Concrete Formwork Svstems - Part 3 potx

These properties include the moment of inertia, crosssectional area, neutral axis, section modulus, and radius of gyra-tion of the design shape in question.. Slab Form Design 51For recta

Slab Form Design

Concrete forms are engineered structures that are required to port loads composed of fresh concrete, construction materials,equipment, workers, impact of various kinds, and sometimes wind.The forms must support all the applied loads without collapse orexcessive deflection ACI Committee Report 347-1994 definesthose applied loads and gives a number of guidelines for safetyand serviceability Based on these guidelines, a number of designtables have been developed for the design of concrete formwork.These tables are useful design tools However, they do not takeinto consideration stress modification factors that are provided bythe National Design Specification for Wood Construction, NDS

sup-1991 This chapter presents a design procedure for all-wood crete slab forms based on NDS 1991 and Plywood Design Specifi-cation 1997

con-The objective of the formwork design is to determine the safespacing for each slab form component (sheathing, joists, stringers,and shores), and ensure that each component has adequatestrength to resist the applied pressure without exceeding predeter-mined allowable deflection

3.1 PROPERTIES OF FORM MATERIALS

The following sections provide an overview of some importantproperties of structural sections that are used in formwork design.Readers familiar with these expressions should start with Section3.3

48 Chapter 3 3.2 PROPERTIES OF AREA

Certain mathematical expressions of the properties of sections areused in design calculations for various design shapes and loadingconditions These properties include the moment of inertia, crosssectional area, neutral axis, section modulus, and radius of gyra-tion of the design shape in question These properties are de-scribed below

1 Moment of inertia The moment of inertia I of the cross

section is defined as the sum of the products of the ential areas into which the section may be divided, multi-plied by the squares of their distances from the neutralaxis of the section (Figure 3.1)

differ-If the section is subjected to a bending moment about

the X-X axis of the cross section, the moment of inertia about X-X is denoted by I xx,

Y i ⫽ distance between element i and X-X axis

If the member is subjected to a bending moment about

axis Y-Y of the cross section, we denote the moment of inertia associated with it as I yy,

Slab Form Design 49

2 Cross sectional area This is the area of a section takenthrough the member, perpendicular to its longitudinalaxis

3 Neutral axis The neutral axis is a line through the crosssection of the member along which the fibers sustain nei-ther tension nor compression when subjected to a loading

4 Section modulus Denoted as S, this is the moment of

iner-tia divided by the distance between the neutral axis andthe extreme fibers (maximum stressed fibers) of thecross section

If c is the distance from the neutral axis to the extreme

5 Radius of gyration This property, denoted as r, is the

square root of the quantity of the moment of inertia vided by the area of the cross section

di-r xx⫽√I xx

A r yy ⫽√I yy

A Here r xx and r yy are the radii of gyration about X-X and

Y -Y axes, respectively.

3.2.1 Rectangular Cross Section

The most commonly used cross section in the design of formwork

is the rectangular cross section with breadth b and depth d (Figure

3.2) These are usually measured in the units of inches or ters

Slab Form Design 51

For rectangular cross section, the formulas discussed in theprevious section take the forms:

3.3 PROPERTIES OF SAWN LUMBER

3.3.1 Classification of Sawn Lumber

Structural Sawn Lumber size classification was discussed inter1 and is summarized below

Chap-1 Dimension: 2 in ⬍ thickness ⬍ 4 in and width ⬎ 2 in

2 Beams and stringers: thickness ⬎ 5 in and width ⬎thickness⫹ 2 in

52 Chapter 3 Table 3.1 Nominal and Minimum Dressed Sizes of Sawn Lumber

3 2-1/2 2-9/12 4 3-1/2 3-9/16 3-1/2 3 3-1/16 5 4-1/2 4-5/8

Table 3.2 Section Properties of Standard Dressed (S4S) Sawn Lumber

Standard X-X-AXIS Y-Y-AXIS

dressed Approximate weight in pounds per linear foot (lb/ft)

