Water Tank Balcony Guard

[NJTelecom]

thread507-326187

I am assessing a water tank balcony guardrail for added load from proposed wireless antenna and other related equipment. As referenced in the thread above, I also beleive the balcony guardrail is a truss spanning between water tower legs These are the only points where the balcony is supported. I am looking for any reference material that can confirm or dispute this assumption. AWWA D100-11 addresses the applied design loads to the top rail, but not the support of the balcony and rails. Any help is appreciated.

 

[RFreund]

Do you have a photo? What type of water tower is it? The catwalk is not cantilevered from the water tower itself correct?

(out of curiosity) how big are the antennas? Usually they are pretty light and there are a couple different ways to justify this situation.

EIT

http://www.HowToEngineer.com

 

[NJTelecom]

(Hopefully) Attached is a photo of the tank from below. I am awaiting tank diameter (approximately 25'), foundation base cl of leg diameter (approximately 42'), and distant photos showing the guardrails. The tank is an elevated, 4-leg water storage tank with a center standpipe. Height is approximately 125' total. Balcony (catwalk) is at approximately 109'. The floor plate is welded to the tanks and toe kick, the only vertical supports appear to be the gussets at the 4 legs (see photo). The total equipment weight is 600lbs then add icing load per industry standards. Design requirement 200 lb concentrated load or 50 plf horiz and vert top rail, or 1000 lbs in 10 square feet on the balcony floor. Must consider cumulative loads of others on guardrail before us. I have dimensions for the catwalk and railing.

 

[RFreund]

Usually the vertical load is not a large concern as the weight is relatively small and spread out over a couple antennas. Usually I see the reaction of the pipe mount on the handrail compared to the 200lb or 50lb/ft requirement. However you make a good point that this may be a truss spanning between supports. I have seen a full analysis carried out for some of these situations (and that does kill the flat fee). Let me look into this and try to get back with you.

EIT

http://www.HowToEngineer.com

 

[msquared48]

Can't tell from the photo, but frequently these water tower cat walks also serve as structural tension rings for the tank. It would be best to get the tower drawings.

I assume the antennas are two to four in each of in three sectors distributed around the tower, weighing 50# or less each including the mounts to the rail. Usually this is not much of a problem.

Mike McCann
MMC Engineering

 

[NJTelecom]

You are correct that this didn't used to be much of an issue. However, the equipment is getting larger and they are now accompanied with multiple other appurtenances. The new loads exceed those that were once a comparison of design loads to actual loads. We now have 8' antennas weighing over 100 lbs, with the added RRHs and other appurtenances, and consideration of additional carriers more analysis or local support needs to be considered.

 

[JStephen]

The balcony is a structural element of the tank, not exactly a tension or compression ring, though. If you treat the tank shell as a circular beam, the balcony is the lower flange of that beam section. It also resists the inward thrust of the legs at that point. Check Gaylord & Gaylord's Structural Engineering Handbook for some discussion in the leg-supported silo section.

Have you tried contacting the manufacturer for whatever information or guidance they have?

Have you tried Google Streetview for pictures of the tank?

Can you add the antenna at a leg location?

FYI, water towers are usually fairly conservatively designed (more so than buildings), but a lot of times, when stuff like this gets added on, nobody checks it, either, they just stick it on there. It increases the weight and the wind load.

 

[JedClampett]

I went to a seminar a long time ago, where a lot of the speakers were attached to Tank industry Consultants. In looking at their website

http://www.tankindustry.com/services

, they provide services exactly like you're asking about. Maybe if you subcontract them, they can come up with a programmatic approach to cover these and other cases.
I know for a fact that telecom companies pay big money for these spaces, so they should make sure the engineering approach is sound.

 

[msquared48]

They should Jed, but the budgets are real thin sometimes. Working on that problem though, real hard.

Mike McCann
MMC Engineering