Page 280 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch06.qxd 9/29/05 5:15 PM Page 6.2

DESIGN OF BUILDING MEMBERS

6.2 CHAPTER SIX

Construction (AISC). This unified specification covers both allowable strength design (ASD) and load
and resistance factor design (LRFD), as reviewed in detail in Chap. 5. Generally, the nominal strength
equations are the same for both methods, but the nominal strength is multiplied by a resistance factor
to determine the “design strength” for LRFD and divided by a safety factor to determine the “allow-
able strength” for ASD. LRFD uses greater (factored) loads, but the final results of the two methods
are about the same. The examples in this chapter are for LRFD, but are easily adapted to ASD.

6.1 TENSION MEMBERS

Members subject to tension loads only include hangers, diagonal braces, truss members, and columns that
are part of the lateral bracing system with significant uplift loads.
The AISC “Specification for Structural Steel Buildings” (hereafter AISC Specification) gives the
nominal strength P n (kips) of a cross section subject to tension only as the smaller of the capacity of
yielding in the gross section,
(6.1)
P n = F y A g
or the capacity at fracture in the net section,
(6.2)
P n = F u A e
For LRFD, factored load may not exceed either of the following:
φ= 0.9 (6.3a)
P u =φF y A g
φ= 0.75 (6.3b)
P u =φF u A e
where F y and F u are, respectively, the yield stress and the tensile strength (ksi) of the material. For
ASD, the load may not exceed
FA
P = yg (6.4a)
.
167
P = FA (6.4b)
ue
200
.
2
A g is the gross area (in ) of the member, and A e is the effective cross-sectional area at the connection.
The effective area A e is given by
(6.5)
A e = UA n
where A n = net area
U = reduction coefficient (see Art. 5.3)

6.2 EXAMPLE—LRFD FOR DOUBLE-ANGLE HANGER

A composite floor framing system is to be designed for sky boxes of a sports arena structure. The
sky boxes are located about 15 ft below the bottom chord of the roof trusses. The sky-box framing
is supported by an exterior column at the exterior edge of the floor and by steel hangers 5 ft from the
inside edge of the floor. The hangers are connected to either the bottom chord of the trusses or to the
steel beams spanning between trusses at roof level. The reactions due to service dead and live loads
at the hanger locations are P DL = 55 kips and P LL = 45 kips. Hangers supporting floors and balconies
should be designed for additional impact factors representing 33% of the live loads. Assume load
factors of 1.2 for live load and 1.6 for dead load, in combination; and 1.4 for live load alone.

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