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Unusual Aspect Ratio

Unusual Aspect Ratio

Unusual Aspect Ratio

I have been directed to engineer a building that will house power generating equipment. The building is approx. 210 feet square in plan with a forty foot high eave line. The roof is essentially flat. The building is envisioned as a steel frame with bracing along the perimeter walls and a few interior braces.

The column lines were layed out by our mechanical designers. To limit the number of interior columns thay have laid out the bays on a 30 feet by 70 foot pattern. I have attempted (without any success) to convince the designers that a bay spacing such as this will present many problems that will result in substantial costs such as oversized footings, large columns, large braces, ponding concerns, etc.

The question is: am I out of line to insist that the bay spacing (aspect ratio) be made more nearly square or is this not a problem?  

RE: Unusual Aspect Ratio

Steve1 - In electric power stations, having minimal interior obstructions is essential because of the sheer size and odd dimensions of the equipment. The building that you describe (perimeter wall bracing, large footings, columns and braces, etc.) sounds completely normal. Suggest that you go along with the Mechanical Engineer's requirements - power stations are, by nature, NOT "cost effective" structures.

www.SlideRuleEra.net idea

RE: Unusual Aspect Ratio

I tihnk you have a duty to explain the cost ramifications of the design direction to the client but the decision to proceed as directed is the owner's.

Don Phillips

RE: Unusual Aspect Ratio

I have done few power generation buildings and mechanical equipment buildings. It is essential, as SRE stated, to accommodate these equipment sizes not only when installed but when maintenance is required and even when time for replacement comes. Aspect ration while desirable, it goes out of the window.

I would work with the mechanical/electrical engineers to maximize the efficiency of the building. I can not tell you how frustrating it is when the building is complete, which normally takes place before equipment is delivered to the site, and then you have to resolve interference between structural elements and the new equipment. That gets very costly to the pocket book and schedule.

Our office invested money in ACAD ABS system to help us mitigate these conflicts during the design. All of our work is in 3-D and it has been a great tool.

In closing, I would not worry about aspect ratio for this type of building. Worry about providing a sound structural system that is free from interferences.


RE: Unusual Aspect Ratio

Thanks for the replies. I guess I have a greater concern over the process (no structural input into the framing concept) than the actual design. Makes it tough to be structural engineer of record when you are told what to engineer without any input into the process.

RE: Unusual Aspect Ratio

Steve1 - In the electric power business, the mechanical engineers and to a lesser extent the electrical engineers take the lead roles. As stated above, there are valid reasons for this - structural does what it takes to "make it so" - leads to some interesting challenges and can be very rewarding - never the same thing twice.

One way to look at the situation is to realized that there is no Architect involved (or perhaps only to select colors, trim work, etc.). The role of an Architect in a typical commercial building project is filled by the ME's on a power station.

www.SlideRuleEra.net idea

RE: Unusual Aspect Ratio

The 70 foot span might add a little extra cost for the overhead traveling cranes you didn't mention.

How are they planning to lift stuff for installation/ repair?

Mike Halloran
Pembroke Pines, FL, USA

RE: Unusual Aspect Ratio

As stated in my original post our mechanical "designers" are the architects for the project, not our mechanical "engineers". I find this to be quite unsettling.

This project is in Massachusettes. According to the Ma building code a project of this size requires that it be engineered by a professional engineer. I belive it to be a violation of the code for the professional engineer not to be involved in determining the basic framing scheme for the structure. I can live with a bay size of 30 feet by 70 feet. I can't live with no input into the process.

As a sidebar, overhead cranes have not yet been addressed.

RE: Unusual Aspect Ratio

This would be a good time to start addressing cranes because:

- It's easier and cheaper to install the crane, or at least the runways, before the roof goes on.

- Either the crane beams or the runway beams will have to span ~70 feet.  This is potentially a big problem because nobody stocks rolled sections that long.

Mike Halloran
Pembroke Pines, FL, USA

RE: Unusual Aspect Ratio

I am not familiar with the MA building code, but I don't see how not being involved in the building arrangement would be a building code violation.  Obviously it is nice to have some input - especially if it is now going to be a more complex design than what you envisioned when you bid it.  However, the professional engineer is signing off on the safety and suitability of the final design, not the design process.

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