concrete column sizes

[archeng59]

I am working on a 3 story medical building. approximately 200 ft x 135 ft. cast-in-place concrete moment frames. seismic design category D, use group III. The column spacing is approximately 30 feet on center in each direction. my analysis shows that 20x20 columns are needed for a special moment frame system. I have designed several multi-story concrete buildings using BOCA, but this is the first under IBC. Using BOCA for a similarly sized building for the same occupancy, I would get 18x18 columns sometimes 16x16 would work. so there is not much difference in column size, but a bit more reinforcing although not alot.

there is a 5 story office building being constructed not far from my office. The columns are appear to be very small, so I talked to the engineer of record who is a friend and former college classmate. he used 12x12 columns for an ordinary moment frame system. his analysis showed the story drift was well within code limits. reinforcing was about 2%. he had a seismic design category B, use group II.

now for my question: does the difference between design category B, use group II and design category D, use group III increase the lateral forces enough to require that much difference in column sizes? what is anyone's experience with that? all of the concrete buildings I have worked on are medical facilities, so do not have a personal frame of reference for comparing if my columns are too large or his are too small. my friend and I are wondering if one of us is messed up! he is rechecking his calcs as am I.

 

[sundale]

Calculating drift with concrete is always fun. Is it cracked or uncracked? For quick and dirty, I typically will simply use 0.5 E (Young's modulus) to account for a cracked moment of inertia to compute my elastic drifts. One then needs to apply the Cd boogie factor to obtain your inelastic drifts. I have heard of software that accounts for cracking (by reducing your moment of inertia) where and only where your moments are larger than the cracking moment, but I don't have such a tool.

As far as comparing column sizes between the buildings, you are perhaps comparing apples to oranges. Namely, I think that you will be require to use concrete special moment resisting frames (SMRF) in SDC D. Your buddy with his SDC B concrete OMF building is not anywhere near this detailing standard in order to absorb both the seismic forces and the large inelastic displacements.

There are a PLETHORA of special seismic detailing requirements for a concrete SMRF contained in Chapter 21 of the ACI. In general, the concept is to detail the beam/column joints and lap locations with many hoops and seismic stirrup such that a plastic hinge will develop where you want it to.

The beams and the columns need to be sized for the probable end shears that may develop due to the probable flexural strength of the member based on 125% fy of the flexural steel (due to its strain hardening). The concept is to make the member fail in bending and not shear. There is also some strong column/weak beam detailing requirements.

 

[archeng59]

thanks, sundale. the analysis was done using 0.35I for beams and 0.70I for columns to account for cracking. the lateral drift was "modified" by multiplying the value by 0.70 to get a service drift.

as far as the column comparison, I agree with the apples vs oranges idea. My friend and I talked about that, but decided to recheck our work just in case. for me, to be more economical if possible. for him, to make sure he didn't undersize anything. as I stated originally, I have never worked on a regular office building as a concrete frame and he has not done a med facility in concrete. so neither of us had a personal frame of reference for comparison.

I always go by the motto: if it looks wrong, it might be wrong. recheck it.

 

[Taro]

There is no such thing as a "service drift" for seismic. You should multiply your calculated elastic drift by 5.5 to get your design inelastic drift.

 

[archeng59]

I wasn't clear what I was talking about. I calculate the inelastic drift as you describe. also, I check lateral drift for wind loads, which I call service drift. perhaps that terminology is wrong, but that's what I call it to differentiate between the two in my mind.

 

[prsconsultant]

archeng59:

We have designed a number of SMFR systems in SDC D with IBC 00/03/06. What beam sizes have you come up with? Have you done the SMF joint shear checks / beam-column strength checks? Typically, for a 30 ft bay, we have never used column sizes less than 24"x24". I will be very surprised if a 16"x16" column works with a 30' bay beam.

 

[archeng59]

prs, we are still working on the analysis at this time. The architect requested 18x18 columns and that does not work. checking 20x20 at this time, but looks like those will not work either. my first concept was for 24x24 columns, and that appears to be where we will be.

 

[AggieYank]

12 x 12 columns in a 5 story office building strikes me as undersized.

I did a quick basic dead load / live load analysis, assuming quite a bit because it was unknown.

Assumptions:
f'c = 5000 psi
As = 4 in2 (2%)
Area of bay carried by column = 625 ft2 (25' x 25')
4 floors and a roof above carried by columns.
DL = 95 psf (concrete pan joist system floors)
LL = 70 psf (50 psf live + 20 psf partition)
DLroof = 20 psf
LLroof = 20 psf

Calcs:
P-1st through P-4th: DL = 59 k, LL = 44 k
P-roof: DL = 13 k, LL = 13 k.
Total P: DL = 250 k, LL = 188 k
Factored P (1.2, 1.6):
P = 600 k

P-all = 549 k

If you reduce live loads, this case probably works. But, if the bay sizes are 30' x 30', then your P = 864 k.

Did I make some incorrect assumptions in my analysis?