Beam Analysis w/ Holes
Beam Analysis w/ Holes
(OP)
I am trying to figure out the best way to determine the stress (bending and shear) in a rectangular beam (not a steel I beam but a wood timber) with a horizontal hole through it. This is the very common situation of plumbing and electrical contractors needing to run lines through a floor system with beams. I have searched far and wide and can only find recommended practices from AITC and APA but not a way to analyze the effected stress and deflection in the beam. Any suggestions?






RE: Beam Analysis w/ Holes
A hole kind of resembles a "knot" and you could justify it based on that.
Or than that, you could use a reduced I (or S) and Area for moment and shear calcs.
RE: Beam Analysis w/ Holes
RE: Beam Analysis w/ Holes
RE: Beam Analysis w/ Holes
surely (?) these are either a matter of "passed by suspicion", like a small hole for an electrical cable to pass thru, or obviously reinforce the web, 'cause you've remove 90% of the shear capacity.
is that an easy way to rationalise things ? ... bending stresses are generally not much influenced by removing web material (so PbI) but shear capacity is directly affected, so derate the beam by the amount of web removed.
RE: Beam Analysis w/ Holes
I would strongly suggest that you get then to stick to circular holes at mid height and less than one third of the beam depth.
If you really do need to use a rectangular hole then I would suggest you google the same topic for concrete sections as the fundamentals will be the same and I have provided some good links to that in the past.
RE: Beam Analysis w/ Holes
A better may be to divide the shear at the location of the hole in two, so that the top carries v/2 and the bottom carries v/2. Then find Q & I of the top section alone and use fv = QV/2Ib. This assumes that the shear is carried in two parabolic distributions: above the hole and below the hole.
I worked it out with a 2"X10" cross section and a 4" hole. This approach yields a shear stress that is 167% that of a solid section.
RE: Beam Analysis w/ Holes
AITC TECHNICAL NOTE 19 GUIDELINES FOR EVALUATION OF DRILLED HOLES
AND NOTCHES IN STRUCTURAL GLUED LAMINATED TIMBER BEAMS, EVALUATION GUIDELINES
http:/
RE: Beam Analysis w/ Holes
RE: Beam Analysis w/ Holes
I politely thanked him for changing my 2x12's into 2x8s and his response was "What?? - what's wrong??"
I explained to him what was going on. He said that his drill wouldn't fit between the joists. My response was "So - go find another drill. We have imported Italian marble on the floor above and we can't have any deflection." Fortunately, he only hit about 4 or 5 and were fixable.
RE: Beam Analysis w/ Holes
Spats- I only think bending above and below the hole is a concern if the hole is so big that "plane sections remain plane" is no longer true. I don't believe that a 3" diameter hole out of a 2x12 makes that true.
RE: Beam Analysis w/ Holes
I personally have always followed the Grading Rules for sizing and (very important, IMO) the spacing of holes. With care to note that the minimum spacing required included the existing holes and knots.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.