Laminating a long beam for strength

Very similar to this question I want to create a balance beam to span the narrow part of a swimming pool which is 16'. I'm thinking of a length of 18' to allow 1' on either side. My initial thoughts around this beam are to laminate together 3 sets of 6-foot 2x4s in this pattern:

• Is there a way to roughly-measure/estimate/guess how much weight this could bear in the middle?
• Will this even work or is there a better way to do this?
• It's going to get plenty of water splashed on it when used. Any considerations with respect to that?
• If I'm able to find longer 2x4s that are straight enough would 9' lengths in the same pattern work better?
• Just to go through the bullets quickly. I don't think you can estimate the weight this might hold without access to someone with the requisite engineering experience or software (and their data set may not actually help in this situation). Yes this could work; as for there being a better way, perhaps, but only if you can buy 18' boards! I would leave this bare in terms of finish; obviously you have to use a 100% waterproof glue. Yes I think 9' lengths would be better, however in either case I think you need to stagger the outside joints to be confident you aren't building in a weakness. Commented Feb 15, 2021 at 5:17
• DIY SE might already have QA on this, but for sure you need to stagger all the seams, even those on the outside. Engineering pieces from smaller pieces of wood is a time-honoured, but actually quite complicated, bit of real world engineering.
– user5572
Commented Feb 15, 2021 at 17:41
• Make your starting pieces 2', 4' & 6', then continue with 6' pieces. You say "balance beam", but I'm not sure if you're talking about something that small kids will walk across as they learn to balance, or if you're talking about gymnasts actually working on beam routines. If the latter, I'd seriously doubt a 4" height would work. A Level 6 girl, age 12 or so, will be closing in on 100lbs. Landing on a 4" beam will cause considerable flex. You want some flex, but not so much that it throws her right back up in the air! Commented Feb 15, 2021 at 17:49
• If it is for beam routine practice, I'd suggest 2x6 or 2x8 (not sure how tall an actual beam is), finger jointing all the ends, then adding some through bolts (counter sink the heads & nuts to prevent injury if someone slips off). If you're expecting this to last a while, upgrade to galvanized or, even better, stainless steel. Commented Feb 15, 2021 at 17:51
• Do you have a good lumberyard nearby? They might sell 18-20' 2x6/2x8. Commented Feb 15, 2021 at 21:52

I just looked at my building code span tables for Glue-Laminated beams (I work at a Architectural/Structural Engineering Firm in Ontario, Canada) and The best options for a 16' beam spanning 14' is 2 - 2x10s which can support 198lbs per linear foot or 3 - 2x10s which can support 298lbs. There is no number on 3 - 2x8s but 2 - 2x8s will only support 84lbs per linear foot at a 14' span.

These numbers represent a deflection of 1/360 of the length so over 14' that would be only 0.46"

Personally I would not use 2x4s. If you don't care to much about deflection you could likely get away with 3 - 2x6s but expect a deflection of 1-2"

• Thanks for this. Does the table assume (or specify) whether the glue lines are in line with the force applied or perpendicular to it? Commented Feb 17, 2021 at 3:05
• For the sizes listed it is referencing the laminated boards resting on their edge. But for the size, length and weight applied the difference would be very minimal for the differing orientations. Commented Feb 17, 2021 at 15:44
• @Marmiketin I would bet that the gymnast using this balance beam will not cause a deflection of 1-2". Commented Feb 17, 2021 at 19:09
• Using some impact force calculation math F = (Height Dropped / Deflection) * Weight of Object. If a 100lb person jumped 1' high and the beam only deflected 1" then it would be (12" / 1") * 100lbs = 1,200 ft-lbs of force that the beam would need to absorb. Commented Feb 17, 2021 at 19:54

EDIT: I reran the sagulator and found an error in my input (first sentence in italics is in error). The deflection is only 1/4". However, I believe the discussion of the dynamic nature of the loads and the behavior of the end points requiring a secure connection remain important considerations.

I just ran this through the Sagulator assuming 200lb total load and got a deflection of nearly 2". Statically, the wood can carry the load, but the problem is that your load is dynamic. Two or three kids could be jumping and shifting weight continuously. Your wood composite beam will probably not crack under their activities (you would need to use a waterproof type of glue to prevent its weakening as it gets soaked). The real problem is not vertical loads, but rather the fact that forces will come at many angles. As the beam sags under the weight it will have a tendency to bow and twist. Enough force in the right direction will cause the beam to rotate around its long access. This will result in the beam bouncing and rotating from the direct line across the pool perpendicular to the edges to something else. It is only a matter of time before it will shift one or both ends to the point it is not supported by the edge. There is great risk of someone getting hurt.
One solution to that is to properly secure the edges to the edge of the pool so that they cannot shift.

I would also consider that someone falling near the edge may land on the pool edge rather than the water. Cracked heads are no fun. I would make certain that at least the edges of the pool are properly padded and that the kids are wearing helmets.

• What numbers did you use?? I just ran the numbers through the Sagulator (with the same weight — central, as sag is significantly lower for distributed load) for a wide selection of softwood species and in most cases I got deflection of under 0.3". Even with the worst-case choices nothing got even close to half an inch. Commented Feb 16, 2021 at 10:10
• @Graphus Yep, I made an input error having 200# per foot rather than as a concentrated load. Thanks for the catch.
– Ashlar
Commented Feb 17, 2021 at 17:28
• +1 for noting that it should be anchored and the pool sides padded Commented Feb 17, 2021 at 18:49