Beam Analysis w/ Holes

[dodson]

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?

 

[MiketheEngineer]

Kind of depends on the size of the hole and where located. IF you read through the grading rules and there are many, each grade (most grades) allow a certain size "knot" located at certain spots.

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.

 

[Ron]

As Mike noted, reduce the section properties and analyze that way.

 

[handex]

I would just draw a cross section of the member with a void wherever the hole is and calculate the stresses from first principles

 

[rb1957]

most of the holes should be near the CL of the web, ie the NA of the beam, no?

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.

 

[csd72]

Be very careful about allowing them to cut rectangular holes in beams as carpenters are notorious for overcuts. An overcut in this situation occurs at the worst spot and is a source of stress concentration.

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.

 

[kipfoot]

I do *not* believe it is quite as simple as finding the properties of the new section. If you find Q & I of the section with the hole and use fv = QV/Ib. Rather than find the max shear stress, you'll find the shear in a solid section at the top of where the hole would be, a false answer and one that I think is not conservative.

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.

 

[boo1]

Hole location may be more important than hole size.

AITC TECHNICAL NOTE 19 GUIDELINES FOR EVALUATION OF DRILLED HOLES
AND NOTCHES IN STRUCTURAL GLUED LAMINATED TIMBER BEAMS, EVALUATION GUIDELINES

http://www.aitc-glulam.org/shopcart/Pdf/aitc_tn_19_ may_2002.pdf

 

[spats]

What you guys are missing is that the shear causes much higher bending stresses in the section above and below the hole. That is why you should stay away from areas of high shear, like put the hole only within the middle third of the span. The bending stresses must also be evaluated.

 

[MiketheEngineer]

I once caught an electrician boring 2'' holes in the bottoms of 2x12's that I had specified at 12'' oc.

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.

 

[Lion06]

I think, generally speaking, if you keep the holes in the middle third of the span and the middle third of the depth you won't have any problems. Even a 3" diameter hole out of the center of a 2x12 is only reducing S from 31.6 to 31.0 (that's pretty much negligible.

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.

 

[woodman88]

Attached is an old (UBC,SBC,BOCA & CABO) guideline (from the WWPA, who print the western lumber grading rules) for notches and holes in conventional light-frame construction. Please note that this was not a referanced in the code and is not update for the IRC/IBC. It also assumes that you designed per the conventional light-frame section of the code and did not engineer the members.
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.