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Shear Load for Large Timber Beams in a Bridge Deck

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Everynameistaken

Structural
Jun 29, 2014
68
Hi All,

Wondering if anyone has any experience with checking large sawn timber products for shear on a bridge deck?

When looking at the SFD we get very high peak shear values associated with the wheel point loads. Which are usually higher then the straight Vr value from most timber codes.

When we look at CSA S6 (Canadian Bridge Code) there is angexpression for a shear load of total shear along the member Vf = 0.82 (1/L (integral V (x))^5)^0.2. This is basically a method for arithmetically adding the area under the blocky SFD and distributing it along the length of the beam or stringer. In the code portion it notes this formula is for Glue-lam beams but in the commentary portion it describes the use of this method for a typical DFL pile cap which we assume is of solid sawn lumber as most pile caps are.

Does anyone have any experience with this clause or similar clauses in other codes?

These types of bridges are common place and the standard stringer and beam size would never work with the normal building style shear resistance calculations?

Thanks
 
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Why do you think the material choice between sawn timber and glulam changes the shear load? The shear load is essentially code defined based on standard loadings and arrangements. Distribution would be based on the reasonableness of the assumptions in distribution.

I believe it points out glulam in particular because glulam resistance is dependent on beam size, which is why it mentions Vr > Vf, when in the wood design manual (not applicable to bridges but still an example of glulam design) there are two formulas for calculating the shear resistance of wood based on the beam volume, and one is compared against a different term (Wf) and not Vf.

I could be wrong. Also, fyi, I don't have a 2014 CSA S6 in front of me so I looked at the previous version, but the integral expression for Vf seems the same.

Great name.
 
Hi,

Thanks for the response.

I agree the shear load is a demand on the element and is a product of the loading geometry.

So I calculate my Vf per the formula and then compare to the Vr determine from the code. The Vf from shear load of coarse evens out the distribution and gives reasonable number to check against.

Thanks for the comment, anyone else have any thoughts?

(Agree, good names we have! :) )
 
I don't know the answer to this question but am very interested in the discussion. My thoughts so far:

1) In S6, they specifically single out glulam for the integral treatment. And that, after they discuss solid sawn separately. While I do not know of a rational reason why glulam should be singled out this way, I would be leery of using the integral method for non-glulam until some form of positive confirmation for that was obtained. Since the name of the game is horizontal shear, perhaps something of glulam's unique nature (glue line shear) makes it more suitable for the integral method?

2) Owing to wood's unique character, in wood design we are very interested in horizontal shear and not at all concerned with vertical shear. Additionally, because wood is very stiff axially, it seems reasonable to assume that horizontal shear stresses could be averaged over the length of the member. This feels very similar to the way we handle shear stud design in steel/concrete composite beams. The studs are spaced uniformly along the beam based on the average horizontal shear demand. All that said, while the integral makes sense to me, I'm at a loss to rationally explain the ultimate form of the S6 equation with all the powers and inverse powers of five etc.

3) As my modest contribution to this, I've attached a copy of the Foschi paper upon which all of this is apparently based, I didn't read it in too much depth but a quick scan really didn't turn up many insights for me. I did get the impression, however, that it's got more to do with the stochastic treatment of a material subject to brittle fracture than any rational mechanics stuff.

4) If this method is okay for bridges, I don't see why we're not also using it for everything else as well. It seems as though it would always be an advantageous provision. I get that it's more important with bridge loadings. But, then, when have we ever relaxed capacity requirements because demand was more severe? Never, that's when.



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.
 
 http://files.engineering.com/getfile.aspx?folder=08c85bc8-e1b5-4a4f-99df-2067f6b3b41d&file=Foschi_Barrett.pdf
Hi KootK,

Interesting points and very similar thoughts to what I initially had.

Thanks for the paper I will certainly read it through.

When I read through the S6 commentary, they work through a very very basic example talking about a pilecap, which are almost always large DFL Sawn members, I don't have it in front of me but I believe the very simple beam shear is 180 in and the shear load closer to 70 kn.

Looking at a particular existing structure (50 years old) if I go through the SFD and compare to the standard Vf to Vr things don't work or even come close! But calculating the shear load for large sawn members the vf shear load works out much close to the vr ??


A good discussion and would love to get some agreement
 
KootK thought #5: in the literature on timber shear, much is made of accounting for checks acting as points of weakness and stress concentration. Glutam doesn't have checks, right?

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.
 
Link

They do have checks but the grading rules are different.

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.
 
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