Design of equipment support towers in Canada

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I am designing towers that support equipment catwalks in Canada. I believe that CSA S37 is the proper design document versus NBC 2015. This has become a major controversy within my company which is Canadian owned and I am located in the U.S. I am a licensed P.Eng. as well as a P.E. The words "Antennas" and "Antenna supporting structures" has them believing that it does not apply. I pointed out that NBC commentary J specifically states that 4.1.8 should not be used be used for seismic design of towers. If not NBC then what is the proper design methodology? I also believe that S37 applies for wind noting the similarities to ASCE 29.5. Feedback on this is much appreciated as my job may depend on it.

 

[skeletron]

It sounds like you are in a bit of a grey area here. The provincial codes govern over the NBC and this should be clarified in the client's specification. It sounds like your project is in an industrial area, which leads me to believe that the specifications will be pretty clear in terms of telling you which code applies.

If you are dealing with lattice structures, CSA S37 will provide some rational guidance on determining the wind loads. CSA S16 will still govern the member design. The seismic design procedure would be dictated by your provincial code because "catwalks" translates to "worker safety" in my brain. CSA S37 specifically deals with communication structures. These typically don't have a "human" load component to them, whereas "equipment catwalks" suggests more frequent servicing. But maybe you could explain more on the specific structure.

 

[modad]

Thank you for your rapid response. The provincial and municipal code only make reference to NBC 2015 for seismic. The customer, of course, makes reference to NBC 2015. The circle continues.
The catwalks, or trusses, carry equipment and personnel access. I decided to use S37 for wind loads because it is very similar to ASCE 29.5 which clearly spells out lattice, or trussed towers. I do not find such a clear reference ins NBC 2015. Having said that, after doing some hand calculations on dynamically sensitive structures, the total wind load is essentially the same as S37.

 

[TLHS]

If it doesn't have an antenna or something similar on it, I would agree that CSA S37 doesn't specifically govern.

However, it also sounds like you're outside the exact written scope of the provincial and national building codes as well. However, I'd say that if the main intent is access and occupancy then it's one of the closest fits unless you can make one of the scaffolding standards fit.

This is generally where, barring specific direction from an authority having jurisdiction, you take all the vaguely relevant codes and try to meet the standard of care that you think they imply. For reasonably frequent personnel access, I think that standard probably is the provincial building code. Specifically for wind load, I would suspect that it depends on elevation and aspect ratio of your towers. There is some guidance for open steel structures in the structural commentaries of the NBCC, but it starts to look like a bit of a hack job if you have to combine dynamically sensitive wind procedure along with the open steel structures stuff. In that case, an alternate solution from S37 might make sense and I would say that a sanity check is never wrong. I'd be watching that the wind design pressures are comparable between the codes though. I haven't used S37, so I'm not sure if it ends up using the same amplified gust procedure as the NBCC and derivitive codes are based on.

It's also completely reasonable to use non-Canadian industry codes when design falls outside of available Canadian references. However, you should try to make sure that you're meeting the broad intents for design level events in the building code or other Canadian standards.

Basically, you're doing something that doesn't fall entirely within any one standard, so review similar standards and use engineering judgement to develop a reasonable level of safety and make sure that you clearly document what you've done. This is well into 'use your engineering judgement' territory. I think we run into that situation a lot up here, especially in industrial situations, since the NBCC doesn't cover those things nearly as well as ASCE does in the states.

I would very much be checking the provincial occupational health and safety standards as well.

 

[modad]

Thank you very much for your response. I think you have done an excellent job of summarizing my dilemma. My highest priority is to ensure that the structure is safe, no matter which direction I go.

 

[Craig_H]

I very much agree with what has already been said above. I have designed industrial lattice tower structures for lifting operations in the past, as we predominantly used the Provincial/National Building Code, but substituted in CSA S37 for ice and wind loads. I think that is a totally rational way to approach the problem. Take care to include the effects of ice accumulation on your wind sail area, and if you're in an industrial setting, beware of additional ice loading caused by nearby chimneys.

 

[modad]

I included ice on all members as shown in S37 to add to the wind area. Thanks for the input.