masonry wall effective width

[Taro]

For working stress design, the MSJC masonry code specifies an effective wall width to use in computing compressive stresses due to concentrated loads (Section 2.1.9.1). Effective width does not appear to be prescribed for strength design. Is it left to the engineer's judgment whether to adopt the WSD provision or some other alternative?

 

[m8589]

I think, effective width will be the same as for WSD.

 

[Taro]

OK, but is that an explicit requirement or your personal judgment?

 

[dik]

I don't know what your codes include, but the Canadian masonry code for limit states design does not stipulate the manner in which the load is distributed. This is likely a consequence of the system being highly redundant or indeterminate and requiring some engineering judgement. It also may change with the next code.

Common practice varies <G> and I don't know what your common codes stipulate. For lintel design, it is common for the design load to include all UDL's within a 45 degree angle from the edge of the support and for point loads to be considered as distributing at a bearing width plus 60 degrees from the horiz downwards to the lintel. The width of the wall affected varies and is generally used to establish the loading on a lintel supporting the wall and point load above.

I have often excluded loads outside a 60 degree angle from the edge of the support, except for 'very big ones' or if the opening is close to an end of wall; I generally look closely at these. If the opening is close to the end of a wall, I have designed the lintel as a cantilever or propped cantilever with a triangular load going from 0 at the interior support to maximum at the outside edge.

Also for heavy point or udl loads, sometimes treat the condition as a 3 pin arch using the height of the arch equal to a 45 degree angle plus the depth of compression I need assuming the allowable masonry compressive stress acting on the face shell only (about 1-1/2&quot. I use the full value of the point load. I then check that the horizontal shear on the blocks is approximately less than 1/4 the allowable (comfy factor) and sometimes grout the whole thing anyway. The tensile steel is then based on the flexure required for the triangular shape of block plus the tension tie for the arch. Seems to work OK. I seem to recall that an earlier book by Amrhein (sp?) uses a similar approach as the first method. Within the text, however, he notes that a realistic arching angle might be 30 degrees from horizontal (from memory and may not be correct... so far back it may have been the first edition).

 

[m8589]

It is the way I read Section 2106 of UBC 97 &quot;General design requirements&quot; which is common for WSD and Strength Design

 

[Taro]

m8589,

Yes, that is why I ask the question. In the 1997 UBC, the effective width was specified for both Working Stress Design and Strength Design. However, the UBC is no longer the governing code for some jurisdictions in which I design.

In the newer MSJC code, the effective width requirement appears to only apply to WSD. I was wondering if I just missed something or if they intentionally deleted the requirement for Strength Design.

I don't want to be unnecessarily conservative if some new research has invalidated the old requirement. But lacking any specific alternate code provision or research paper, I will probably stick with the old UBC restrictions for now.