2 Column Canopy Roof Question

[VAStrEngr]

I am in the preliminary stages of designing a 2 column cantilevered canopy (similar to what you see in small gas stations) and what is discussed here:

http://www.eng-tips.com/viewthread.cfm?qid=232846.

I noticed looking at pictures of similar structures and by using satellite images from a few around my area, most appear to be designed with the framing exposed and a roof deck fastened to the bottom side of the framing. Closer inspection from the ground shows downspouts running down through the column.

Two questions:

One, is this typical to leave the steel framing exposed without any decking or roof membrane above it? If so, why? I would expect that you would want to fasten the decking/membrane material above the framing as with other roof structures.

Two, some articles I have read suggest that running downspouts through the column allows the possibility for water to collect at the base of the column and corrode the member. This makes sense to me. Are the examples I am seeing simply outdated and are newer structures typically built with downspouts fastened to the outside of the column?

Thanks!

 

[paddingtongreen]

I have designed toll plazas for several states' toll roads but we were not required to go cheap and nasty, so that experience may not help. We did run the drains down the columns, I find the corroding steel framing argument unconvincing, a driving rain will put water there more often than a broken downspout.

I don't know where the ones you looked at are, but if snow load controls, the deck may be figured to provide lateral stability for the compression flange.

I used cellular deck on the top flange, that would span from support to support. It was covered with shaped fill and an elastomeric membrane roofing. The shape of my supports was a modified "T" with a slight slope and toward the center and crickets to aim the water at the drains.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[MiketheEngineer]

In colder climates, I might worry about getting ice in the bottom of the internal downspout and therefore inside the column. Not usually a good thing as I have seen such structures literally shatter from the ice pressure.

 

[msquared48]

In answer to your first question, yes, it is normal.

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

 

[VAStrEngr]

Thanks for all of the info...

Any particular reason why you leave the framing exposed?

 

[RacingAZ]

It's the cheapest way to do it. The deck attaches to the bottom of the framing, which doubles as the finished ceiling.

Not that I agree with it. It's a huge snow collector in my opinion. Putting a standard roof is not the norm unless the owner or architect is aiming for that look.

 

[msquared48]

There are a lot of structures with exposed framing such as towers, signboards, etc. You just have to design for it.

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

 

[OHIOMatt]

This is antecdoatal, but I have done quite a bit of work in the aftermath of hurricanes. It seemed that there were an awful lot of gas station canopies that just tipped over. The ones that I looked at showed signs of severe corrosion on the inside of the column. The columns would fracture, not yield at the concrete slab line.

Do a quick bing images search for " gas station canopy collapse". As stated before, this is also antecdotal, but my preference is to design the downspout outside of the column if at all possible. The concern that I always have is that if the owner is pushing for the absolute cheapest solution, there approach to maintenance such as painting, downspout repairs, etc. will follow the same path.

 

[VAStrEngr]

I really appreciate all the advice.

Despite being the cheap alternative, I believe I will most likely propose to do some sort of decking above the framing. I need to check some estimates, but I can't see it adding a whole lot to the project for the size of canopy I am working on.

In consideration for the maintenance at this facility as it is, I am very wary of putting the downspouts through the column. It seems like an unnecessary risk since the owner will probably not care about seeing downpouts.

Thanks again for all of the help.

 

[hokie66]

Good for you, VAStrEngr. Just because it's good enough for others doesn't mean that is the only way to do it. I agree with you that downspouts should not be hidden in columns.

 

[paddingtongreen]

I said, above, that I had designed toll plazas. When I did my first, I asked about the heavy concrete protective bumps on the "Inbound" side. I was assured that they do need them.
Here is proof:

http://www.youtube.com/watch?v=voTYHtezv4I

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SWComposites]

Perhaps the decking is put on the bottom side to eliminate locations for birds to roost and poop on the customers?

 

[dhengr]

Paddington:
Sure you asked about conc. protective bumps, but I’ll bet you never designed the catching system on the other end. If the design shown is similar to your design, were you assuming they could charge a higher toll if the car went through the gate airborne?

 

[paddingtongreen]

dhengr, the standard we used had vertical plow fronts, not ramps. If anyone hit them at the same speed as that woman, they would be very lucky to get out alive. If they went airborne, they would be doing backflips.

Michael.
Timing has a lot to do with the outcome of a rain dance.