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Re: Tube shape LRFD design

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richard lewis wrote:

> I am designing a HSS18x6x5/16 beam with combined bending and torsion,
> simple span.  It spans 30 ft and is unbraced the full length.  I am
> looking at AISC Specification for the Design of Steel Hollow Structural
> Sections, Sect. 5.1 (pg. 7).  b/t ratio is less than lambda(p) for the
> strong axis and greater than lambda(r) for the weak axis.  I have reduced
> the section modulus of the weak section as detailed in the specification.

An HSS18x6x5/16 has a strong axis moment capacity of 168 k-ft (ASD) or 252
k-ft (LRFD) based on formula 5.1-4. The weak-axis moment is 50 k-ft (ASD) or
75.2 k-ft (LRFD) based on Formula 5.1-6 using an effective section modulus of
21.78.  The torsional capacity is 80.8 k-ft (ASD) or 121.5 k-ft (LRFD).

> At the bottom of the page it has a paragraph dealing with unbraced
> length. It states Lb (unbraced length) is not limited for HSS shapes.
> designed by elastic analysis and is limited by some equations for plastic
> analysis.  Since I have a simple span beam I have an elastic analysis.
> Am I reading this right, there is not unbraced length limit for this HSS
> shape?  This shape is designed for Mp in the strong axis and Mn in the
> weak axis?

Essentially, HSS have such an elongated distance between Lp and Lr that the
differences in moments at these points are less than 10% of the member
capacity. An HSS18x6x5/16 has an Lp = 14.31' and and Lr = 282' for strong
axis and an Lp = 73.9' and an Lr = 1265' for the weak axis. Don Sherman
recommends bracing HSS at 24 x depth. This is 36' for this member. The strong
axis moment capacity with a braced length of 36' is 248 k-ft (LRFD) which is
only 1.78% less than the fully braced capacity.


Davis G. Parsons II, PE RA FAEI
a practical architectural engineer
in Fort Worth, Texas

"Education is what you get from reading the fine print;
 Experience is what you get from ignoring it."

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