Simple Strut and Tie Question: Depth of Horizontal Strut, Anchored by Bearing Area and Strut

[canwesteng]

I have a simple question here, but I don't feel confident enough in my interpretation of the problem. Effectively, I have a diagonal strut and horizontal strut anchored by a bearing area, and don't know what the depth of the horizontal strut should be. As this is a D region of a beam, I'm using the compression region from flexural analysis, but that doesn't neccesarily seem correct. See attached sketch, d.a is the dimension in question.

Thanks

 

[KootK]

As this is a D region of a beam, I'm using the compression region from flexural analysis, but that doesn't neccesarily seem correct.

Why not? My understanding is that you can use that value or any greater value that you may find advantageous so long as you're consistent in your analysis. Often, your D region is bordered by a B region and it makes sense to use the B region flexural compression block depth for continuity in analysis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

I guess the only downside to an infinite depth for this strut in a deep member is a reduced lever arm between the top chord (strut) and bottom chord (tie) then? It seems reasonable from a fundamental standpoint but it always seems like there are some empirical limits when working with concrete.

 

[KootK]

I guess the only downside to an infinite depth for this strut in a deep member is a reduced lever arm between the top chord (strut) and bottom chord (tie) then?

Precisely. I believe that was kind of the genesis of the whole thing. Essentially acknowledging reduced flexural depth in situations where proportions allowed shear to be resolved via arching action. That whole no free lunch thing...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.