Permanent Glulam deflection cause 5

[Triangled]

I am visiting a site soon in the aouthwest non-snow region where the client says he has measured a permanent deflection of 4“ in roof Glulam beams. Ponding has been an issue and he intends to repair the roof as required.

I did not think that Glulam would deflect that far without springing back. And although certainly dangerous for overloading, I didn't really think of ponding lasting long enough to instill "creep" in a Glulam.

The original beams, based upon the very sketchy plans I have, date from early 60s, are 4.25 x 16.25 (thank you msquared for confirming the existence of this size in another post), furnished with 1” camber, are spaced 8’ o.c. and appear to span about 34' to 36’. The roof composition is not apparent at this time and is very close to flat.

I would sure appreciate advice on what to look for and what might be at work in this situation.

 

[JAE]

Maybe got wet over the years and lost some integrity?

 

[AELLC]

I think 1" camber is insufficient. I ran a quick analysis and it appears deflection controls in this design. Stress appears OK, assuming nominal Fb=2400 psi.

Is this in an area of high temperatures, such as in the Phoenix area? I have seen glulams that are even slightly deficient in depth tend to creep dramatically because of the sustained high temps we get all summer and fall here.

Do you think the dead load is about 14 psf? If it was re-roofed many times without scraping off previous re-roofs, dead load may be higher.

 

[msquared48]

Also, if wood has a high moisture content due to high humidity and is loaded, then the humidity and moisture content lowers while still under load, it will retain the deflection. This cycle could have been done many times over the years. Ponding could have done this.

Mike McCann
MMC Engineering

 

[Triangled]

Climate-wise, the building is about 20-25 blocks from the ocean not far from San Diego, CA, so not as hot, and certainly not as dry, as Phoenix.

I am hoping for somewhere in the 14 psf dead load range based upon the building style in the sketchy plans I've received, but won't know until I am there. I certainly would not be surprised to discover multiple re roofs on a 50 year old building.

Based upon age appropriate (did I say that?) data tables from msquared, it seems the maximum stress for these beams might be 2200 psi, and so yes, bending could be very much be an issue as well, but again.

Msquared, the cycling nature you indicate is intriguing. I can imagine this beam was designed right on "the edge" and is currently carrying several too many re roofs, do you think a beach environment could provide the cyclic moisture retention and moisture release you mentioned?

 

[msquared48]

Damn skippy bro.

Mike McCann
MMC Engineering

 

[woodman88]

The "Wood Handbook Wood as an Engineering Material" a free download pdf from the below link

Gives some (but not much) good information about creep.

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.

 

[Triangled]

thank you

 

[Triangled]

OK... data from the field.

The typical beam deflection is 2.5" not 4" as reported before.
The 4.25 x 16.25 glulam beams are approximately 40' long, approximately 34' from bearing to bearing with 3' cantilevers each end.
The beams do not look bad.
The bearing point at one end of each beam is level with the bearing point at the other end of each beam.
The entire roof is about 40' wide x 56' long with four roof drains located about 6' in from each corner.
The fascia all around creates a nice lip to increase the depth of a pond by about 1".
There is on obvious spot, perhaps 4'x6' oblong in one portion of the ceiling, somewhat towards the center.
There is a roof drain visible from the inside, in the center of the room ceiling, elbowing then extending along the underside of the ceiling to the outdoors.
I spoke with the owner who also was present when this building was built in the early 1960s. He said that upon completion of the roof, he flooded it with water and it immediately ponded. He therefore made the contractor install that drain in the center of the roof, which is also visible in the center of the open ceilinged room from below.

OK...
I am suspecting that the design team never designed for drainage properly, or the contractor didn't follow the plans, and that the engineer specified 1" camber just to level out the exposed beams after dead load deflection.
I am suspecting that the contractor installed them upside down explaining the immediate ponding, then the beams deflected another inch or so just like the engineer anticipated.
Everything is painted so I cannot see "TOP" stenciled on the beams, but then again, I don't know that laminators stenciled "TOP" in those days, and also, I suppose there was a reason why they always (I hope) stencil "TOP' on laminated beams now.

