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?

 

[SAIL3]

At first glance, I would use a value of GCp=-2.0(ASCE notation) and then using engineering judgement and analysis work my way up or down from that value...I personally would stick to that value.

As for the possible venturi effects created by the stair towers....if the clear distance between the towers is less than 3 times the width of the towers then I would certainly look into it....from your description, it is difficult to determine how far apart they are, but, would hazard a guess that you are in the range of 10 times(or more) the width of the towers apart and in that case I would not worry about it.
If I remember correctly, that 3 times the clear distance apart came from cylindrical shapes....in your case with a rectangular shape of the towers, I would use 5 or 6.

 

[ajk1]

Sail3 - attached is a schematic sketch - plan and section - of the balconies. My concern is the Venturi effect for wind in the east-west direction. I don't think the stair "spacing" affects that direction much ... I think the enclosed stair shafts create a speed-up of the wind in the east-west direction parallel to the building wall.

 

[hokie66]

As the wind goes through those "tunnel" areas, I would think it tends to transition more to positive pressure, thus decreasing the suction. Agree with others that the wall suction and ceiling suction at the junction are the same, without discontinuities such as you have.

 

[pmblair]

ajk1....The tables are in ASCE7. I thought someone mentioned 120 mph storm,but regardless here in Florida i would use ASCE 7.

Good question on the pressure being either single action or a component of top and bottom. From my best memory of going over it before, it is a component of the suction on the upper part and the wind rushing up the wall.

In that regards i would think it would be conservative to use the full Gcp from the overhang tables.

Secondly in terms of the vortex, venturi,... or whatever is already taken into consideration in the tables.

The GCp values are based on tributary areas, because in wind tunnel testing and real world testing there are hot spots that show up. You know those multi colored results you see ranging from blue to red. Well in real world and testing environments during a storm there are small areas of very high pressures. That is why the wind tables in ASCE have lower GCp values for larger tributary areas. For smaller areas you may experience a hot spot of high pressure. With a member with larger tributary area the different pressure would average out to lower values.

 

[SAIL3]

I see your problem now...you are certainly asking the right questions.
Here is how I would approach the problem.
To get an upper bound increase in wind velocity, assume an effective venturi.
A1V1=A2V2
V2=A1V1/A2..say V1=90mph,width of tower=25ft(scaled from sketch)
=(25/2+15)x90/15=165mph
So how reasonable is this?
An effective venturi has a certain profile in order to eliminate turbulance, separation and minimize the buildup of boundary layers at design air flow velocity.
Your setup certainly does not meet those parameters.
So as the wind velocity increases, I would expect an increase in turbulance, flow separation and the buildup of the boundary layers leading to a choking affect which limits the the magnitude of the air velocity thru the tunnel.
To get a handle on the velocity at which this begins to occur is the big unknown, hence the need for wind tunnel testing to get an accurate answer.
With wind tunnel testing off the table, I would tend to go with the 165mph...others on the forum may have a better handle on this so please weigh-in on the subject.
If feasible, I would rather go with the following design modifications:
A/..extend the sidewalls of the stairtowers in the south dir
to the bldg wall itself and provide doors for egress E-W
onto the balcony...eliminating the venturi effects completely.
B/..If still in design phase..make the stairtowers an open
structure.

A few comments on the design layout.
A 15ft balcony seems excessive to me?
Also, one would think that internal stairwells would lead to a
more integrated design versus the external towers that look like an afterthought.

 

[ajk1]

To SAIL3: Your comments are excellent! But a bit more of the background may help your understanding - this building comprises a number of phases starting in 1956 and large editions to the east and west in 1961 and 1969 respectively. It is a racetrack and casino (the balconies are on the non-track side of the structure) so this public building use is likely why the balconies were originally so wide even though they do not overlook the track. The stair towers were constructed about 1999. That is about when the linear aluminum ceilings were installed in the balconies. Periodically pieces of the ceiling dislodge and fall to the ground. We are about to install some temporary restraining battens but are now starting to look for a more permananent solution but one which is cost effective, as the areas are realtively large and our calcualtions indicate that the various components of the ceiling are generally good for only relatively small wind suctions < 15 psf). Toronto reference wind is probably about 85 mph, but I am not sure because the Canadian Code (National Building Code of Canada) is set up in terms of reference pressures, not wind speed and also we don't use the 3 second wind as is used I belive in American Codes. Our reference wind pressure is 0.53 kPa (11.0 psf) which is based on a 1 in 50 return period. (I believe this is mean hourly wind but would have to check that).

Our mapping of the dislodged 8 inch wide panels indicates that there is a higher incidence of them within several feet of the stair towers (but beyond the stair tower itself), although these are certainly not the only locations where panels have dislodged.

Because this is a repair, we want to come up with a cost effective solution and strengthen only those suspending members that require it. So although adopting a very conservative approach may be fine for new construction, it is not a particualrly satisfying way to approach this particular repair problem.

If the wind is 165 mph as you suggest might be the case, this would give about 4 times the suction that the 85 mph Toronto wind would give and the entire ceiling system would have been ripped out long ago even at much lesser wind velocity.

However you suggestion of extending the sidewalls of the stair shafts southward and putting doors in the extended walls is precisely what I had been thinking and your suggestion gives me greater confidence to suggest this to the architect, though there may be architectural snags like they may have to temper the air in summer so the enclosed space does not become too hot, fire rating issues etc.

This will still leave the remainder of the ceiling to be strengthened, but based on the Vickery paper and other suggestions on this forum to design for suction equal the the wall suction or the suction on the roof edge, I come up with about 30 psf design suction unfactored wind load (or times 1.4 to get the factored load to use in Ultimate Limits States Design).

 

[SAIL3]

ajk1..nothing is as simple as it first appears.
Extending the sidewalls of the tower will add additional area both to the main bldg and the stairtowers for wind E-W.
If one does not consider shielding from tower to tower, this could add up to a significant additional wind load.
May have to check existing structures for this additional load.
May have to determine if this qualifies as a significant change to an existing structure, therby, requiring the whole structure to be analyzed per latest codes.
In the US this check has really become more strict in recent years..if the proposed change to an existing structure causes an increase in load of more than 5%, then, it requires the application of current codes.

 

[ajk1]

To Sail3 - good thought but may not be a problem in this case for a couple of reasons:
a) the stair towers are separated from the balconies by expansion joints and the stair towers have their own self-contained independent bracing;
b) the original building design was done in 1955 to Allowable Strength Design Procedures but current steel design in Canada is by Ultimate Limit States Design and has been since 1977, so this should gain us something; on the other hand in 1955 they allowed a 33% increase in allowable stress for wind load - this is no longer allowed.

I know of no Canadian Code limit of 5%, but we usually allow 7% in our office, and use some engineering judgement as well. I suspect it is ok but we will have a look at how it works out. Thanks for raising the issue.