Wind load on exposed beams below deck 1

[RFreund]

How would you account for wind load on exposed beams below a deck.

Couple examples:

Wood deck wind is perpendicular to the joist direction, say 12" deep @ 16" o.c. The wind will load the facia/rim joist but will it also partially hit each joist as the underside of the deck is exposed? Or will the air 'flow' over all of them with no force? Maybe somewhere in-between but what is a reasonable value?

Take the previous example a step further. Say it is a large steel canopy and the beams are 24" deep @ 6' on center or something to this effect. When do u start applying the full wind load to each beam?

Thanks in advanced!

EIT

 

[Ron]

RF....Some aerodynamic shading occurs on all members that are aligned. Some codes allow consideration, some do not. I believe BAretired noted in another thread that one of the Canadian codes gave some factors for the shading.

For canopy structures, I've considered full shading....with only the exposed face taking the projected windward load and the opposite side taking the leeward load.

 

[SAIL3]

normally I would consider wind load on items spaced more than 3 times their depth(in your case). However, each situation is diferent and requires engineering judgement ie:
Since this is based on approximation the final decision would depend on how sensitive the whole structure is to the above assumption...if have only wind on the deck to deal with, then you could be talking about 50 to 100% increase in wind load, depending on your assumtion,..in this case I would take wind load on each joist(more conservative).
If the deck is part of an overall structure the change in total wind load, either way, could be much less, say 10%...in this case I would tend to consider shielding.
Again, it is a matter of engineering judgement and that is what engineering is all about.

 

[Lutfi]

I would recommend, if your code permits shielding, to use what is recommended for use in marinas with boats in slips parallel to each other. It goes like this:

• First boat berthed in row 1, use 100%
• First boat berthed in row 2, use 50%
• First boat berthed in row 3 and beyond, use 30%

This conservative and it comes out of Tobiasson's Marinas And Small Craft Harbors


Regards,
Lutfi

 

[ishvaaag]

See section 2.4.5.1 from IAP 2011, code for road bridges in Spain.

http://www.fomento.gob.es/NR/rdonly...DF-EE28810BFCF9/107316/LIBROIAPparalinea1.pdf

 

[RFreund]

Thanks for the references guys.

I guess if using ASCE7 which I don't believe allows shielding would you need to apply the wind load to each joist even for tight spacing such as 16" oc? Maybe some engineering judgement needs to be applied?

EIT

 

[DST148]

@RFreund: Ref ASCE 7-05, section 6.5.2.1 - Shielding - My interpretation of this section is for a global behavior i.e. no reduction in velocity pressure due to shielding afforded by adjacent buildings/structures/terrain. MWFRS analysis would be done as if the adjacent structures do not exist. However, the code does not preclude the determination of shielding effects and the corresponding reductions in velocity pressures by means of wind tunnel procedure (as given in the commentary). Also IRC indirectly seems to allow shielding in cases of townhouses separated by a common 1-hour fire rated wall. You may see thread507-309907 for more info.
Although reduction in velocity pressure is not permitted due to shielding, increase in snow loads on the roof still needs to be accounted for due to shielding / sheltering.
As far as shielding of the interior joists of a deck, you may refer to thread507-115123, last post by Vmirat.

 

[SAIL3]

agree with DST on interpretation of the ASCE code on shilelding...
in OP's case I probably would take wind load on 50% of the joists...
the codes can not cover every situation,even though they are increasingly trying to, that is where engineering judgement comes in..

 

[ToadJones]

I have recently run into a very similar situation with a T-shaped canopy with the canopy roof having some slope to it. Basically a series of several T-shaped frames that are cantilevered from the foundations and having a roof system spanning from T-shape to T-shape.
I am having a tough time trying to determine the effects of wind loading on the side of the system.
I know the wind loading against the sloped roof causes upward and downward pressures but I am having a tough time determining how to put wind load on the T-shaped frames and the roof framing members.

Ideas?

If I am hi-jacking thread let me know and I will make new post.

 

[RFreund]

TJ - I don't think your hi-jacking this seems very similar/relevant.

So the T-Shaped frames are T-Shaped columns and T-shaped beams with say channels framing between the frames?

Here is a guess:
For the sloped roof (as you mentioned) apply ASCE7-10 Fig 27.4-4.
For columns Section 29.5 (other structures)
For side of roof (facia) apply Section 27.6.2 (parapets).
Now for the topic of the thread - if the bottom is exposed how much wind is catching on the frames or roof framing. What is the spacing?

For reference this was vmirat's post from the thread DST referred to:
"I recently designed a pergola for an outdoor patio of a restaurant in Colorado that is about 10,000 square feet in covered area. It has 2x2 wood slats at 3 inches on center supported by 2x8 rafters at 24 inches on center. At first, I was just going to apply the wind load to the horizontal projected area of the rafters, which seemed inadequate. I did some research and found a European building code that requires wind load to be applied at a 10 degree angle of attack. This seemed logical since wind can't be counted on to blow horizontally. I applied a wind pressure to a portion of each of the 2x8 rafters. I figure this accounts for the drag the wind will have against the whole structure."

EIT

 

[Ron]

TJ....Parallel to the slope you will have the projected area of the slope. Perpendicular to the slope, you will only have the facia and any exposed beam/column area beneath. As an engineering judgment call, I neglect the area of interior beams unless they are quite deep.

Check the oblique condition on this one....you can get some pretty good torque on those cantilevered T-bents.

 

[ToadJones]

Ron-
The oblique is the one I am having trouble trying to determine. Is this covered by the wind torsion cases in figure 6-9?

Problem is, I was thinking of designing the frames per 6.5.13 of ASCE 7-05 as an "open building with a monoslope roof" for wind parallel to the slope and then using 6.5.15 for "wind loads on other structures" for wind perpendicular to the slope.
Is this permitted?

 

[ToadJones]

Other question is, do you treat the wind pressures acting on the roof as only acting normal to the surface as you would with an enclosed building or do you actually put windward and leeward pressures on the roof on the projected area?

 

[ToadJones]

RF-
I am using 7-05, so I cant reference the figures from 7-10.

 

[hokie66]

Pressures always act normal to the surface.