Building Design and Construction Handbook - Merritt - Ventech!

7.44 SECTION SEVEN
filler blocks—usually a lightweight tile. Some of these variations fall in the heaviest
floor classification; also the majority require substantial forms and shoring.
7.14 ROOF FRAMING SYSTEMS
These are similar in many respects to the floor types, discussed in Arts. 7.13 and
7.13.1. In fact, for flat-top tier buildings, the roof may be just another floor. However,
when roof loads are smaller than floor loads, as is usually the case, it may be
economical to lighten the roof construction. For example, steel joists may be spaced
farther apart. Where roof decking is used, the spacing of the joists is determined
by the load-carrying ability of the applied decking and of the joists.
Most of the considerations discussed for floors in Art. 7.13 also are applicable
to roof systems. In addition, however, due thought should be given to weather
resistance, heat conductance and insulation, moisture absorption and vapor barriers,
and especially to maintenance.
Many roof systems are distinctive as compared with the floor types; for example,
the corrugated sheet-metal roofing commonly employed on many types of industrial
or mill buildings. The sheets rest on small beams, channels, or joists, called purlins,
which in turn are supported by trusses. Similar members on the sidewalls are called
girts.
DESIGN OF MEMBERS
In proportioning of members, designers should investigate one or more or a combination
of five basic stress or strength conditions: axial tension, axial compression,
bending, shearing, and member element crippling. Other conditions that should be
investigated under special conditions are local buckling, excessive deflection, torsion
and fatigue. Until the early 1990s, such analyses were based on allowable
stress design (ASD). More recently, a method known as load and resistance factor
design (LRFD) has come into use because it permits a more rational design. It takes
into account the probability of loading conditions and statistical variations in the
strength, or resistance capability, of members and connection materials.
The use of LRFD design procedures will result in a savings of material, generally
in the range of 15 to 20%, and on major structures, some elements may show a
savings of up to 25%. Such weight savings generally means a lesser cost for the
structural steel. However, except for major structures, when serviceability factors
such as deflection and vibration are considered in the proportioning of the individual
members, the nominal savings of LRFD procedures versus ASD procedures is more
likely to be approximately 5%.
7.15 BASES FOR ASD AND LRFD
ASD is based on elastic theory. Design limits the maximum unit stress a member
is permitted to bear under service loads to a level determined by a judgmental, but