Open building definition

[bvbuf]

I am designing a freestanding canopy for a gas station in a hurricane region. It has 10 interior columns and a 50'x160' roof. It does not have walls. When looking at wind load, I'm wondering if this is an "open building" (ASCE 7 6.5.13) or "other structures" (ASCE 7 6.5.15). There is no requirement for uplift in "other structures".

If I must design as an "open building", will I need to design the columns for the downward acting pressure shown in figure 6-18A (ASCE 7)in the combination D+0.75Lr+0.75W?

 

[hokie66]

I am not familiar with the ASCE standard, but can guarantee that there will be both uplift and downward loading conditions on a freestanding canopy.

 

[msquared48]

Those things act like a wing and will DEFINITELY have uplift. If not due to the wind, certainly due to the rising gas prices.

Seriously, as Hokie says, there is uplift as with any canopy structure.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[strucguy]

You need to consider both uplift (0.6D+/-1.0W) and downward thrust for open structures. See page 66 of ASCE 7-05 for a better idea.

 

[GalileoG]

Canopies will attract uplift like an umbrella on a windy day.

I have done many canopies, but per NBCC 2005, which does not address wind load for canopy-type structures. I have used an internal pressure coefficient of +1/-1 (I have heard of others multiplying the external pressure by 2, should almost give the same magnitude of pressure.) I determine the wind load on the fascia as a "sign structure." Of course, this is per NBCC 2005, which does not really help you.

I'd exercise extreme caution if you have a single line of columns as opposed to columns that are doubled up along the length of the canopy. Also, most canopy failures occur at the base plates.

I've attached a journal on canopy failures.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

Clansman,

Is it customary to provide a deck only on the underside of framing members as shown in the pictures in your link? I don't think that is a good idea at all.

BA

 

[Ron]

Yes, it is an open structure as defined in ASCE 7. It is subject to uplift, and in your area, a bunch of uplift.

Because of the depth of the "skirting" that hides the roof structure, it also picks up a significant lateral load, so the columns usually have a high moment at the footing.

BAretired...yes, it is customary on gas station canopies to leave the top open, have a reflective underside on the flimsy decking, and hide the structure from bottom and side view.

 

[msquared48]

Agree with Ron.

Done a few of these in the past. Bad design in high snow load areas though.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

I guess that accounts for why I have never designed any of these.

BA

 

[strucguy]

Ron -

Any reasons for providing deck only on the underside??
Thanks.

 

[BAretired]

strucguy,

The reason is to save money. It is a bad idea in my opinion. It could get filled up with leaves, bird poop and all kinds of other things, not to mention snow in the winter. It is simply an unacceptable solution. The top should be enclosed with deck.

BA

 

[Ron]

Yes, the reasons BAretired gave are correct. In addition, all drainage is typically through the columns or downspouts attached to the columns...in either case, it produces more corrosion of the columns where they are embedded in concrete (and they almost always are embedded).

 

[bvbuf]

Thanks everybody. I appreciate all the help. I do have a follow up question.

The wind loads normal to the roof of an open structure include upward and downward forces. When looking at D+.75Lr+.75W for beam and column design, do you include the downward force of the wind with the dead and roof live loads? This can significantly affect the sizes of the members.

 

[BAretired]

You always include wind effect if it adds to the load. You never include wind effect if it reduces the load.

BA

 

[JLNJ]

Be careful and conservative with these structures. They are flexible, yet the wind loading is sensitive to deflection.

When the weather channel shows failures the first thing to go seems to be the gas station capopies. The second are the pre-engineered buildings.

 

[GalileoG]

BAretired, single skin canopies are very popular. I share similar concerns. I've seen some canopies where the 'deck' overhangs past the side steel beam (that runs parallel to canopy length) by almost 1.2m and then you would have the fascia framing bolted to it as opposed to the steel beam. Now THAT is more cause for concern.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.