Web Stiffener Design 1

[jheidt2543]

Question:

When checking the member strength of web stiffeners acting in combination with a length of a member’s web, the computed active length of the web is given by 25*twc (25 x the thickness of the column’s web). Ref: Salmon & Johnson p.895 and LRFD-K1.8

• Why isn’t the full beam depth D used?
• Why isn’t the beam length T used?

Member Section Depth Web T 25*tw

Column W10 x 33 9.73” .290” 7.50” 7.250”

”Beam W12 x 40 11.90” .295” 9.25” 7.375"

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[HDStructural]

You need stiffener plates because the web isn't strong enough to take the compression (or tension) load from the flange force at the moment connection. It is failing in compression.
The 25*tw is saying how wide of a section of the column web you can use to resist the compressive force in combination with your stiff plates.
You would design for this compression force and you would have a + shaped section resisting it. The left to right section ( - ) is the web thickness, by 25*tw long. The up and down section ( | )of the + is your two stiff plates on each side of the column web.
The 0.75*D value is used to calculate the kl/r of the + shaped section in compression. For deep beams or columns, you will have a strength reduction as kl/r gets larger.
You should read further in chapter J.10 of AISC and look up design examples.

 

[jheidt2543]

HDStructural:
Thank you for your detailed reply. I was thinking along those lines, but if we take an extreme case where 25*tw is say half the depth of the beam and the web stiffeners are in line with the top & bottom beam flanges, the length of the web is too long. Would multiple web stiffeners placed at 25*tw distance be an acceptable solution to stiffen the web?

 

[HDStructural]

If the half the beam depth is 25*tw, you would not have issues since the two stiffener sections would not overlap.

For your application at a moment connection, you would only need stiff plates at the beam flanges (assuming no panel zone issues). If these stiff plates are spaced less than 25 * tw apart, then I would think you should use their spacing instead of 25*tw when determining the effective area. That is based on engineering judgement, I am not sure if that is anywhere in AISC.

 

[jheidt2543]

It's nice to know "engineering judgement" is still an acceptable attribute. Thanks for your quick response!

 

[cliff234]

AISC 360-22 (and earlier versions of the AISC Specification) discusses the potential need for stiffeners in two locations – Section J10, Flanges and Webs with Concentrated Forces, and Section G2, I-Shaped Members and Channels.

Stiffeners are sometimes required per Section G2 because of high shear forces and tension field action in plate girders and other deeper flexural members with large h/t ratios. The stiffeners, if required, are required at spacings calculated per Section G2 along the span until shear stresses diminish below the available strength.

Stiffeners are required per Section J10 where large, concentrated forces occur (such as flange forces at beam-to-column moment connections, or where columns bear on transfer girders). In both cases large, concentrated forces are trying to crunch the webs of columns or the webs of flexural members, or excessively bend the flanges of supporting beams or columns. Section J10 stiffeners, if required, are required only at the concentrated loads.