Result of Temporary Bolt Holes in Permanent Wood Beam

[AndBre44]

I'm currently reviewing a situation where my GC needs to put up a temporary wood guardrail around a balcony area as a means of fall protection/so that they can store materials outside. The framing is already installed to the contract drawings, and will be able to handle the expected live loads & resulting shear loads coming from the guardrail. The balcony was going to have a guardrail in its finished condition, however the GC is not at a point where he is able to install the final guardrails, instead looking for this temporary condition for the time being. The plan was likely to use a similar detail to the fascia mounted detail from this link:

https://www.sandiegocablerailings.com/framing-considerations-for-cable-railings/

Once the temporary posts are removed the presence of the holes in the beam concerns me; especially since in its final condition its going to be a uniformly loaded beam. The guardrail posts would need to be installed at 4ft O.C., and all holes would be 1/2" dia.; I'm just picturing in my head all of the holes and stress concentration around them. One heavy load gets placed near it in the balconies final condition is where my head is going. I haven't found any kind of definitive formula/methodology to quantify the lack of strength, so I figured I'd turn to those with more knowledge & experience than myself. Any thoughts/advice on the matter would be appreciated.

As an EIT, I'm open to being wrong now if it means being right when it counts.

 

[Enable]

I suspect you already know the formulas to check for strength reduction, if you were so inclined. Moment and shear capacity are related to sectional properties (in particular section modulus and net area). Find out what those differences are and use them in the formulas to find the new moment/shear capacity.

But unless your perimeter beams are an engineered product, or your beam is especially shallow in dimension then a couple of 1/2" holes every 48" are not going to cause you problems. If they did, then you didn't design the structure right anyways. Recall that when designing the beams you probably didn't account for the deck boards above, right? Well those will span the problem areas in terms of load distribution/shear and if sufficiently connected will act compositely with the beam and help with moment. There's lots of redundancy in wood that we don't ever consider!

You have a much larger concern with durability than with strength. You don't want water to get into the holes once the fasteners are removed and degrade the beams from the inside out (or freeze thaw). I would consider epoxying the holes after they have served their purpose. As an additional benefit, this will help restore your sectional strength in the compression zone at least.

 

[KootK]

I wouldn't sweat this. With or without the bolts, the holes surely do decrease the capacity of the beam a bit as a result of the impact of stress concentrations. However:

1) More than anything that follows, an engineer has to pick their battles strategically. And this isn't a worthy one in the grand scheme of things.

2) It's common to put all kinds of little holes into beams for connections of various types -- and beam lamination -- and we rarely sweat that.

3) The issue would be predominantly a tension stress / facture mechanics thing. Empty holes will be little worse in this regard than holes with bolts in them.

4) The rated strength of your members is probably based on grading rules that assume the presence of knots etc that are bigger than bolt holes anyhow.

 

[SnTMan]

Well, if the holes are just "temporary"...

LOL

Regards

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand