Construction Type for 7-Story Residential Bldg

[ETThompson]

Hi

I'm working on a 7-story residential R-2 apartment bldg. (I've posted about it before).

I'm using provision 510.2 for a podium bldg. I have 2 stories below (S-2 parking garage, lobby and fitness), and 5 stories above. The building above the podium is all R-2, we have indoor/outdoor roof decks (each space under 750sf, so classified as R-2) (at least for now until the client increases the size of the rooftop amenity).

The building height is 79'-6" from grade plane to avg roof. Our floor plate per floor is about 10,800 for floors 3 - 6, and floor seven is about 9,000 (don't have actual #s on me at this time), in round numbers 52,000 total.

It appears to me the project works under IIB. Height allowed is 75', so we comply. Stories is 5 for either A or B, so OK. Allowable area is 48,000, so times 3 = 144,000sf. divided by 5 stories I'm allowed about 28,000 sf, without even taking anything for frontage. So we comply.

Usually in my office when we do these types of projects I expect a 1hr rating at floors and apartment walls, and exterior walls too, though many of our buildings are wood. Are there other provisions that I should watch out for that could kick me up to IIA? Or am I good? I haven't worked on a project that's had basically no ratings so want to make sure I'm not missing something.

If so, I basically have no rating requirements. Our building is load-bearing steel stud walls, we're using a UL between apartments which is already 2 hr (we're using it because the owner wants STC of 60). We had been planning to use UL G548 for the floors. So we could drop the Type X GWB, and use non-rated dampers and such for the ductwork going through this assembly?

For exterior walls, all my fire separation distances are more than 10' (mostly to centerline of street, one side on park with no-build easement). So, per Table 602 I have no required rating for IIB either.

Seems too good to be true - am I missing something?

Thanks

 

[ETThompson]

Actually now I'm not sure. I was thinking the height I had to meet for the podium approach was above the 3 hr podium, but if I'm reading item 6 in 510.2 right, I have to meet the height limit in 504.3 (which is 75' for IIB), for my entire height - including the podium. So my height, from grade plane to average roof of 79'-6", is not in compliance. But do I have that right? The wording in the code is confusing.

6. The maximum building height in feet (mm) shall not exceed the limits set forth in Section 504.3 for the building having the smaller allowable height as measured from the grade plane.

 

[ETThompson]

My podium is basically 20'-0" tall, and the building above the podium is 59'-6", and the total height as noted above is 79'-6".

 

[ETThompson]

OK, another correction, I found that section 420 tells me I need 1 hr at walls (1/2 hr at corridors), and 1 hour at floors, can be reduced to 1/2 hr for IIB. So can't eliminate dampers and such, even if I can get IIB.

 

[RLGA]

Actually now I'm not sure. I was thinking the height I had to meet for the podium approach was above the 3 hr podium, but if I'm reading item 6 in 510.2 right, I have to meet the height limit in 504.3 (which is 75' for IIB), for my entire height - including the podium. So my height, from grade plane to average roof of 79'-6", is not in compliance. But do I have that right? The wording in the code is confusing.

6. The maximum building height in feet (mm) shall not exceed the limits set forth in Section 504.3 for the building having the smaller allowable height as measured from the grade plane.

Yes, the height is measured from the grade plane, so your building is currently non compliant.

 

[ETThompson]

Thank you.

 

[cda]

And you are not a high rise ????

 

[RLGA]

And you are not a high rise ????

High rise is measured from the lowest fire vehicle access to the highest occupied floor. Assuming the grade plane is roughly where the fire vehicle access is, then the highest occupied floor would be less than the 79.5 ft. measured to the average roof height.

 

[cda]

Just checking

Thanks

 

[ETThompson]

Right, what RLGA said is what we'd come up with. Only thing above that 79'-6" is elevator overrun and roof maintenance access.

 

[RLGA]

So, what are you going to do to overcome the Type IIB height bust...reduce the roof height or go to Type IIA construction?

 

[ETThompson]

Type IIA. That's what we had come up with in SD, so not a change or problem even though much has changed since SD. We'd already assumed 1 hour floors etc. We are all noncombustible construction anyway (steel stud bearing walls interior and exterior). Walls between apartments are using a 2-hour assembly already since client wanted it for sound. Floor is steel bar joists on composite concrete deck ,so adding rated GWB and dampers on mechanicals gets us our 1 hour rating.

 

[ETThompson]

We'd only look at reducing roof height if we get into cost problems. We do have very high floor to floors, the owner wanted very high ceilings and we advised them that this would impact cost.