Where does a Moment Frame ends and a Shear Wall begins ?

[elinwood]

Let me start with a story - My client has recently bought a hotel building. The building was bought along with an empty plot of land behind it. The previous owner intended to expand the hotel on the plot of land and has already paid for the architectural & structural design to commence. The expansion was planned to be a U-shaped 4 story reinforced concrete building. My client is planning on constructing the building expansion based on the issued drawings. Please keep in mind that this project in a relatively high seismic zone - Ss of 1g and S1 of 0.4g.

I'm tasked with reviewing the structural drawing along with a senior engineer. The senior engineer brought it to my attention that the "columns" are inappropriately sized for seismic application. The "columns" are T-shaped and L-shaped with a thickness of 140mm and "legs" ranging from 300mm to 700mm. This is based on ACI 318's provision Chapter 21.6.1.1 for Special Moment Frame Systems. The only thing is - I'm not sure if this is considered a "Special Moment Frame System". There are a couple of shear walls in the building for the elevator shafts. I couldn't find any reference in ACI to the minimum thickness of a Shear Wall and I'm wondering if 140mm is thick enough. From a constructibility standpoint, there is very little room for the rebars at 140mm :

10mm(Transverse Reinforcing) x 2 = 20mm
19mm(longitudinal Reinforcing) x 2 = 38mm
20mm (Concrete Cover) x 2 = 40mm

There is a 40mm space in between the longitudinal reinforcing. When there's a column splice, the whole thing is going to be full of rebar and it doesn't really make sense.

Is there a minimum thickness on shear wall? Can the "columns" be considered a shear wall or are they "columns" ?

 

[hokie66]

It is not feasible to provide reinforcement in both faces of a 140 cast in place wall.

 

[CELinOttawa]

Hokie's right... If this is indeed called out to have two layers of reinforcing, you can't built it as a poured wall...

Sounds a bit like some of the tilt-up I've worked on. Is this a precast / tilt-wall / tilt-slab extension?

 

[elinwood]

I'm reviewing another building and this time it's around an elevator shaft:

150mm thick wall
10mm x2 transverse reinforcement
13mm x2 longitudinal reinforcement
20mm x2 cover

10mm+13mm+20mm )x2 = 96mm
We have 150 - 96 = 54mm to play with, which isn't much.

We're trying to get the structural engineer to change it to a rectangular column instead of a shear wall, but I believe there will be some problems in the ETABS model due to the seismic lateral load if we do this. For some reason, my senior engineer doesn't think that a single layer of rebar in the shear wall will work. Is there a minimum on a shear wall's thickness ? These are two designs from two different projects from two different structural engineering consulting companies in Indonesia. I'm beginning to doubt myself

 

[TXStructural]

Most shear walls in high seismic areas here have two layers of reinforcement, tied to each other so they confine the concrete core between and continue to work as a unit even after the concrete cracks. 150 mm is a very thin shear wall for any serious seismic demands, and will only allow a single layer of reinforcement. Bar diameters are nominal, so the deformations make the outside diameter larger (11 mm and 14 mm, respectively for size 10 and 13 bars.) (You are calling out US domestic, soft metric sizes, which are an inaccurate approximation, and we are phasing these out in the US by the end of this year.)

 

[IngDod]

Im confused, The only things resembling a shear wall in both buildings are the elevator shafts? If this is the case perhaps they are not designed to function as such... and the building is indeed a moment frame (or at least was intended as such). I dont have much experience but I have never seen a concrete building where the only lateral force resisting system are the walls surrounding the elevator shaft. Also I would say that the concern should not be having a problem in the etabs model.. but having a problem with the stability of the actual structure (which is not an etabs model..) Also most seismic codes require different design methods (response spectrum) for braced frames (or frames with shear walls) than for moment frames, could you not simply contact the engineer who did the calcs and ask him?

 

[IngDod]

Also, some codes do allow a mixed system of shear walls and moment frames.. in my country you can get a way with a moment frame capable of resisting 25% of the lateral load and the rest taken by the shear walls... Also, Take a look at the shear reinforcement of the columns (the tie bars) if this a seismic moment frame the spacing between the bars (specially close to the joints) should be much closer than a non seismic frame, this is done to confine the concrete during the cyclic loading experienced during an earthquake.

 

[Ron]

IngDod...using the elevator core for shear resistance is common.