Vortex Shedding of Balcony Railings
Vortex Shedding of Balcony Railings
(OP)
Allow me to propose a hypothetical situation:
Suppose there was a 20-story reinforced concrete oceanfront apartment building, of typically rectangular shape, assume 2:1. Each unit has a balcony and each balcony is enclosed with an aluminum railing system. The aluminum railing system is very typical: 48" high vertical posts embedded into the balcony slab every 4' to 5', with top and bottom horizontal rails and vertical intermediate picket rails every 4" between the top and bottom rails. Aside from the main vertical posts embedded into the slab, the majority of the connections are via clips and tek screws. Except for the picket rails, which are welded to the top and bottom rails. (This is a very detailed hypothetical situation, I know!)
Here's my question: Is it possible for vortex shedding to cause the intermediate picket rail welded connections to fail at some of the corner balcony units?
I don't think so and here is why: even if a consistent wind speed was such that the vortex shedding frequency equaled the natural frequency of the intermediate picket rails, a properly installed welded connection should be greater than the force created by vortex shedding. (Although I'm not sure how to calculate the force on the connections created by the vortex shedding.) I believe that some of the aluminum welds were the issue, due to the higher complexity involved with welding aluminum versus steel.
Any opinions/thoughts?
Suppose there was a 20-story reinforced concrete oceanfront apartment building, of typically rectangular shape, assume 2:1. Each unit has a balcony and each balcony is enclosed with an aluminum railing system. The aluminum railing system is very typical: 48" high vertical posts embedded into the balcony slab every 4' to 5', with top and bottom horizontal rails and vertical intermediate picket rails every 4" between the top and bottom rails. Aside from the main vertical posts embedded into the slab, the majority of the connections are via clips and tek screws. Except for the picket rails, which are welded to the top and bottom rails. (This is a very detailed hypothetical situation, I know!)
Here's my question: Is it possible for vortex shedding to cause the intermediate picket rail welded connections to fail at some of the corner balcony units?
I don't think so and here is why: even if a consistent wind speed was such that the vortex shedding frequency equaled the natural frequency of the intermediate picket rails, a properly installed welded connection should be greater than the force created by vortex shedding. (Although I'm not sure how to calculate the force on the connections created by the vortex shedding.) I believe that some of the aluminum welds were the issue, due to the higher complexity involved with welding aluminum versus steel.
Any opinions/thoughts?






RE: Vortex Shedding of Balcony Railings
It is very likely that poor welds were the culprit, coupled with thermal movement.
Many manufacturers will punch the lower rail channel or tube, and insert square pickets through the channel or tube and weld on the bottom side where it is not seen. The weld is usually just a small tack weld. When the picket expands linearly from thermal expansion, it puts the tack weld in shear. This is at least a twice daily repetitive load, so fatigue will happen as some point.
Check the shadow lines on the building. You might find a correlation to the failures.
RE: Vortex Shedding of Balcony Railings
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Vortex Shedding of Balcony Railings
More importantly though, it is a big nono to ever embed aluminum into concrete due to chemical actions between the two materials leading to the denegration of both materials.
In my structural notes, I always have a clause inserted that aluminum conduit is not to be embedded in any concrete slabs flor exactly that reason.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
http://www.cement.org/tech/cct_dur_corrosion.asp
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
OK. Enough of beating a dead horse here...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
So do you think it is better to attach the vertical picket rails to the upper and lower horizontal rails with tek screws and/or clips in lieu of weld? I'm not sure how that would've helped with the thermal expansion problem. Although a screw connection is more giving than a welded connection.
Do you think there is any correlation between the failure of the aluminum welds with the location of the balconies being at the corners of the building?
Regarding the issue of embedding the aluminum posts into the concrete balcony slab, would it make a difference if the post was completely surrounded by a layer of non-shrink, non-metallic grout?
RE: Vortex Shedding of Balcony Railings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
As for embedding aluminum in concrete...it makes no difference if the grout is non-shrink or non-metallic...if it contains portland cement, it will react with aluminum. You MUST isolate the aluminum, usually with a dip coating of clear lacquer or epoxy.
The failure correlation at the corners of the building could be related to thermal or other movement associated with the corner conditions. As I mentioned before, check the shading lines of the building. If the majority of the railing is shaded, and the corner is in the sun, the thermal movement alone can cause issues.
RE: Vortex Shedding of Balcony Railings
RE: Vortex Shedding of Balcony Railings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Vortex Shedding of Balcony Railings
RE: Vortex Shedding of Balcony Railings