huangyhg

i-beam support by foundation walls
i have 8" thick poured concrete foundation walls which support an i-beam (that in turn supports floor joists). this is new residential construction. i think the i-beams should be well supported by a significant portion of the 8" thickness of the foundation. yet my builder tells me that the ends of the i-beam are satisfactorily supported by only 2" of foundation wall (this is how much the beams protrude into pockets in the foundation wall).
does this sound at all acceptable? the beams are spanning 12' between supports.
how long are the floor joists (to get an idea of the reaction at the end of the beam)? any idea what size the beam is (w8x??, w12x??)?
2" isn't enough - you should bear a minimum of 3" - preferably 4.
the aisc 9th edition uniform load tables r1 values are based upon 3.5 inches of bearing.
the values from those tables are based upon the k1-3 local web yielding formula or the k1-5 web crippling formula.
the only limit to length of bearing i see is "not less than k for end reactions".
as long as the allowable bearing stress in the concrete is satisfied, it seems to me that the bearing length may be reduced to the k distance of the beam.
designing only 2" of bearing would require a rather tight tolerance for the installation the concrete walls, but especially for residential construction, may be acceptable.
jae thinks 3" is the minimum. i'd like to ask jae, "is there a specification that is guiding you in that response or are you basing your response on personal experience?"
regards,
jpj
3" min. looks familiar. in my view, it was not necessarily based on stress, rather, it's from experience. a minimum bearing length can accommendate construction offset, movement, etc, and avoid spalling of concrete in the cover layer.
answers to question from sperlingpe:
the beam is a w8 and the joists overlying it are 9.5" on 19.2" centers. they each span 10 feet for reaction determination.
thank you,
codys
question to virtualengineer: you wrote "it seems to me that the bearing length may be reduced to the k distance of the beam." what is the k distance of a beam (the meaning)?
thanks to everyone for you advice on this!
"k" distance is the length of the fillet between the flange and the web; it's not a 90 degree joint. note that it isn't a true radius; the length of the fillet horizontally isn't the same as the length of the fillet veritcally.
cody: virtualengineer's advise is wise since he is backing up his answer with aisc..
1prsplmps1
cody: you can find the "k" distance of any beam in the dimensions and properties section of the aisc manual of steel construction. the w8 "k" distances range from 5/8" for a w8x10 to 1 7/16" for a w8x67.
i understand that the "k" dimension on many beams have been revised upward and the newer dimensions can be downloaded from the
codys,
it seems to me that a bigger concern is going to be the spalling of the concrete. aci states that the shear plane will tend towards the nearest free edge. the pca notes on aci 318-02 gives an excellent diagram illustrating this mode of failure. one example they gives approximates this angle at about 20 degrees. so as you can imagine, you don't get much of a shear plane when you are only two inches from the nearest free edge. also with only two inches of bearing there is not much of a chance that any effective shear friction reinforcement is present.
of course if you have an anchored bearing plate cast into the wall then there should not be a problem. if you don't then you may want to consider a sturdy angle and some hilti bolts under your beam.
~waytsh

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