Glulam Purlins In Swimming Pool Trusses û Moisture Movement 1

[countervail]

I have a design carried out by another engineer which I need to finish off.

We have steel trusses spanning 25m over a swimming pool. The top truss cord is braced at 2m centres by glulam purlins (eventually back to a horizontal wind truss).

The wet environment is likely to cause the glulam to expand and the timber could split at the bolted connection to the truss.

I can’t uses slotted holes to allow for this movement as I will lose my lateral restraint to the top cord of the steel trusses.

Has anyone come across this before? Am I unduly worrying about dimensional changes in the timber?

 

[tmills]

Sounds like you are worrying too much on the glulam expanding, but talk to the manufacturer. It is not a wet environment but a high humidity environment. Probably 50% RH at 84 degree F. The glulam must be speced and treated for the pool enviroment. Use stainless plates and bolts if you can.(I assume chorline pool) Research the type of stainless to spec (there is some controversy). The steel trusses are exposed I assume, and will be epoxy coated??

The issue with pools is adequate air infiltration management the location of the vapor barrier and proper HVAC design.

The Architect must be dogmatic about air and vapor barrier location and detailing. the Air barrier has to almost be perfect and the HVAC has to control the RH and keep the space under slight negative presure or there will be condensation problems. That's what will destroy your glulams and any other wood.

pools = lots of lawsuits. Be careful.

Tim
TAM Consultants

 

[msquared48]

If the temperature and humidity are constant, where is the expansion?

Just put the bolts on hand tight in slotted holes, and tighten them down a month or so after the pool is filled with water and the building heated. Most of any initial expansion will take place during this timeframe. Make sure you spec this too.

You know, too, putting a vapor barrier paint over the glulams, assuming they are fully cured, would help with any moisture problem too.

Mike McCann
MMC Engineering

 

[hokie66]

Timber splitting at bolting connections is usually caused by corrosion of the bolts rather than loading or volume change of the glulam. Volume change in the longitudinal direction is not much. Good advice by tmills to use stainless connections to the glulam. I think the steel trusses are the more likely corrosion problem. Make sure to use a high performance coating system designed for swimming pool service.

 

[countervail]

Interesting from design guides I have managed to track down, the moisture content of timber not directly in contact with water in a swimming pool environment is only 15% so it should be much of a problem.

Many thanks all