Negative moments reduction under interior support in wood beam?
Negative moments reduction under interior support in wood beam?
(OP)
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






RE: Negative moments reduction under interior support in wood beam?
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.
RE: Negative moments reduction under interior support in wood beam?
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?
RE: Negative moments reduction under interior support in wood beam?
RE: Negative moments reduction under interior support in wood beam?
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!
RE: Negative moments reduction under interior support in wood beam?
RE: Negative moments reduction under interior support in wood beam?
Do not use moment redistribution with wood, period
RE: Negative moments reduction under interior support in wood beam?
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!
RE: Negative moments reduction under interior support in wood beam?
I think your formula is okay for an interior support. It is more conservative than taking clear spans.
BA
RE: Negative moments reduction under interior support in wood beam?
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.
RE: Negative moments reduction under interior support in wood beam?
My only question would be "Why?"
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Negative moments reduction under interior support in wood beam?
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.
RE: Negative moments reduction under interior support in wood beam?
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Negative moments reduction under interior support in wood beam?
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?
RE: Negative moments reduction under interior support in wood beam?
BA
RE: Negative moments reduction under interior support in wood beam?
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.
RE: Negative moments reduction under interior support in wood beam?
Michael.
Timing has a lot to do with the outcome of a rain dance.