Bolt holes effect on glulam strength. 1

[RARWOOD]

An engineer asked me today about a simple span glulam that had posts added after installation to reduce the glulam deflection over a window. Now the owner wants to remove the posts. As a simple span beam under design load the beam is stressed to the maximum allowable stress.

The question I was asked was, would the bolt holes that would be left when the post were removed, cause load carrying capacity problems. The bolt holes are 3" & 6" up from the bottom of the beam.

My gut feeling is that they would not have a significant effect. I do know however that stress concentrations would occur at the bolt holes.

I thought while I mull over the question and do some research I would post the question to get some advice from other engineers out there.

I think the basic approach would be to treat the problem as a question asking if a 13/16" diameter hole could be drilled 3" up from the bottom of an existing beam to allow for the passage of sprinkler pipe.

 

[SlideRuleEra]

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

http://www.SlideRuleEra.net

 

[JAE]

Nice response SRE - I just seem to be always visualizing you with little sparks coming out of your fingertips every time you need a link reference and once you touch the keys...voila!

 

[RARWOOD]

SRE the link you posted to the AITC web site is a very good reference. I am always amazed at the vast range of your kwonledge and always find your posts useful.

After reading the AITC publication I've changed my thoughs on the issue. I was trying to relate the problem to placing a 3/4" diameter bolt 3" above the bottom of a member to hang a light or sprinkler pipe.

The N.D.S. has a provison prohibiting the hanging of heavy or medium concentrated loads below the neutral axis of beam unless reinforcement is provided to resist tension perprndicular to the grain. The N.D.S. Commentary indicates that concentrated loads less than 100 pounds spaced 24" apart may be considered light loads.

The above provision doesn't address the question of fastener size or type. My understanding of what is intended would be lightly loaded joist hanger nailed to the beam.

I started my approach to this problem by looking at the load capacity of a 3/4" bolt loaded perpendicular to the grain. With 1/4" steel side plates. For a 5 1/8" Douglas Fir glulam the allowable load would be 1500 pounds. Minumum edge distance would be 4 diameters which is equal to 3".

The thing I forgot was that you are also required to check the shear at the connection using section 3.4.3 of the 2005 N.D.S. In most cases the shear would probably be too high to allow the location of the bolt only 3" above the memeber bottom.

My conclusion was that trying to compare the problem to the question about adding a bolt in that location, is not applicable. The problem really is a question regarding the loss of strength of the beam due to the addition of a hole.

With out a way to check the stress concentration around the hole and based on the N.D.S. and AITC publication. I concluded that it should be assumed that the hole would have a major impact on the load carrying capacity of the member.

 

[SlideRuleEra]

JAE & RARSWC - Thank you both for your kind comments. Little sparks? Must have spent too much time at electric generating stations...

http://www.SlideRuleEra.net

 

[woodengineer]

APA had put out a bulletin some time ago titled "Field Notching and Drilling of Glued Laminated Timber Beams." As it pertains to horizontal holes the technical bulletin read: [...Field-drilled horizontal holes should be used for access only...Examples of access holes included those used for passage of wires, conduit, small diameter sprinkler pipe...Field drilled horizontal holes should be spaced at least 2 feet on center in the non critical zones (d/2) in the compression zone. Under no circumstances should holes be drilled in the outer four laminations of a flexural member on the tension side in the zone or zones stressed to 50% or greater of design stress. Holes with a diameter larger than the thickness of one lamination (typically 1 1/2") or 1/1 of the beam depth, whichever is smaller, are not recommended. However for glulam members that have been oversized for architectural reasons, this limit may be increased based on engineering analysis.]

These are just guidelines. Net section should be checked for flexure and horizontal shear. The above artical was considering uniform distributed loading.

 

[RARWOOD]

Both the APA and AITC tech notes are good sources of information regarding the field notching and drilling of glulam beams.

The problem encountered in practice is that some one has drilled a hole, or cut a notch in an area that doesn't fall within the AITC guidelines. An example would be a 2" hole drilled at the midspan of a beam 4 1/2" from the bottom tension lam. At this location the hole would remove 1" of the third and fourth tension lams.

It is always easy to take the position that the hole violates the AITC guidlines and should be replaced. Replacing a girder beam, which has purlins framing into it after all the roof deck and roofing is in place, is not an easy job.

As engineers we are always being asked to solve the type of problem above. In most cases it requires some sort of field repair such as adding steel plates to the member. There are times where based on our engineering judgement and experience we can solve the problem by plugging the hole and have the contractor relocate the hole in a more acceptable location.

Problems like the one above makes engineering so challenging and interesting. As someone who works with wood day in and day out, I see a lot of designs that violate the general rules of glulam detailing. There are numerous times where the engineer who developed the orginal design, is unwilling to take my advice on improving their design.

When he or she insists on doing it their way I am forced to decide if it is just a bad detail or if it will cause significant performance problems in the beam.