Nominal size (S4S) Area of Section Moment Section Moment of piece when density of wood equals:

size b ⫻ d Section modulus of inertia modulus of inertia

Table 3.2 Continued

Standard X-X-AXIS Y-Y-AXIS

dressed Approximate weight in pounds per linear foot (lb/ft)

Nominal size (S4S) Area of Section Moment Section Moment of piece when density of wood equals:

size b ⫻ d Section modulus of inertia modulus of inertia

56 Chapter 3

3 Posts and timbers: Cross section is approximately 5 ⫻ 5 in

square or larger, and width ⬎ thickness ⫹ 2 in (not more)

Decking: 2 in ⱕ thickness ⱕ 4 in with load applied to wideface of board

All sizes referred to in the previous classification are the inal , or stated, sizes However, most lumber is called dressed lum-

nom-ber, which means the members are surfaced to a standard net size.Structural computations to determine the required size of mem-bers are based on the net dimensions (actual sizes), not the nomi-nal size Sizes of members is further discussed in Section 3.3.2

3.3.2 Sizes of Structural Lumber

Most structural lumber is called dressed lumber In other words,the lumber is surfaced to the standard net size, which is less thanthe nominal, or stated, size This is shown in Figure 3.3

Dressed lumber is used in many structural applications ever, some architectural applications may call for larger members

How-that have a different texture Such members are commonly sawn to dimensions that are close to the standard net size Thecross-sectional dimensions of these timbers is about 1/8in larger

Slab Form Design 57

than the standard dressed size A less common method of

ob-taining a rough surface is to specify full-sawn lumber Since

rough-sawn and full-rough-sawn lumber are not frequently used, their sectional properties are not included in the NDS

cross-Below is an example of the differences between nominal,dressed, rough-sawn, and full-sawn sizes of lumber Consider an

8 ⫻ 12 member (nominal size ⫽ 8 ⫻ 12 in.):

1 Dressed lumber: Standard net size 71/2⫻ 111/2in

2 Rough-sawn lumber: Approximate size 75/8⫻ 115/8in

3 Full-sawn lumber: Minimum size 8⫻ 12 in (generally notavailable)

3.3.3 Mechanical Properties of Lumber

The mechanical properties that will be used in the design of

form-work are compression parallel to grain (F c), compression

perpen-dicular to grain (F c⊥), tension parallel to grain (F t), and tension

perpendicular to grain (F t⊥) Figure 3.4 helps clarify the direction

of forces which produce these different types of stresses

3.3.4 Design Values of Mechanical Properties

Design values for the different types of stresses are dependent onthe type of lumber The design values given in these tables are to

be adjusted to fit the conditions under which the structure will beused.Tables 3.3through 3.6 give the design values along with itsadjustment factors that are specified by NDS for dimension lum-ber, southern pine dimension lumber, timber (5⫻ 5 in and larger)and decking Table 3.3a through 3.3d gives the design values alongwith its adjustment factors for all species except Southern Pine.Design values for Southern Pine are shown inTables 3.4athrough3.4dand Table 3.5

Size Factor

Stresses parallel to grain for visually graded dimension lumbershould be multiplied by the size factors provided in Tables 3.3aand 3.4a

58 Chapter 3

When the depth d of the beam, stringer, post, or timber ceeds 12 in., the tabulated design value F b shall be multiplied bythe following size factor:

Table 3.3 Design Values For Visually Graded Dimension Lumber

Design values in pounds per square inch (psi) Tension Shear Compression Compression Modulus parallel parallel perpendicular parallel of Grading Species and Size Bending to grain to grain to grain to grain Elasticity Rules commercial grade classification F b F t F v F c⊥ F c E Agency

DOUGLAS FIR-LARCH (NORTH)

