Covered Walkway

[slickdeals]

Folks,
I have a covered walkway (15' wide) with wood rafters (4.5 in 12 slope) extending beyond the edge of a 3 story building.

I was thinking of using the Figures 6-19A (Monoslope free roof) and Figure 6-18A for calculating wind pressures.

Is there any guidance in ASCE 7-05 regarding designing entry canopies or covered walkways for uplift/downforce design?

 

[msquared48]

Looks good to me. Just tie all the forces through to the foundation...

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

 

[slickdeals]

Mike,
Would you be using a C&C pressure for the design of the front columns/beams that support the rafter or would you use MWFRS?

The column has a triburary area of 90 SF.

 

[a2mfk]

There's another thread just like this one, not sure if your situation is attached to the building? My two cents were, for a canopy attached to a building on one side and columns on the other...

Overhang or not an overhang, its a discussion I've heard throughout the years, especially in Florida. I was hoping ASCE/FBC would have cleared this up by now...

Why is this not the same as an overhang? Because of the difference in height of the two roofs? Its mounted to the building on one side, and on the other side it is supported by posts. If it was cantilevered out from the building would it be an overhang?

I have always treated them (and I'm not alone), perhaps conservatively, as overhangs. Think of how the wind hits the canopy, it goes over top of the roof and underneath. The wind underneath hits the wall and then is forced up to cause upward pressure on the ceiling/underside of the roof (same pressure as the wall according to FBC). So you have surface loading on both sides of the canopy roof, just like an overhang. I know a 10-15 ft wide canopy adjacent to a building is not EXACTLY the same is a 3ft overhang, but its somewhere in between probably?

Perhaps the step in the two roofs reduces the pressure on the canopy upper surface, but I have not seen that cited/explained in a reliable source.

Modeling it as a free-standing open canopy (like a gas station or park structure) would result in very low pressures compared to an overhang, which I am not convinced is correct. I would like to know if there is a source out there treating this specific situation...

 

[slickdeals]

Andrew:
I responded to your post in the other thread. I apologize.
thread507-284667

 

[slickdeals]

IMHO it should be treated as an open building with obstructed wind flow (Figures 6-19A and 6-18A)

 

[msquared48]

MWFRS.

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

 

[slickdeals]

Mike,
In Figure 6-18A, for a roof angle of 15 degrees for obstructed wind flow (wind direction 180 degrees)
CASE A
Cnw = 0.4
Cnl = -1.1

CASE B
Cnw = 1.2
Cnl = -0.3

In Figure 6-19A, for similar situation with effective wind area > 4a2
Zone 3,2,1
Cn = +1.2/-2.1

Based on MWFRS calcs, there is never an uplift on the windward edge. However, there is uplift (a pretty significant one) based on C&C loading.

Does that make sense?

 

[csd72]

I would design it as the worst case of a cantilevered canopy or a free roof.

It is the blocking of the wind that makes all the difference, one has pressure purely from the altered flow of the air above and below it the other has pressure from the wind hitting the wall of the building and having nowhere else to go but upward or sideways.