Is the top beam helping?
Is the top beam helping?
(OP)
In the attached, the load is resting on a smaller beam. Does the smaller beam actually assist the bigger beam by resisting the 500# to keep the deflection within the allowable 1/2"
If so then can it be said that at the 1/2" deflection only 1000# plus the wall weight is being transferred to the bottom beam?
If so then can it be said that at the 1/2" deflection only 1000# plus the wall weight is being transferred to the bottom beam?






RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
Is Iu/(Iu+Il) a constant regardless of the proportionate size of the two beams?
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
Bottom line here is that your problem cannot be answered with any degree of certainty as it is not specific enough.
Mike McCann
MMC Engineering
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
I don't think the capacity of the beams matters for the load distribution. That's a capacity problem. The beam may, or may not, be able to take the load, but it will attract it based on the relative stifnesses. Point taken, though, that if one of the beams fails prematurely that the other beam is doing all the work.
Toad-
I considered the diapgragm behavior of the wall, but discounted it when I saw the load and aspect ratio of the wall. The wall is longer than it is tall so the plf of the diaphragm would be greater than the load plf, which is significant for a wood diaphragm.
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
Lion:
I would have to disagree as the detail is dimensionless, only subjective in nature. I do get your drift however.
It is interesting to note though that if the studs transfer the vertical force with no plywood, then the size of the top and bottom beams (speaking of the larger and smaller context of the OP) will not matter. However, with the plywood, and the aspect of a deep plywood beam between the two solid beams, then the shear stress in the end of the plywood wall will be greater if the smaller beam is placed on the top since a greater portion of the load will have to be transferred through the plywood wall to the larger beam below. However, in the end, regardless of the placement of the beams, all three will deflect the same.
In essence, with the plywood, there will be three beams of different relative stiffnesses.
Mike McCann
MMC Engineering
RE: Is the top beam helping?
Which part do you not follow? I did say this was a relative stiffness problem, no? Iu/(Iu+Il).
RE: Is the top beam helping?
RE: Is the top beam helping?
RE: Is the top beam helping?
You might flesh out this problem for us by telling us why you are asking.
RE: Is the top beam helping?
Can I then subtract that amount from the overall load and design my lower beam based on the result?
I've been told to ignore the (in some cases small amount) that the upper beam can resit and design the lower beam to carry the whole load. That just didn't make sense to me.
Thank you all for your time.
RE: Is the top beam helping?
It will not make a difference in the plywood wall shearing stress whether the larger beam is placed at the top or bottom as all three mechanisms deflect at the same rate and together. When I thought about it further, a difference in stress made no sense.
And, Hokie, you are correct. The answer ti the OP is yes. We were beating around the bush.
Mike McCann
MMC Engineering
RE: Is the top beam helping?
If the smaller beam deflects 1/2" when subjected to the 500# and the larger beam deflects 1/2" when subjected to 1000#. Then a 1500# load applied at the top beam will be shared in a ratio of 33.3% and 66.6% as long as the transferring elements (being the wall frame) has the ability to help share the loads.