Select Structural 2 ″ -4 ″ thick 1300 800 95 625 1900 1,900,000

* West Coast Lumber Inspection Bureau

** Western Wood Products Association

*** Northeastern Lumber Grading Agency

From National Design Specification for Wood Construction 1991

* Northeastern Lumber Manufacturers Association

** Northern Softwood Lumber Bureau

NORTHERN WHITE CEDAR

on wide face of (nominal) and 2-in (nominal) and

2-in (nominal) lumber C H thicker lumber C H thicker lumber C H

1 / 2 ⫻ wide face 1.67 1 / 2 ⫻ narrow face 1.67 1 / 6 ⫻ narrow face 1.67

3 / 4 ⫻ wide face 1.50 3 / 4 ⫻ narrow face 1.50 1 / 4 ⫻ narrow face 1.50

1 ⫻ wide face 1.33 1 ⫻ narrow face 1.33 1 / 3 ⫻ narrow face 1.33

1- 1 / 2 ⫻ wide face or more 1.00 1- 1 / 2 ⫻ narrow face 1.00 1 / 2 ⫻ narrow face 1.00

* Shake is measured at the end between lines enclosing the shake and perpendicular to the loaded surface.

Table 3.4 Base Design Values For Visually Graded Mixed Southern Pine Dimension Lumber

Design values in pounds per square inch (psi) Tension Shear Compression Compression Modulus parallel parallel perpendicular parallel of Grading Species and Size Bending to grain to grain to grain to grain Elasticity Rules commercial grade classification F b F t F v F c⊥ F c E Agency

MIXED SOUTHERN PINE

8 in and wider lumber

* Shake is measured at the end between lines enclosing the shake and perpendicular to the loaded surface.

Table 3.5 Base Design Values for Visually Graded Southern Pine Dimension Lumber

Design values in pounds per square inch (psi) Tension Shear Compression Compression Modulus parallel parallel perpendicular parallel of Grading Species and Size Bending to grain to grain to grain to grain Elasticity Rules commercial grade classification F b F t F v F c⊥ F c E Agency

Non-Dense Select Structural 2100 1100 90 480 1750 1,700,000

No 1 Dense 2 ″ -4 ″ thick 1650 875 90 660 1800 1,800,000

Non-Dense Select Structural 1850 950 90 480 1750 1,700,000

No 1 Dense 2 ″ -4 ″ thick 1450 775 90 660 1750 1,800,000

Non-Dense Select Structural 1750 900 90 480 1700 1,700,000

No 1 Dense 2 ″ -4 ″ thick 1350 725 90 660 1700 1,800,000

SOUTHERN PINE (Dry service conditions—19% or less moisture content)

Dense Structural 86 2-1/2 ″ -4 ″ thick 2600 1750 155 660 2000 1,800,000

Dense Structural 72 2200 1450 130 660 1650 1,800,000 SPIB Dense Structural 65 2 ″ & wider 2000 1300 115 660 1500 1,800,000

SOUTHERN PINE (Wet service conditions)

Dense Structural 86 2-1/2 ″ -4 ″ thick 2100 1400 145 440 1300 1,600,000

Dense Structural 72 1750 1200 120 440 1100 1,600,000 SPIB Dense Structural 65 2-1/2 ″ & wider 1600 1050 110 440 1000 1,600,000

* Southern Pine Inspection Bureau

From National Design Specification for Wood Construction 1991

Table 3.5b Shear Stress Factor C H

Length of split on wide Size of shake* in 5-in.

face of 5-in (nominal) (nominal) and thicker

1 / 2 ⫻ narrow width 1.67 1 / 6 ⫻ narrow face 1.67

3 / 4 ⫻ narrow width 1.50 1 / 4 ⫻ narrow face 1.50

1 ⫻ narrow width 1.33 1 / 3 ⫻ narrow face 1.33

1 1 / 2 ⫻ narrow width 1.00 1 / 2 ⫻ narrow face or more 1.00

* Shake is measured at the end between lines enclosing the shake and perpendicular to the loaded surface.

