Area supported

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Column dead loads

(2) 8

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Column imposed loads

1117 Wind directions

MULTI-STOREY DESIGN EXAMPLE

BS 5950: Part 1: 2000

2 6

Building wall height H = 46 m

Distance to sea = 80 km (upwind @ 210 deg ± 45 deg)

Terrain = town

Logarithmic interpolation is optional)

Note: normally, either all wind directions should be checked to establish the highest effective wind speed or a conservative approach may be taken by using a value of

S d = 1.0 together with the shortest distance to sea irrespective of direction A lower value

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Distance to sea = 80 km Terrain = town

Site exposure type = A

Example to table on page 85

(1) (4.37 ¥ 1688) + (4.37 ¥ 158) + (1.5 ¥ 189.6) = 8351 kN

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BS 5950: Part 1: 2000

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F is the total force applied at that level The factored minimum force ex dead load (F¢)

is clearly less than the value of factored wind load and will be ignored in further calculations.

Loads are divided by 4 in the sub-model analysis to represent the force applied to one braced tower To justify this approach the behaviour of the whole building was analysed

was sufficiently accurate One of the vertical braced towers was then analysed using CSC

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Notional horizontal forces (NHF)

To allow for the effects of practical imperfections such as lack of verticality, notional horizontal forces are considered.

At each level,

F NHF = 0.5% of the factored vertical dead and imposed loads at that level.

For simplicity, calculations for the NHFs do not include any % imposed load reductions.

NHF = 0.5/(100 ¥ 4) ¥ [(1.4 ¥ 8351) + (1.6 ¥ 3164)] = 20.9 Plant room

NHF = 0.5/(100 ¥ 4) ¥ [(1.4 ¥ 9361) + (1.6 ¥ 13292)] = 43.0 Office floors (typ.)

NHF = 0.5/(100 ¥ 4) ¥ [(1.4 ¥ 7118) + (1.6 ¥ 9072)] = 30.6 Ground

NHF = 0.5/(100 ¥ 4) ¥ [(1.4 ¥ 4375) + (1.6 ¥ 9072)] = 25.8 Basement 1

NHF = 0.5/(100 ¥ 4) ¥ [(1.4 ¥ 4873) + (1.6 ¥ 4852)] = 18.2

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direc-in the positive and negative direction to give the worst effect

Sway stiffness P–d effects

All structures (including portions between expansion joints) should

have sufficient sway stiffness, so that the vertical loads acting with

the lateral displacements of the structure do not induce excessive

secondary forces or moments in the members or connections Where

such ‘second order’ (P–d) effects are significant, they should be

allowed for in the design of those parts of the structure that

con-tribute to its resistance to horizontal forces.

Sway stiffness should be provided by the system of resisting

hori-zontal forces Whatever system is used, sufficient stiffness should be

provided to limit sway deformation in any horizontal direction and

also to limit twisting of the structure on plan.

Except for single-storey frames with moment-resisting joints, or

other frames in which sloping members have moment-resisting

every storey.

has been determined from:

d = storey drift due to NHFs

lcr=h / 200( ¥ d)

2.4.2.5

2.4.2.6

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Because of symmetry one of the four braced towers only was modelled as a space frame

Basic load cases

1 Dead load

2 Reduced imposed loads

3 Wind loading (Y direction)

4 NHF (Y direction)

Results from NHF load case

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method may be used.

This gives an amplification factor for NHFs in the Y direction of:

(for clad structures, provided that the stiffening effect of masonry

infill wall panels or diaphragms of profiled steel sheeting is not

explicitly taken into account.)

Cautionary notes to analysis approach

1 Results are for sway only in the Y direction using NHF in the

Y direction from the gravity combination This analysis must

normally be repeated for sway in the X direction using NHF in

the X direction The choice is then to either:

applied to loads in both directions, or

and applied to each appropriate combination.

2 The results from a simple tower constrained out-of-plane at all

levels take no account of out-of-plane deflections and their

influence on P–d effects An analysis of the whole building is

required to spot this effect and determine whether it is

signifi-cant In this example the tower constrained out of plane is only

a reasonable assumption owing to the very symmetric nature of

the building as a whole If the whole building is considered then

there could be other columns (even simple ones not in braced

bays) that lean over more These will have a detrimental effect

on the braced areas That is, whilst not affected themselves

(being simple columns), they will increase the P–d effect on the

braced towers Taking the worst sway index for all columns also

helps to compensate for any out-of-plane deflections.

k amp= lcr /(1 15 lcr-1 5 ) =1 14