Page 238 - Civil Engineering Formulas

172 CHAPTER SIX

For wood screws,
W WC D C M C t (6.44)
(6.45)
Z ZC D C M C t C d C eg
where C is the end-grain factor.
eg
For lag screws,
(6.46)
W WC D C M C t C eg
(6.47)
Z ZC D C M C t C g C C d C eg
For metal plate connectors,

Z ZC D C M C t (6.48)
For drift bolts and drift pins,

(6.49)
W WC D C M C t C eg
(6.50)
Z ZC D C M C t C g C C d C eg
For spike grids,
Z ZC D C M C t C (6.51)

ROOF SLOPE TO PREVENT PONDING

1
Roof beams should have a continuous upward slope equivalent to 4 in/ft
(20.8 mm/m) between a drain and the high point of a roof, in addition to
minimum recommended camber to avoid ponding. When flat roofs have
1
insuffic-ient slope for drainage (less than 4 in/ft) (20.8 mm/m) the stiffness
2
2
of supporting members should be such that a 5-lb/ft (239.4 N/mm ) load
1
causes no more than -in (12.7 mm) deflection.
2
Because of ponding, snow loads or water trapped by gravel stops, parapet
walls, or ice dams magnify stresses and deflections from existing roof loads* by
1
C p (6.52)
3
4
1 W L /
EI
where C factor for multiplying stresses and deflections under existing loads to
p
determine stresses and deflections under existing loads plus ponding
W weight of 1 in (25.4 mm) of water on roof area supported by beam, lb (N)
L span of beam, in (mm)
2
E modulus of elasticity of beam material, lb/in (MPa)
4
4
I moment of inertia of beam, in (mm )
*Kuenzi and Bohannan, “Increases in Deflection and Stresses Caused by Ponding of Water on
Roofs,” Forest Products Laboratory, Madison, Wisconsin.