Wind Suction on Balcony Ceiling 2

[ajk1]

I am investigating a linear aluminum ceiling that is suspended about 2 feet below the underside of the reinforced concrete structural slab of a long balcony. The balcony is about 500 feet long and about 15 feet wide. At the south side of the 15 feet is the building; the 500 foot long north side of the balcony, as well as the 15 foot east and west sides, are open from the ceiling to the top of the solid balustrade on the balcony below (the space between the ceiling and the slab above is closed by cladding). This condition repeates on several floors. The building is situated in an open field type terrain.

Question #1: How can I determine the suction (i.e. downward) force on the ceiling only. The overhang provisions of the National Building Code of Canada (and other Codes/Standards) can perhaps be used to determine total forces on the slab/ceilign assembly (although even this is questionable as to whether it applies to balconies) but I cannot to-date find anything that would allow determiantion of the suction on the underside only (in this case the ceiling. Any suggestions, other than a wind model test?

Question 32: If there is no procedure or data for these type ceilings, what is the standard practice with respect to the wind load that they are designed for?

 

[ron9876]

If I understand your question properly I use the stagnation negative wall pressure.

 

[ajk1]

Why would you use stagnation suction on a "wall" to determine the suction on a "ceiling"? One acts vertically, the other horizontally.

 

[WillisV]

If you pull a negative pressure on a closed box, doesn't the same suction act on both the "wall" and the "ceiling"...

 

[msquared48]

Where are questions #2 thru #31?

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

 

[ajk1]

Yes of course but neither the balcony nor the wall of a building is a closed box. There are edge effects etc. that signifcantly affect the negative pressure. If a closed box was all there was to it, the suction on a building roof would be the same as the suction on a building wall. The diagrams in the various codes and standards show that they are not the same. The suction on the top edge of a wall is very different than the suction of the edge of a roof. Are you saying that these edge effects do not apply to the balcony ceiling?

 

[ajk1]

Question #32 should say question #2. If I knew how to edit my original message, I would fix it, but I am new to the forum and don't yet see how to make that revision

 

[WillisV]

I am saying the zone of negative pressure for the wall is the same zone of negative pressure for the balcony. See attached for a paper concluding that the negative pressure on soffits correlate closely with wall negative pressures. I would feel comfortable applying the same logic to a balcony.

 

[ajk1]

To WillisV: This paper looks very interesting indeed. I have printed it out and will read it now. Thanks to all the respondents and especially to WillisV for pointing me to this paper. It is enormously appreciated. If I have any questions, I may be back. Thanks again.

 

[msquared48]

ajk1:

I was playing with you. Get used to it. No worries...

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

 

[ajk1]

I have read the paper to which WillisV sent me the link earlier this afternoon and find it very useful.

But now for step 2: There are several large enclosed stair shafts extending northward from the north exterior edge of the balcony, at locations along the 500 foot length of the balcony, each stair shaft being about 30 feet square. This creates a sort of tunnel effect for the wind, between the north wall of the main building and the south wall of the stair shaft. I expect that the wind passing through these tunnel-like regions (walls at north and south sides, but open at east and west sides) will speed up and create greater suction on the ceiling. Any suggestion of how to account for that in evaluating the suction on the ceiling?

 

[pmblair]

What am i missing here?

Why wouldn't you use the Component and Cladding overhang tables?

 

[Ron]

When the wind is blowing parallel to the length and no pressurizaton of the open space occurs, then the wind doesn't care whether it is blowing over or under the subject surface...it still creates an "uplift" or when inverted, a "downdrag". This assumes there is no air gap continuity between the top of the roof and the underside of the canopy.

When the wind is blowing perpendicular to the length and against a vertical wall, the canopy ceiling is "pressurized" with uplift that can be considered like an overhang condition.

 

[SAIL3]

It all depends on the local effective wind velocity vector and any flow separation that may occur locally.
In flat open terrain with no obstructions one can assume the wind velocity vector is parallel to grade. Once you place an obstuction in it's path, then all bets are off. Depending on the shape of the stucture, it's local shape..so-called dead pockets or protrusions, then locally at these areas the wind velocity vector could possibly be in one of many directions with accompaning flow separation.
In addition to wind parallel and perpendicular, how about wind from 0 to 20deg off-parallel and what type of air flow would one get at the balcony overhang?.
Short of wind tunnel data, I would personally go with the most conservative wind pressure values.
Even though values for "component & cladding" are for local areas, I would consider applying it to this entire balcony mostly because of the difficulty of visualizing all the possible local wind velocity variations and resulting flow separation.

 

[pmblair]

The wind will change direction during a 120 mph storm.

And I would think(dont hold me to this), the wind perpendicular to the overhang with the wind rushing up the wall would be the governing case.

I would use the tributary area of the member your designing to get the GCp from the overhang table,and use the component and cladding design pressure.

 

[ajk1]

Thank you all for the responses. No one seems to want to tackle step 2 of my question, namely what happens where the venturi effect is created by the stair towers which are located at the exterior edge of the balcony at intervals and create a tunnel effect for the wind parallel to the bulding wall. Note that there are 3 floors that each have these 500 foot long balconies. Maybe my description of this was not easy to follow so I will see if I can make a sketch that I can attach tomorrow.

 

[Ron]

ajk1..that condition is not covered by the code. You have two choices...engineering judgment or wind tunnel. Chances are, if you use the ASCE guidelines, you'll be more conservative than wind tunnel testing.

 

[ajk1]

Thank you Ron. Wind tunnel testing was the conclusion that I had come to before I posed the question to this forum. At least with wind tunnel testing I will be able to sleep nights. But if the client will not pay for the wind tunnel tests, then I will have to come up with another approach and the comments on this forum are helpful in that regard. The paper by Vickery that WillisV put me onto was interesting in that it concluded (based on wind tunnel testing at the boundary layer wind tunnel at the University of Western Ontario) that the suction on the ceiling can be taken (within a few percent) as the same as the suction on the building wall, but that was for an eave with an overhang of only about 10% of my balcony width. It seems to me that there must be some width at which the suction on the ceiling becomes similar to the suction on the roof surface - the ceiling would be like the roof flipped over. The suction at the edge and corners of a roof surface is relatively high.

 

[ajk1]

To pmblair - this is toronto and we don't have 120 mph design storms

 

[ajk1]

to pmblair - to which Standrad are you referring when you say "Component and Cladding Overhang Tables"? The User's Guide to the National Building Code of Canada does have some data on overhangs but it is for the SUM of the top surface and soffit surface; since I am checking the ceiling only I am interetsed in the wind on the soffit ONLY. Does "Component and Cladding Overhang Tables" give the wind on the soffit ONLY, and if so, which Standard is it in?