BEAM SIZE

[SEAL]

I want to build an open pavilion. There will be 4-4x6 or 4-6x6 support posts. On the support posts will be 2 beams to support the roof load. The beams will be 26 feet each in length with a 20 foot open span with 3 foot sticking out on each end. The beams will be 16 feet apart with 3 foot eaves as part of the roof structure. This gives a total of 26x22 feet under roof. What would be the best type beams for this purpose and what size would be appropriate for the snow loads here in Virginia.

 

[tony1851]

it depends on;
(a) whaat type of timber you are using;
(b) the type of roof deck you will be using; and
(c) the snow load for your area (this will probably be stipulated in your local Code.)

For that length, you may end up having to use LVL. If the structure is free-standing, your posts will need to be well-fixed in the ground.

 

[SEAL]

Thanks for the response. The type roof deck will be metal with a 4/12 pitch. The snow load here is 25 psf. I am not sure what you mean by what type timber as that is my original question unless I am misunderstanding something. The structure will be free standing and the posts will be set in a good bed of concrete.

 

[tony1851]

Assuming your metal deck rests on the main beams, and a dead load for the deck of, say 20lbs/ft2, and assuming the timber was Douglas Fir, (or equivalent, of bending strength 1000lbs/in2,) you would be looking at a beam depth of between 15" and 18", depending on width (4" -6"). (or maybe slightly less owing to the reverse stress induced by the cantilver)
This is taking into account the modifiation factors used here, but I know some factors differ in the US.
This size won't be easily available or economic; you might need a
LVL beam, or could you use a steel channel instead of timber?

 

[SEAL]

Steel beam is a non starter (personal preference and cost). The problem with LVL's is that they are intended for inside dry applications as far as I can tell. This is going to be an outside picnic pavilion that will be exposed to the weather. The beams will be protected from direct rain but the humidity is quite high by the lake.

 

[tony1851]

Have you thought of using a flitched beam, in which a thin steel plate is sandwiched between 2 timmber beams> The principle is that the steel carries most of the stress, while the timbers prevent it from buckling. If you know of a local SE who will be up to speed on your loadings /permissible stesses etc, it is fairly straightforward to size these.
Example pic from a structures book;

Untitled

• tony1851
• 14 Jun 2012
• 1

 

[AronSearle]

Steel beam is a non starter (personal preference and cost). The problem with LVL's is that they are intended for inside dry applications as far as I can tell. This is going to be an outside picnic pavilion that will be exposed to the weather. The beams will be protected from direct rain but the humidity is quite high by the lake.

LVL beams are typically manufactured using external glues.

They are not suitable for external exposure due to the use of non durable timbers, and the amount of glue line that is exposed to direct wetting (other than glulam, it's still not a good idea to expose those glue lines).

As long as they are protected from direct wetting, the humidity shouldn't be an issue (although I say this based a UK climate).

 

[SEAL]

I have any number of large red oak trees on my property. If I cut a solid red oak beam that was 26 feet long and 5-1/5 inches wide how tall would it have to be to carry a snow load of 25 PSF and 15 PSF static load with a 20 foot span and 3 foot cantilevers.

 

[tony1851]

Untitled

• tony1851
• 18 Jun 2012
• 1

 

[SEAL]

Thanks Tony. I guess my next project is tree selection.