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Vortex Shedding of Balcony Railings

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?

RE: Vortex Shedding of Balcony Railings

dcStrucEng....very doubtful that vortex shedding would cause such failures.  While on occasion you will hear sounds emitted from railings as a result of wind, such vibrations are short term and not of a consistent enough nature to cause failure, even in the small picket welds.

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

The lowest natural frequency of a properly supported handrail/guardrail is much too high for vortex shedding to be your problem.

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

RE: Vortex Shedding of Balcony Railings

Considering the salt environ, salt spray, humidity and everything, I would look to either chemical action between the concrete and aluminum, or galvanic action between the aluminum and steel beams.

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

Although it mainly pertains to steel rebar in concrete, alujminum is mentioned here too.  Good article...

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

(OP)
Thanks for all the opinions so far, on a Saturday no less!

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

I believe that Ron mentioned a special coating that could be used to embed the aluminum in the string I quoted.

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

RE: Vortex Shedding of Balcony Railings

Sorry about that misplaced prepositional phrase - definite  occupational hazard leading to a possible misinterpretation.

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

RE: Vortex Shedding of Balcony Railings

Attaching the pickets with screws would be cost prohibitive.  Welding is fine...just make sure they are good welds, not just a poor tack weld.

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

The railing in areas with most thermal movement could fail first, but the welding is the problem.

RE: Vortex Shedding of Balcony Railings

Sounds like a combination of problems contributing to a failure at the corner columns to me.

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

RE: Vortex Shedding of Balcony Railings

If you have the option to twist the pickets like old fashioned metal fences, the twist shape will serve to dampen vibes like modern car antennas and sub periscope shafts.

RE: Vortex Shedding of Balcony Railings

(OP)
Twisting the pickets isn't going to be possible, but good suggestion for future projects.  The next step of this "hypothetical scenario" is to come up with a fix (assuming a complete removal and replacement isn't in the cards).  I don't want to add more weld to such thin aluminum members because I think over-welding might have caused some of the problems, too.  I'm considering suggesting some type of weather-proof, epoxy adhesive to fill the small cracks between the pickets and top and bottom rails, like a Sika product.  Any opinions on that remediation?

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