telecommunication towers on top of a Roof 1

[Saedhalteh]

If we want to design a telecommunication tower on the top of an old building's Roof what will be the procedure or main steps for that ?

how can we verify that the slab of the roof can stand or support the tower sufficiently (because when the building was designed there was no consideration for towers to be modeled on top of the building ) and do we consider the tower as a dead load or a live load on top of the roof ?

I am confused about the workflow and would really appreciate if someone can guide me through it

 

[Tomfh]

You need to make sure the building is strong enough to support the weight of the tower and any loads on it - typically wind loads.

I knew someone whose office did this work and they said it was fairly cavalier at times. A lot of “engineering judgment”.

 

[Blackstar123]

As far as design check of existing building is concerned, this is not a really complicated matter. You just need to make sure that in all structural elements the required capacity is less than the provided capacity for additional loads from the tower.
What you need to be careful about is how to model the existing condition of the building such as material models, stiffness of individual structural components and building as a whole.
In my opinion if tower load is not specifically given as dead and live load, then you could treat it as dead load.

Euphoria is when you learn something new.

 

[Klitor]

Often spreader beams are designed to 'smear' load into larger roof area.
Also drilling anchors into roof is frowned upon (rain water intrusion)

 

[BridgeSmith]

The selfweight of the tower would normally be considered dead load. However, typically the selfweight is small issue compared to the lateral wind load, which is separate from live load in the codes I'm familiar with. Usually, the only true live load would be due to a maintenance worker who may need to climb the tower.

The first thing I would look at is whether this will a guyed tower or a freestanding cantilever. For a tower of any significant size, a guyed tower would typically be the preferred type for most roofs, since the moment capacity for a freestanding tower isn't usually available without a huge base for the tower.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[azcats]

Do you have the TIA-222 code? It's the governing code for telecom structures and is helpful for wind load development.

The scale here matters. Is this an actual tower that needs to get higher above the roof? Or is the roof high enough so that they only need mounting points for antennas & equipment? If the latter, there are pre-manufactured products that may work.

Where is the support equipment located? The battery cabinets can be really heavy and usually need an engineered platform unless you're given space inside the building.

 

[skeletron]

A tower, a ballast mount, or an attachment? They are all fairly the same.
1) Design the component for the vertical and lateral load at height. Treat the component as independent of the building first.
2) Take the resultant forces and apply them to the building. Determine if the building can handle those loads. In order to proceed with Step 2, you need building drawings and a field review of the conditions. In my experience, the building's construction is usually the most difficult part of the task, as most building owners/management companies don't have or don't want to give up the building drawings for inspection and you are limited in budget, time, effort, etc. in surveying the construction.
3) I treat the tower+equipment as dead load. Check the structure supporting the tower for vertical, lateral, and overturning. Design the attachments to the structure. Attachments have some difficulties because the unknowns of the structure start to emerge. Most building owners don't want to see through-bolted plate systems. Most contractors just want to drop in expansion anchors or adhesives. You need to use your engineering judgement to decide what strategy to use.
4) In my opinion, you need to consider deflections of the tower so that your antennas remain in line. I rarely considered the deflection at the top of the structure as additive in a mathematical sense; that is, I would rarely if ever consider the building's top deflection and the tower's deflection all at once. You end up having to beef up the telecom structure which defeats the purpose of having low-weight construction thirty floors up. And, deflection for antenna service is something that the RF designers should be specifying but rarely do. Look at the rotation more than the absolute deflection.
5) Consider if you are in a seismic zone as well. In my area, the jurisdiction passed a new-ish building code that required the positive seismic restraint of all non-structural components. It really changed how telecom structures were treated around here because they couldn't just put a light weight mounts loaded with antennas on the roof. These components now need to tie into the structure to have seismic restraint. It starts to blow up with building envelope and diaphragm problems, so beware. Keep an eye on your contractor and construction.

In Canada these structures fit under CSA S37. I believe in the States TIA 222 governs and provides really good guidance on how to compute the wind loads. But there is A LOT of ambiguity on how these structures pertain to the building codes. My opinion is that, they generally do and you need to follow those guidelines. Some designers take the ambiguity as a signal of "do what you want" which I don't agree with.

 

[JoshPlumSE]

Start with the TIA codes. If you plan on getting into these projects on a regular basis, then you should probably purchase a program like TNXTower.

Note: I used to work with the owner (PeterC) of TNXTower back when the program was known as RISATower. So, You can take my "plug" of tower programs with a grain of salt.

My point is really that this is a specialty field and those programs (there are others, but only only familiar with TNXTower) really do a lot for you (libraries of dishes and such, libraries for Schifflerized angles, and various common "truss legs"). They really do simplify the process if you're doing a lot of this work.

 

[Atmane]

TIA-222-H provides more details how to calculate wind loads on a top of roof. A rooftop wind speed-up factor (Ks) was introduced in the equation of wind pressure (art. 2.6.7). I am used to work with TSE Telecom program and there is an option to include this factor in the wind load definition.

 

[dik]

Is deflection an issue?



Dik

 

[msquared48]

Deflection could be an issue for antenna alignment. The tower owner or carriers would have to specify their requirements. The deflection in the roof structure would only amplify the tower deflection, which gets worse with taller towers. Being on top of a building, though, I would not expect a tower much over 40 feet, or two tower sections.

Also, if memory serves me correctly, I believe that TIA 222 uses unfactored loads, so you will have to separate out dead, wind and icing loads. Seismic rarely controls.

Mike McCann, PE, SE (WA, HI)