Title: /
Modification of Compression Bar Internal Project Limits
Date: / 08/22/2008Contact: / Name: / Larry Hiner
Company: / Chicago Bridge & Iron Company
Phone: / 815-439-6125
E-mail: /
Purpose: / Limits on interior compression bar projection are too restrictive
Source: / CB&I email dated 8/22/08.
Revision: / 1- Dated Dec 29, 2008, (Passed to publication version)
Impact: / This change reinstates previous rules for compression bars allowing compression barsfor large cylindrical tanks to be supplied using reasonable thickness.
Background: / Agenda item 620-254 dated October 31, 2002 identified a misprinting in Figure 5-6 in the equation for the interior and exterior compression bars projection limits. The equation in the 6th Edition was Wh L 16t. Where Wh is the internal projection and L is the external projection. This equation did not limit Wh except for the separate limitation (see Figure 5-5) of and to require Wh L
Aprinting error,in the symbol , occurred after the 7th Edition Rev 1 resulting in what appeared to be a cross between a minus sign and an arrowhead as seen in the following:
This eventually became Wh - L 16t. This effectively applied an additional restriction on Wh of 32t (when L =16t), however, it no longer limited L 16t. The failure to limit L 16t lead to the agenda item 620-254
The current standard,resulting from agenda item 620-254, requires L 16t and Wh 16t. The 16t limit is considered correct (see AISC 360-05, case 3) for the exterior projection edge of the bar (L) because its outer edge is unsupported. As tank diameters and design pressures have continued to increase it is now apparent that the 16tlimit for Whis too conservative for the interior projection. The interior edge of the compression bar is attached to and supported by the roof plate (and sometimes radial stiffeners or rafters). Compression bar loading results from internal pressure load on the roof which, inturn, results in tension stresses in the roof. Thecombined underside pressure and plate tension significantly increases the effective width of the interior compression bar.
Proposal: / Change limits for Wh and L for details e-g of Figure 5-6 from:
L 16t and Wh 16t to L 16tandL Wh ,Wh 32t.
Rationale: / AISC 360-05 Table B.4.1, case 14 identifies the limiting b/t of other stiffened compression elements as 1.49. Therefore Wh = 43t for A36 material. Because fixity of the compression bar at the roof plate may have some variability, this proposal will reaffirm the previous limits of the compression bar such that Wh 32t and also reimpose the requirement that Wh L
Figure 5-5 shown only to provide formula for calculation of Wh. No changes required in this figure.
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