Page 375 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.29

LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.29

Tensile yielding also contributes a significant amount to the inelastic deformation and energy dis-
tribution of the concentric braced frame system. This tensile yielding is controlled by the yield stress
and the gross area of the brace. The inelastic deformation capacity in tension may be limited by net
section fracture, which is controlled by the tensile strength of the steel and the effective net area of
the brace. The effective net area, A e , of a brace is typically smaller than the gross area, A g , and so
therefore there is rational concern that tensile fracture may occur before the brace develops its full
elongation in tension. Therefore, the 2005 AISC seismic provisions require that the required tensile
capacity of the brace should be greater than the expected yield strength of the brace. That is, this
requirement implies that
(8.26a)
R y A g F y <φA e F t
While the goal of this check is very rational, the actual application of this equation is not totally
logical, because the gross area and effective net area occur within the same steel section, and extreme
variations in material properties are not expected. Further, this requirement leads to extreme conser-
vatism in the design of the net section. As a result, a special exemption is permitted when the gross
area and net section occur within the same member. For these cases,

(8.26b)
R y A g F y < R t A e F t
Bracing contributes most of the lateral strength and stiffness to concentrically braced frames. As
noted earlier, bracing also dissipates energy through postbuckling compressive and tensile yielding.
Bracing systems that resist too large a portion of the seismic shear force of the frame through either
tension or compression sustain greater pinching of their hysteretic behavior and greater deterioration
of resistance. As a result, all bracing systems must be designed so that at least 30%, but no more than
70%, of the base shear is carried by bracing acting in tension, while the balance is carried by brac-
ing acting in compression.
Local buckling is also a major concern. Brace elements that are too slender may sustain local
buckling, which results in deterioration in resistance or early fracture of the brace. As a result, the
local slenderness of angle bracing elements is restricted for special concentrically braced frames to

b E
.
≤ 030 (8.27)
t F y
where b and t are the leg width and thickness of the angle, respectively. The slenderness of bracing
for hollow rectangular and circular tubes of high-strength steel are likewise limited such that

b h E
and ≤ 064 ( for hollow rectangular tubes) (8.28)
.
t t F y
D E
≤ 094 (for hollow circular tubes ) (8.29)
.
t F y
where t is the wall thickness of the tube, D is the diameter of a circular tube, and b and h are the width
and depth in compression for a rectangular tube. Beyond the restrictions noted above, the bracing
may be compact or noncompact but must not exceed the limit for slender members in the AISC
specification.

Connection Strength. The strength of the connections should be stronger than the members them-
selves, because connection behavior is more complex and less predictable than member behavior,
and premature failure of the connection may result in significant reduction in structural ductility.
Therefore, the nominal connection resistance (φR n ) for all expected behaviors must exceed the lesser
of the expected tensile resistance of the brace (R y A g F y ) or the maximum load effect that can be

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