Installation — business terrible  1 part
September 8th, 2015
The joist span selected will be based on the spacing of the stringers. We can follow the same procedure that is used for analyzing joists. Again, an integer or modular value is selected for stringer spacing.
After calculating the stringer spacing, the span of the stringer is checked against the capacity of the shore. The load on each shore is equal to the shore spacing multiplied by the load per unit foot of the stringer. The maximum shore spacing will be the lower of these two values (based on joists loading or shore spacing).
Type 
One span 
Two spans 
Three spans 
Bending moment (in.lb) 
M = wl 96 
m = wl 96 
m = wl 120 
Shear (lb) 
V = wl = 24 
1 1 ‘O 
V = wl = 20 
Deflection (in.) 
5wl4 Д = 4608EI 
. wl4 Д = 2220EI 
wl4 Д = 1740EI 
Table 3.15 Design Equations for Different Support Conditions 
Notation: l = length of span (in.) w = uniform load per foot of span (lb/ft) E = modules of elasticity (psi) I = moment of inertia (in.4) Source: Reproduced from the 1998 edition of Construction Methods and Management by S. W. Nunnally, with the permission of the publisher, PrenticeHall. Table 123, pp. 340341. 
EXAMPLE
Design formwork to support flat slab floor of 8in. thickness and conventional density concrete. Sheathing will be plywood that has the following characteristics: [10]










































































Notation:
l = length of span, center to center of supports (in.)
Fb = allowable unit stress in bending (psi)
FbKS = plywood section capacity in bending (lb x in./ft)
Fc = allowable unit stress in compression parallel to grain (psi)
Fcl = allowable unit stress in compression perpendicular to grain (psi)
FJb/Q = plywood section capacity in rolling shear (lb/ft)
Fv = allowable unit stress in horizontal shear (psi)
= actual unit stress in compression parallel to grain (psi)
= actual unit stress in compression perpendicular to grain (psi)
= actual unit stress in tension (psi)
= area of section (in.2)[11]
= modulus of elasticity (psi)
= moment of inertia (in.4)*
= applied force (compression to tension) (lb)
= section modulus (in.3)*
= deflection (in.)
= width of member (in.)
= depth of member (in.) w = uniform load per foot of span (lb/ft)
*For a rectangular member: A = bd, S = bd2/6,1 = bd3/12.
Reprinted by permission of Prentice Hall
SOLUTION
Design loads:
• Dead load = weight of concrete + weight of the formwork.
= 8/12 in. of concrete x 150 + 5.0 (assumed) = 105 lb/ft2
• Live load = 75 lb/ft2 (according to ACI = 347).
• Total vertical load = 75 + 105 = 180 lb/ft2
Sheathing:
• Thickness = 11/8in. sanded panels.
• From Tables 3.12 to 3.14, we get the following properties:
Grade Stress Level S2.
A = 3.854 in.2 (KS) = 0.820 in.3/ft.
I = 0.548 in.4
lb/Q = 9.883 in.2/ft.
Fb = 1200 psf Fv = 140 psf.
E = 1.2 x 106 psf •




















Load/ft of joist = 180 x 2 = 360 lb/ft Bending:
F’b = Fb( Cf)(Ct)(CD)
= 1350 x 1.3 x 1.0 x 1.25 = 2193.75 lb/ft From Table 3.16
Shear:
FV = F„ (Ch)(C,)(Cd) = 80 x 2 x 1.0 x 1.25 = 200 psi
= 13.5 x 200 x 8’25 + 2 x 5.5 = 72.875 in.
Deflection:
E’ = 1.4 x 106 psi


Stringers:
Load on a stringer is = 180 x 6 = 1,080 lb/ft Use 2 x 8 (two stringers)
• b = 1.5 in.
• d = 7.25 in.
• I = 47.63 in.4
• A = 10.87 in.2
• S = 13.14 in.3
• E = 1.4 x 106 psi
• Cf = 1.2
Bending:
F’b = 1350 x 1.2 x 1.0 x 1.25 = 2025 psi
Shear:
I = 13.3 x Fv^Aj + 2 x d
_ 13.3 x 200 x (10.87 x 2 ^ two stringers) 1080
+ 2 x 7.5 in. = 68.045 in.









Shear governs. Maximum span <68.045 in. (5.67 ft). Take stringer spacing = 5.5 ft.
Check for Crushing (joist on stringer):
Force transmitted from joist to stringer is equal to load of joist/ ft multiplied by the span of the joist, force = 6 x 360 = 2160 lb.
Area through which this force is transmitted = 1.5 x 3 = 4.5 in.2
Crushing stress = 2160/4.5 = 480 lb/in.2 From Table 6.a, we get the following properties:
• Fc± = 650 lb/in.2
• Bearing area factor = 1.25 (b = 1.5 in.)
• Temperature factor = 1.0
F’c± = Fcl (Cb)(Ct) = 650 x 1.25 = 812.5 psi since 480 < 812.5 ^ safe in crushing.
Shore strength:
Required shore spacing = (stringer span)
Shore strength = span of stringer x load of stringer = 5.5 x 1080 = 5940 lb
So we use shores whose strength is larger than 6000 lb.