Concrete Formwork Svstems - Part 3 potx

3

Slab Form Design

3.1 Properties of Form Materials

3.2 Properties of Area

3.3 Properties of Sawn Lumber

3.4 Properties of Plywood

3.5 Slab Form Design

3.6 Design Steps

Concrete forms are engineered structures that are required to sup￾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 or

excessive deflection. ACI Committee Report 347-1994 defines

those applied loads and gives a number of guidelines for safety

and serviceability. Based on these guidelines, a number of design

tables have been developed for the design of concrete formwork.

These tables are useful design tools. However, they do not take

into consideration stress modification factors that are provided by

the National Design Specification for Wood Construction, NDS

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

The objective of the formwork design is to determine the safe

spacing for each slab form component (sheathing, joists, stringers,

and shores), and ensure that each component has adequate

strength 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 important

properties of structural sections that are used in formwork design.

Readers familiar with these expressions should start with Section

3.3.

48 Chapter 3

3.2 PROPERTIES OF AREA

Certain mathematical expressions of the properties of sections are

used in design calculations for various design shapes and loading

conditions. These properties include the moment of inertia, cross

sectional 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 differ￾ential areas into which the section may be divided, multi￾plied by the squares of their distances from the neutral

axis of the section (Figure 3.1).

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 Ixx ,

Ixx

n

i1

AiY 2

i

where

n total number of differential areas

Ai area of element i

Yi 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 Iyy,

Iyy

m

j1

Aj X 2

j

where

m total number of elementary areas

Aj area of element j

Xj distance between element j and Y-Y axis

Slab Form Design 49

Figure 3.1 Moment-of-inertia calculation.

2. Cross sectional area. This is the area of a section taken

through the member, perpendicular to its longitudinal

axis.

3. Neutral axis. The neutral axis is a line through the cross

section 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 and

the extreme fibers (maximum stressed fibers) of the

cross section.

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

50 Chapter 3

fibers in inches, one can write:

Sxx Ixx

c

Syy Iyy

c

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

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

r xx √

Ixx

A

ryy √

Iyy

A

Here rxx and ryy 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 millime￾ters.

Figure 3.2 Rectangular cross section.

Slab Form Design 51

For rectangular cross section, the formulas discussed in the

previous section take the forms:

Moments of intertia: Ixx bd3

12 , in.4 or mm4

Iyy db3

12 , in.4 or mm4

Radii of gyration: rxx √

Ixx

A d

√12, in. or mm

ryy √

Iyy

A b

√12, in. or mm

Section modules:

Sxx Ixx

c bd2

6 , in.3 or mm3

here c d

2

Syy Iyy

c db2

6 , in.3 or mm3

here c b

2

The section properties for selected standard sizes of board,

dimension lumber, and timbers are given in Table 3.1. The values

given in this table can be used to calculate the properties given

above. Table 3.2 provides section properties of standard dressed

(S4S) sawn lumber.

3.3 PROPERTIES OF SAWN LUMBER

3.3.1 Classification of Sawn Lumber

Structural Sawn Lumber size classification was discussed in Chap￾ter 1 and is summarized below.

1. Dimension: 2 in.  thickness  4 in. and width  2 in.

2. Beams and stringers: thickness  5 in. and width 

thickness  2 in.