Negative moments reduction under interior support in wood beam? 1

[drile007]

I'm wondering if it's possible to reduce negative moments under interior support (also wood) in continious wood beam...like common practice in reinforce concrete? Why not?

Appreciate any comment

 

[J2CUB612]

I assume you're referring to redistribution of moments in concrete beam design.

It's not acceptable to use this same procedure for wood.

Redistribution of moments for concrete is an issue related to plastic design. At ultimate strength you can reduce the amount of steel for negative bending over the support. When yielding starts the positive moment strength at midspan starts to take up load. The total system supports the entire design load this way.

A wood beam won't work the same way due to material behavior and the inability to tailor the cross sections strength as one would when choosing rebar.

 

[drile007]

I was thinking on reduction accounting on support width? I found that in reinforce concrete I can reduce the interior moment by amount as follows:
M'=R*t/8
M'...amount of reduced moment
R...reaction under support
t...width of support
This reduction has no relation with nonlinear redistribution...it's independant of the type of analysis we used?

 

[nutte]

The concrete beams and column create a rigid joint, where the column also resists moment. This is not the case with your wood beam, sitting on top of a wood column. If your wood beam is that close, I suggest a bigger beam.

 

[drile007]

nutte:
That's also not an issue not to take into account the formula I wrote! That's something else!

These are the words from Eurocode 2 (concrete):
Ragardless of the method of analysis used, where a beam or slab is continuous over a support which may be considered to provide no restraint to rotation (e.g. over walls), the design support moment, calculated on the basis of a span equal to centre-to-centredistance between supports, may be reduced by M'=R*t/8!

 

[nutte]

If it was accepted practice, don't you think you would have come across it in some documented form?

 

[dcarr82775]

Are you asking about taking the clear span instead of the centerline span? If so I see nothing wrong with it, but don't use some equation out of a code, just calculate the moment based on the clear span. The code provision you note appears to me to account for clear span v centerline span, nothing to do with moment distribution.

Do not use moment redistribution with wood, period

 

[drile007]

nutte:
You're right, but that's why I'm using this forum...to get with argument supported negative (or positive) answer. Till now all the answers are related to something else. Hope I'll get it!

 

[BAretired]

007,

I think your formula is okay for an interior support. It is more conservative than taking clear spans.

BA

 

[J2CUB612]

The 2005 edition of the NDS for wood says that the span length can be taken as the clear span plus 1/2 of the required bearing length at each end. For wide supports this may help reduce the moment from that calculated to the center of support.

What doesn't appear to be addressed by the NDS is the effect on column moment. I would apply the beam reaction to the column at the center of required bearing as well in order to apply the resulting column moments.

 

[msquared48]

If you neded to reduce the negative moment, which would increase the positive moments in the center of the spans, and the beam is existing in place in the field, all you need to do is drill a series of small 1" diameter holes vertically thru the beam over the column. This will relieve most of the negative moment.

My only question would be "Why?"

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[drile007]

dcarr82775:
I don't know if the M'=R*t/8 formula is related to clear span...maybe indirectly since the original moment is calculated on the ceterline span basis. The reduction is based on support width (t) which one is related to clear span.

 

[paddingtongreen]

That is simply considering the support reaction as a UDL over it's width instead of as a point. The moment is still about the CL. The upward moment is (R/2)(t/4)=Rt/8.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[drile007]

paddingtongreen:
You are right, but can I use it in the wood construction.

I'm scared that stiffness of support has huge influence on the formula. If support is very stiff the formula is OK but if you have soft supporting material (like wood) formula evolve to something else?

 

[BAretired]

The end grain of the supporting column is stiff compared to the side grain of the beam. I think the bearing stress will tend to be fairly uniform.

BA

 

[rscassar]

In concrete construction, it is saying to perform the analysis from centreline to centreline of support, but allows the engineer to use the negative moment at some distance away from the centreline for flexural design (moment at the face of the support).

The 2005 edition of the NDS for wood says that the span length can be taken as the clear span plus 1/2 of the required bearing length at each end. For wide supports this may help reduce the moment from that calculated to the center of support.

I don't have a copy of NDS so I can't verify the accuracy of the code requirement, but I would think this is similar to what you are referring to in concrete design. Instead of calculating moments at the face of the support; for wood, the design is based on a span from edge of support to edge of support + the minimum length required for bearing.
Shorter spans = smaller design moments.

 

[paddingtongreen]

I agree with BA, and even if we are wrong, I would expect the greatest pressure to be on the edges of the column. Any "bite" won't be significant, this is wood, after all. If there is sinkage at the column, it will relieve the fixing moment in the beam.

Michael.
Timing has a lot to do with the outcome of a rain dance.