Class Exercise $(15)$: Formwork Slab Analysis and Design

Stress Control

E$\mathrm{.}34.$ The formwork for a $6-$in$.-$thick reinforced concrete floor slab of normal weight consists of

$\frac{3}{4}$ in$.$ Class II plyform sheathing supported by $2$ x $6$ in$.$ joists spaced $2ft.($o$.$c$),$ which in

turn are supported by $28$ in$.$ stringers spaced $4ft.($o$.$c$).$ The stringers are themselves

supported by shores spaced $4ft.$ on centers.

The joists and stringers are constructed with Douglas fir, coastal construction, and are to

be used many times. The allowable flexural stress in the fir members is $1875,$ the

allowable horizontal shear stress is $180,$ and the modulus of elasticity is $1,760,000.$

For the plyform the corresponding values are $1330,72,$ and $1,430,000,$ with the

stress applied parallel with the face grain. Using a live load of $150$ psf$,$ a$)$ Check the

bending and shear stresses in the sheathing, joists and stringers; b$)$ Check deflections

assuming that the maximum permissible deflection of each of these elements is L$/360$ for

all loads.

Joist Spacing

E$\mathrm{.}35.$A timber formwork system utilizing $4$ x $10$ S$4$S joists spans a length of $14$ feet $($simply

supported$).$ The system carries a load of $150psf($dead load plus live load$).$ At what

spacing should the joists be placed? Assume Douglas fir$-$Larch No$.2(f_b=1,450).$

Doing this for a civil engineering course.