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Re: Compression flange bracing

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> From: "Lutz,James" <JLUTZ(--nospam--at)>

> The issue of compression flange bracing has come up in my practice on
> numerous occasions with respect to rafters supporting the roofs of steel
> water storage tanks. Typical practice is to not weld the rafters to the roof
> plates, so compression flange bracing is only provided by friction of the
> roof diaphragm. This is specifically permitted by AWWA D100, which allows
> rafters to be designed as if continuously braced.

The pre-eng industry has been around the block on this in the past with
standing seam roofs. Bottom line:
1) compression in the top flange is caused by gravity loads that provide
direct contact bearing of the supporting panel on the compression flange
2) there must be nothing supporting the panel above the compression
flange or reducing the friction
3) the panel tends to wrap itself around the compression flange
providing added restraint

Static load testing indicates that for light gauge C and Z sections that
the compression flange may be considered fully restrained in the above
conditions. MBMA promotes it and Factory Mutual, for one, accepts it.

I suspect that under light gravity loads and gusting wind (dynamic)
loads that it may be possible to create a failure situation resulting
from rapidly alternating uplift and gravity loading of the panel.
Probably a rare condition.

> From: Charles Greenlaw <cgreenlaw(--nospam--at)>
> The bottom, tension flange wants to stay in a straight line. The top flange,
> to go sideways, has to twist the whole cross-section. To do so it lifts one
> top flange edge. Then the load bears on the lifted flange edge and puts an
> eccentric moment in the beam that twists it back where it belongs. Stability
> results. No friction is required.

This works with W sections and other symmetric sections (ships hulls)
within limits. See Yura/Helwig as suggested by others. However, the
friction can also provide an important restraining force, otherwise C's
and Z's would fail.

Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)> <>