Building Design and Construction Handbook - Merritt - Ventech!

7.48 SECTION SEVEN
ing stress ƒ b and therefore is nearly proportional to F y, a beam of steel with F y �
100.0 ksi would have to be twice the depth of a beam of steel with F y � 50.0 ksi
when each is stressed to allowable values and has the same maximum deflection.
Vibration of large floor areas that are usually free of physical dampeners, such
as partitions, may occur in buildings such as shopping centers and department
stores, where pedestrian traffic is heavy. The minimum depth-span ratios in Table
7.10 suggested for ‘‘heavy pedestrian traffic’’ are intended to provide an acceptable
solution.
One rule of thumb that may be used to determine beam depth quickly is to
choose a depth, in, not less than 1.5% of F y times the span, ft. Thus, for A36 steel
depth, in, should be at least half the span, ft.
Ponding. Beams for flat roofs may require a special investigation to assure stability
against water accumulation, commonly called ponding, unless there is adequate
provision for drainage during heavy rainfall. The AISC specification gives
these criteria for stable roofs:
where Cp � 32Ls /107 4 Lp Ip 4
Cs � 32 /107 SLs Is Cp � 0.9Cs � 0.25 (7.1)
4 25S
Id � 6 10
(7.2)
L p � column spacing in direction of girder, ft (length of primary members)
L s � column spacing perpendicular to direction of girder, ft (length of secondary
member)
S � spacing of secondary members, ft
I p � moment of inertia for primary members, in 4
I s � moment of inertia for secondary members, in 4 . Where a steel deck is
supported on primary members, it is considered the secondary member.
Use 0.85I s for joists and trusses
I d � moment of inertia of a steel deck supported on secondary members,
in 4 /ft
Uniform-Load Deflections. For the common case of a uniformly loaded simple
beam loaded to the maximum allowable bending stress, the deflection in inches
may be computed from
where F b � the allowable bending stress, ksi
l � the span, in
E � 29,000 ksi
d/l � the depth-span ratio
5 Fl
b
� � (7.3)
24 Ed/l
Drift. AISC Design Guide No. 3 (Table 7.9) suggests that the lateral deflection
of a building frame (drift) be limited to a value which does not damage other
structural or architectural components when subject to a 10-year recurrence interval
wind pressure. The 10-year wind pressure can be reasonably estimated at 75% of
the 50-year wind pressure.