I'd appreciate your further thoughts on this one. Thanks!

 

[msquared48]

Hopefully these are V8 beams, otherwise the tension flange could be severely overstressed if they were inverted.

Mike McCann
MMC Engineering

 

[Ron]

Dead level or flat roofs were common in the 60's since most commercial roofs were built up systems using either coal tar or dead level (Type 1) asphalt. Code mandated slope requirements did not show up for many years. The process the owner described of flooding the roof and putting a drain where the greatest ponding occurred was also common.

Creep is a distinct probability, but you might also consider shear flow in the glue. Seems a little esoteric, but the potential is certainly there and, much like creep, would not be reversible.

 

[AELLC]

I think the shear flow in the glue is actually common. That is what I meant when I opined the GLB's creep a lot at sustained high temps.

 

[Triangled]

Was originally thinking of post tensioning to restore to flat condition and going from there. Then I observed that the "diaphragm" is one of those thick insulated "boards" (wood not visible), and, that those boards are supported on 4x4 purlins at 4’ o.c., And those 4x4 purlins are bearing, but not blocked, over the Glulams, and therefore proving questionable compression face stability at best 4' o.c.
The idea of adding compression to that beam via the post tensioning process plus the potential for a 50% extreme fiber in tension allowable for an upside down beam is creeping me out even if the beam / glue is not creeping. I'm not sure engineer creep is reversible either.

 

[dhengr]

Triangled:
Same bldg. owner for more than 50 years, and smart enough to ask questions when the thing was being built, and now again. Maybe he was smart enough to save a copy of the plans and specs., check and see. Who was the original engineer, any plans there or at the local bldg. dept.? Check to see if the deck mat’l. isn’t a product called ‘Tectum Decking?,’ it was a wood fiber decking product which would span 4', or a like product. The 4x4 purlins are attached to the beams somehow and would offer some stability, it’s been standing for 50 years and doesn’t show any instability, does it? In the early 60's I don’t recall GlueLam beams which had different Lam strengths on the top than those on the bot. plys, necessitating top and bot. markings on the beams. That came along later than the early 60's. Of course, the couple top and bot. plys were a better grade than the interior plys, but those outer plys were likely the same grade. Usually, some detailing oriented the beams, t&b.

I would look further at post tensioning, that was not bad first thought. Yes it adds a small P/A compression component. But, if you harp the rods from near the beam top at the two bearings, to a few inches below the beam at about the third points, maybe 12' from both bearings and 10' in the middle, maybe with another harp point at the center line; you do reduce the bending normal stress on both the top and the bot. fibers, while still adding a small P/A stress. A few calcs. should show if that’s a reasonable trade off. You might also jack the existing beams up while you a tightening the turnbuckles. And, compare the starting and final deflection of the beams as you do the work.

 

[Triangled]

Dhengr,
Thank you for the insight. I hope to perform a review of this project Tuesday and perform the preliminary calcs. I'll look for the information mentioned and revisit the PST tensioning concept.

 

[Sawbux]

1" seems like too little camber to begin with. Typically beams were cambered for 1.5 x dead load delta, figuring that the .5 would come out as creep.

Other problems for glulams from the 60's include the use of casein glue (which softems in moist environs) and scarf joints (which can separate). But these do not seem to be the problem.

I have post tensioned glulams before with cables. Typically I tension them to not exceed the dead load on the beam, so as to not put tension in the top or compression in the bottom that would cause lateral instability. Anchorage of the cable ends is usually the tricky part, it's hard to get enough bolts in the end anchorage to develop the cable. I have many times used steel plates across the ends of the beams with threaded swaged ends on the cables to tension them. Puts the ends of the cables at the top of the glulam and harp the cables to the bottom near center span. They can have 1 or 2 harp points (king or queen post style).

Another fool proof option would be to put a new line of columns down the center - if it doesn't interfere with the building function!