78 Chapter 3 Table 3.6 Design Values for Visually Graded Decking

Design values in pounds per square inches Species and

commercial grade Size classification F b F c⊥ E

Douglas Fir-Larch

Select Dex 2–4 in thick 1750.0 625.0 1,800,000 Commercial Dex 6–8 in wide 1450.0 625.0 1,700,000 Hem-Fir

Select Dex 2–4 in thick 1400.0 405.0 1,500,000 Commercial Dex 6–8 in wide 1150.0 405.0 1,400,000 Redwood

Select, Close Grain 2 in thick 1850.0 — 1,400,000

Commercial 6 in and wider 1200.0 — 1,000,000

2 in thick, Deck heart 4 in wide 400.0 420.0 900,000 and

2 in thick, Deck common

6 in wide 700.0 420.0 900,000

Southern Pine (Dry service conditions—19% or less moisture content)

Dense Standard 2–4 in thick 2000.0 660.0 1,800,000

Dense Commercial 1650.0 660.0 1,600,000 Commercial 2 in and wider 1400.0 565.0 1,600,000

Southern Pine (Wet service conditions)

Dense Standard 2 1 / 2 –4 in thick 1600.0 440.0 1,600,000

Dense Commercial 1350.0 440.0 1,400,000 Commercial 2 in and wider 1150.0 375.0 1,400,000 From National Design Specification for Wood Construction 1991

Slab Form Design 79

Table 3.6a Wet Service Factor (C M)*

of the member shall be multiplied by the wet service condition

factor C M, as is specified by the National Design Specification forWood Construction (NDS) The wet service factor (⬍1.0) is used

to decrease the allowable stresses to account for the weakening

of the member due to the increase in its moisture content An ample of the wet service factor is given in Table 3.3c

ex-Also, moisture content adds additional weight to the lumber.NDS specifies that the following formula shall be used to deter-mine the density (in lb./ft3) of wood

m.c ⫽ moisture content of wood, %

Load Duration Factor

Wood has the property of carrying a substantially greater mum load for short duration than it can for long duration of load-

maxi-80 Chapter 3

Table 3.7 Load Duration Factor (C D)

Load duration C D Typical design load

Permanent 0.9 Dead load

10 years 1.0 Occupancy live load

2 months 1.15 Snow load

10 minutes 1.6 Wind/earthquake load

Impact 2.0 Impact load

From National Design Specification for Wood Construction 1991

ing The tabulated design values given by NDS apply to normalload duration Normal load duration is defined as the application

of the full design load that fully stresses a member to its allowabledesign value for a cumulative period of approximately 10 years.Values for load duration factors are given in Table 3.7

Bearing Area Factor

Tabulated compression design values perpendicular to grain F c⊥apply to bearings of any length at the ends of the member, and toall bearings 6 in or more in length at any other location For bear-ings less than 6 in and not nearer than 3 in to the end of a mem-

ber, the tabulated design values perpendicular to grain F c⊥ shall

be permitted to be multiplied by the bearing area factor C b Values

of C are given in Table 3.9

3.4.1 Exposure Durability Classification

Plywood is classified as interior or exterior The classification ismade on the basis of the resistance of the glue bond to moisture,which is affected by the adhesive used, the veneer grade, and thepanel construction Plywood is made in four exposure durabilityclassifications: Exterior, Exposure 1, IMAGE (Exposure 2), andInterior Exterior type is made with 100 percent waterproof glue

3.4.2 Veneer Classification

Plywood is graded based on the appearance and defects in the

veneers Veneer is divided into the following five grades: N grade

is free from defects, knots, and restricted patches and is suitablefor work where natural finish is desirable such as cabinet work

A gradeis smooth, free of knots, and paintable N and A are

consid-ered the highest grade levels B grade is similar to A grade except that knots, patches, and sanding defects may be found C grade

allows larger knots and knotholes; it is the lowest grade allowed

in exterior-type plywood C-plugged is a repaired or improved C grade D grade may have larger knots, knotholes, and a number

of repairs, holes, and sanding defects; this grade is not permitted

in exterior panels

3.4.3 Plywood Grades

Any combination of the above-mentioned grades are available asface and back surface for the plywood panel