228 CHAPTER NINE
In continuous composite construction, longitudinal reinforcing steel may be
considered to act compositely with the steel beam in negative-moment regions.
In this case, the total horizontal shear, kip (kN), between an interior support and
each adjacent point of contraflexure should be taken as
(9.72)
whereAsrarea of longitudinal reinforcement at support within effective area,
in^2 (mm^2 ); and Fyrspecified minimum yield stress of longitudinal reinforce-
ment, ksi (MPa).
PONDING CONSIDERATIONS IN BUILDINGS
Flat roofs on which water may accumulate may require analysis to ensure that
they are stable under ponding conditions. A flat roof may be considered stable
and an analysis does not need to be made if both of the following two equations
are satisfied:
(9.73)
(9.74)
whereCp 32 LsLp^4 /10^7 Ip
Cs32SL^4 s/10^7 Is
Lplength, ft (m), of primary member or girder
Lslength, ft (m), of secondary member or purlin
Sspacing, ft (m), of secondary members
Ipmoment of inertia of primary member, in^4 (mm^4 )
Ismoment of inertia of secondary member, in^4 (mm^4 )
Idmoment of inertia of steel deck supported on secondary members,
in^4 /ft (mm^4 /m)
For trusses and other open-web members, Isshould be decreased 15 percent. The
total bending stress due to dead loads, gravity live loads, and ponding should not
exceed 0.80Fy, where Fyis the minimum specified yield stress for the steel.
LIGHTWEIGHT STEEL CONSTRUCTION
Lightweight steel construction is popular today for a variety of building compo-
nents (floors, roofs, walls. etc.) and for entire buildings.* Such construction
takes many forms, and a number of different designs of structural members and
framing systems have been developed.
Id 25 S^4 /10^6Cp0.9Cs0.25VhAsrFyr
2*From Merritt—Building Construction Handbook, McGraw-Hill. Formulas in this section are
based on the work of F. E. Fahy, who, at the time of their preparation, was Chief Engineer, Product
Engineering, Bethlehem Steel Company.