Multi Story RC Building With Soft First Story

[Quanski]

Hello everyone,

I am relatively new to mid rise building design and am wondering if someone can provide some general guidance on how to deal with soft story at the ground level of a 7 story RC building. The SFRS for the upper stores are concrete or masonry shear walls. The ground story is mostly open retail spaces with no room for shear walls. The building foot print is 93m x 18m. There is an elevator shaft but likely not sufficient alone as SFRS. See attached dwg.

The current concept is to have concrete moment frames in the long direction (along the perimeter) and steel moment frames in the short direction. The steel moment frames in the short direction will be 2 bay frames.

The general structural system will be ICF perimeter walls with precast slabs running in the short direction. There will be an interior beam line or load bearing walls to support the precast slabs.

Any suggestions would be much appreciated.

Thanks in advance.

 

[JLNJ]

What jurisdiction are you in? Which building code governs your situation? What is the seismicity of your site?

A soft story might not be permitted, depending on which code is in force.

 

[Quanski]

Hi JLNJ,
The project is located in Ontario Canada.
Site class is unknown at this time.,
Below are seismic date for the site:
Sa(0.2) = 0.25
Sa(0.5)=0.16
Sa(1.0)=0.088
Sa(2.0)=0.028
PGA=0.1

The Ontario Building Code would govern the design.

 

[Quanski]

JLNJ,

To my knowledge, there is nothing in the Ontario Building that says soft stories are not permitted. Therefore, a soft story is allowed but dynamic analysis may be required and the frame system needs to be designed accordingly for the large lateral forces?

 

[Shu Jiang PEng SE]

the structure seems to be type 1 irregularity and per 4.1.8.7 c of NBC, equivalent static analysis method is not applicable to the structure. It will make the whole design process complex.

 

[Shotzie]

Is there any way that you can switch the upper floor concrete/masonry shear walls to be moment frames to better align the columns with the structure on the first floor? Or see if the shafts will suffice as your lateral system? Trying to carry a shear wall on an open structure below seems very difficult (although not impossible).

 

[Quanski]

Shu Jiang,

Thank you for your comment. I believe I can still use equivalent static analysis as the overall bldg height is under 20 or 22m.
I am still in discussion with the owners and architect trying to persuade them to implement some shear walls on the ground level to eliminate vertical irregularity. Fingers crossed.

 

[Quanski]

Shotzie,

Thank you for your comment. No, I cannot change SFRS for the upper stories. The shaft alone is not sufficient.

 

[Agent666]

Requirements for excluding having a soft storey is sometimes disguised within codes as not being allowed to have a vertical irregularity in storey shear strength. i.e. storey under consideration (ground floor) in this case not having a strength and/or stiffness of less than the storeys above.

Specifically allowing/designing for a soft storey sounds like a recipe for a disaster if you are in a seismic region. You have to appreciate that your design level seismic event is just that, an arbitrary number based on probabilistic techniques, bigger events can occur. By following the code you impart some robustness to resist higher than anticipated forces. While you might be able to show the frames work essentially elastically under the design case without forming the soft storey mechanism, one day a larger event might come along that is sufficient to form the collapse mechanism.

I don't know what its like in Canada, but most other countries require some consideration of capacity design principles to ensure desirable mechanisms (Strong column/weak beams theory for MRF's for example) can only form to achieve the desired level of ductility to dissapate the seismic energy.

Just because a code might not say specifically to avoid having a soft storey cannot be inferred as meaning its ok to have one. You have to use some judgement and go back to first principles and look at what you are trying to achieve in avoidance of collapse and hence achieving life safety requirements. Generally you are trying to preclude any brittle collapse mechanism, a soft storey is one example of this.

You need sufficient structure to preclude the likelihood of the formation of a soft storey. The scheme might need to be adjusted.

 

[Quanski]

Agent666,

Thank you and I agree with your comments. My intent is to use moment frames of sufficient stiffness in the ground level so that the soft story can be eliminated. (i.e. ensuring the frame stiffness is at least 70% of the stories above.).

These types of buildings are becoming increasingly popular and I am looking for insight from peers who has done similar projects. I just want to make sure I am not completely out to lunch on the SFRS as moment frames are very expensive.