Upgrade glu-lam systems for new codes 2

[jamie2000]

I am currently investigating a school roof built in 1968 for compatibility with current codes. The roof will need to be strengthened because of design snow load changes since 1968. The roof system consists of a laminated (vertical laminations ) deck supported on double tapered glu-laminated beams whose span is about 75 feet. The beams are spaced about 16 feet apart. Any ideas of the most efficient way to upgrade the roof?

 

[Ron]

Assuming that aesthetics is not major issue, you can "flitch" the glulam bents (using steel or aluminum plates on the sides or by contouring angles to fit the undersides and wrapping to the sides) to increase capacity. You can also add to the decking if deflection is too high, by adding 2x members in a vertical orientation to the underside of the deck. Do this by applying continuous adhesive and by screwing the new members into the existing, using care not to penetrate the roof system from the bottom side. Use either hangers or a ledge on the bents to support the ends of the added deck members.

With a bit more effort you can add to the glulam members by adhering additional sections onto the bottom/inside of the bents. Make sure adhesive is continuous and supplement with mechanical fasteners.

 

[JAE]

Not trying to question your validity about snow load increases, but I am surprised that it has. In most cases, where snow loading has been in use for a number of years, the code required snow has actually diminished from what local engineers usually used. For example, in our area, engineers for years used to place 30 psf snow on all roof designs. Under the current UBC, the actual calculated snow is 21 psf. With an additional 5 psf for rain on snow, you get 26. If you are in a specific area where snow has increased, I would be interested to know what region your project is located.

 

[jamie2000]

JAE
The project is in northern Maine where the ground snow load is 100 PSF, resulting in a roof snow load of about 70 PSF. Typically, in this area, roofs were design for 40 PSF roof snow load in the 1960's. Therefore, the code changes significantly affects roof upgrades in this area.

 

[MikeTheBike]

In my area of Great Britain the design roof snow load has been reduced from 15 psf to 12 psf which matches the general trend throughout Europe as far as I know.

I would be very dubious of any Building Code which nearly doubled the design roof snow load from 40 psf to 70 psf "overnight".

(1) How long was the old 40 psf Building Code in operation? (2)How many structures designed to this code collpased?

In all probability the answers are (1) a very long time and (2)not very many.

Regarding the original question about the gluelam roof I would be asking the following before proceeding with strengthening:-
Do you have a real problem ie excessive deflections and localised damamge under heavy snow loads or a paper problem - compliance with a modern bBuilding Code?

Is the quality of construction good enough to justify higher design stressses in the beams?

Are there alternative loadpaths should there be a failiure?

The current design code - is it a "one size fits all from Florida to Alaska" or a local code more appropriate to the local circumstances?

What is the local micro-climate - is this area the last to have any major snowfalls and the first to thaw?

Are there any features in the immediate vicinity of the bulding which would encourage a build-up of snow on the roof or does most of the snow get blown away by the prevailing winds?

If you work at it you may find that the roof does not need strengthening - yes the risk of it collapsing may be greater than that of a more modern building but the level of risk could still be acceptable. Determining the level of risk and its acceptability or not requires a great deal of engineering judgement.

I was taught that Building Codes were for the blind obedience of fools and the guidance of wise men.