Lateral load resisting frame question 1

[Enhineyero]

hi, is it possible to use a flat slab + RC column as lateral force resisting frame? (without shear walls). if so what "R value" will be used for the seismic load computation. will this kind of system follow the requirements of SMRF?

 

[slickdeals]

Are you in a zone with moderate to high seismic risk? If so, what you are proposing is not advisable. An OMRF requirement can be used and if pushed, I would only use it in areas with R=3.

 

[PicoStruc]

NO !

 

[JAE]

I've used flat slab plus RC Columns before in a frame system. This is essentially Chapter 13 of ACI 318 using the equivalent frame method. (six story courthouse)

slickdeals is probably correct that in higher seismic conditions such a system would either not be allowed or not acceptable.

 

[Michel60]

Good morning Enhineyero,
I think the comments already provided are pretty much on point. It's system sometimes used but is really limited in seismic performance potential. In a normal configuration it has initially low and quickly degrading stiffness for lateral displacements. And the potential failure mechanism can be petty catastrophic, punching shear failure at the slab-column joint. Under the best of circumstances it should be considered only as a last chance back-up to something more robust in seismic application.

That said it is interesting that when you look at the lateral behavior particularly for highrise building using a shear wall core and flat slab/plate floors the cumulative effect of the slab frames acting as outriggers can be pretty significant. So even if you designate the lateral forces to another system it's worth looking at the behavior and interaction of slab frames with the LFRS.

regards,
Michel

 

[mikesg]

This structural system is forbiden in our code (Bulgaria), the reason being you cannot detail the slab adequately to dissipate energy and develop plastic hinges. AFAIK, the use of this system was the cause of many building collapses in a Turkey quake several years ago.
I would try to find a system where the slab is not part of the moment resisting frame.

 

[msquared48]

I do not like flat slabs. The reason why? When they fail, everything below them gets very flat. That will NOT be me!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Enhineyero]

Thank you everybody for your meaningful reply.

Assuming i am in a moderate to high seismic zone, say zone 3 or 4
So if I add a shear wall in the flat slab + RC column framing system. will that type of structural system work? if yes, will it follow special detailing requirments such as those with SMRFs?

FYI, I have mostly dealt with my projects using SMRF's. or dual system SMRF + shear wall. Hence, i am not that familiar with other lateral systems

 

[DST148]

@Enhineyero: Ref ASCE 7-05 & ACI 318-11. In response to your first post - It is possible to use flat slab + columns (moment-resisting frames only and no shear walls) as LFRS. OMRF( R = 3) is permitted in low seismic risk zones, SDC A and B. IMRF( R = 5) is permitted in low to moderate seismic zones, SDC C and lower. SMRF(R = 8) is permitted in all seismic zones. Seismic detailing requirements of ACI Chapter 21 are required for frame members in SDC B and higher but not for SDC A.
SDC A & B ~ UBC Zones 0 & 1, SDC C ~ Zone 2, and SDC D, E, & F ~ Zones 3 & 4.

 

[hetgen]

Hi All,


Can we generally define areas with peak ground acceleration < 0.8 m/s^2 as low hazard areas.


@DST148... for those of us outside US we're lost in acronyms. This is what I could translate:

(SMRF) San Marcos River Foundation
(IMRF) International Maritime Rescue Federation
(OMRF) Oklahoma Medical Research Foundation

Thanks.

 

[DST148]

@hetgen: Sorry about those acronyms.
OMRF - Ordinary Moment Resisting Frame
IMRF - Intermediate Moment Resisting Frame
SMRF - Special Moment Resisting Frame
LFRS - Lateral Force Resisting System
SDC - Seismic Design Category

 

[Enhineyero]

@ DST - is flat slab + rc column considered as a OMRF?

follow up question, In order for me to utilize the R value for dual systems (say RC frame + shear wall) does the shear wall + RC frame need to be present at every bay of the structure for the direction of the EQ load being considered??? because it is quite difficult to have a shear wall at every bay of the structure (esp if it is irregular in shape or if the architect does not allow it).

 

[DST148]

@Enhineyero: R.C. Columns in combination with flat plate / flat slab can be considered as OMRF. Seismic detailing requirements per section 21.2 /ACI 318-11 would apply.
Shear wall + Frame need not be present in every bay. Typically the concrete slab acting as a diaphragm can carry the lateral loads to vertical elements of the LFRS.

 

[Enhineyero]

DST - thanks for your reply. I have a question for diaphragm action, how would one check the adequacy of the diagram if can transfer the loads to the LFRS? if it is relatively thin how would one check if the thickness is adequate? Usually the RC slab for a framed structure is just checked for bending & shear due to vertical loads. I am not quite familar how slabs or flat slabs are checked for lateral loads.