OCBF Height Limits 1

[mzaitz]

All

A question on the height limits (ASCE 7-10 - Table 12.2-1). I have a fire station with apparatus bay on the right and a 2 story fire station/police department on the left. The plan is to use steel framing. Per Table 12.2-1, the height limitation for an OCBF in SDC D is 35 feet. Most of the building is under this height limit except for an architectural tower (20 ft x 20 ft) that goes to about 45 feet. The tower is tied to the building and can be entered at the second floor and is also tied in at the roof level. Above the roof of the building it is open on 2 sides and some architectural screening on the other 2 sides.

Since the tower is not part of LFRS, can I still use the OCBF and not have to go to a SCBF?

 

[KootK]

I'd be fine with it but I don'T know of a specific code clause that allows it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

In the unlikely event that you're challenged on this, I think that you could argue that the height of the building is about where the center of mass of the diaphragm + tower is. By that measure, you're probably okay.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tstructural]

What is the lateral resisting system for the 45' tall tower? Can you argue that the tower has it's own lateral resisting system that exerts it's reactions on the main building. Then you adjust those reactions for the "R" of the main building and your main building meets the 35' limit.

 

[SAIL3]

if the tower is laterally supported by the bldg, then the bldg's lateral bracing system is the only one being activated in a seismic event and experieces only an applied load from the tower and would, therefore, meet the height restriction...if not, then the tower should have it's own lateral bracing system...

 

[mzaitz]

The tower's LFRS above the roof will be moment connections as it is open on 2 sides and then on the other 2 sides are translucent panels. I read thru ASCE 7-10 and the 2003 NEHRP and it did not address this. If the tower stopped at 35 feet and I had a 5,000 lbs mechanical unit on it then it falls under the normal aspects of the code. I am thinking of classifying as an Architectural Component.

Thanks for the comments.

 

[sandman21]

Your height is "The vertical distance from base to the highest level of seismic force-resisting system of the structure..." Section 11.2. Is the hose tower under two stories and less than 10% of the total weight? If yes than it would be considered a penthouse/rooftop structure and the R of the tower would not need to be considered for the structure, section 12.2.3.1.

If this were my building I would not even consider using OCBF, I am surprised that the fire authority allows the use. Had they wanted a braced frame system, I would select a buckling restrained brace frame system, R of 8 easier connections than SCBF, small footings, and better performance during and after a seismic event.