shear wall - frame interaction: large link beam forces

[nkat]

Hello:

Can anyone give advice on my problem below?

Under lateral seismic, I am developing very large shears and moments in short beams (about 1 meter length) connecting shearwalls and the framing system. My lateral force system is only shearwalls, so the framing system is designed for gravity loads. That is, the framing system has to be able to deform under the seismic load without yielding. The current connection I have between the beam and the wall is a moment connection.

My concerns are below:

1. Is the moment connection right, or do I use moment releases?

2. To me, the large forces on these beams makes it seem like a "coupled wall" type of response.

3. If it is ok to model these beams as hinged, how are they detailed in that way? Also, is this detailing better done at both sides of the beam, or only on the side of the wall?

Thanks

 

[JAE]

You might clarify if this is a concrete or steel frame.

 

[nkat]

Sorry, for not stating this clearly, this is a concrete frame

 

[rowingengineer]

nkat,
What about wind, do you need to have these shears walls acting as coupled shear walls under wind loading?

1. I think your moment would be correct.

2. I agree if you are taking high moment and shears from one shear wall to another it is a coupled response.

1b & 3. I don't want to answer 1b and 3 since i live in a world where we don't have high requirements on detailing for seismic events. It would be helpful to know where you are, the codes that you need to comply with ect. Then we may be of assistance.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[nkat]

rowingengineer:

1. Thanks for your response. No, wind is not an issue. This is a 4-storey building in a medium-to-high seismic area.

2. Design is with Eurocode 8, which requires that all secondary members (not primary resisting elements) must be able to safely carry gravity loads under seismic load. Actually, all major seismic codes require this.

3. Please note that I did not mean a beam connecting two shear walls to form a coupled shear wall. What I meant is the beam that connects the shear wall and the rest of the concrete moment framing system.

Hope my problem is clearer.

 

[hokie66]

The short beam is trying to act as a coupling beam whether you want it to or not. If you don't need a coupling beam to make your building work, why not take it out?

 

[rowingengineer]

Agree with hokie, easiest answer is to remove it. This ensures that the plastic hinge will form in the beam/slab and not the columns.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[nkat]

Thanks guys, I guess it seems my only choice is to try and avoid it. I'll access the best way to take it out.

 

[InDepth]

If it is an exterior condition, you could always detail it out. You could assume that a beam very lightly reinforced will plastically yield early and in effect decouple the shear walls (i.e. model without the beam). And you could detail it such that it is pinned to support gravity loads and make sure it has enough shear strength at the ends to support the gravity load once the lateral displacement of the wall causes cracking. Try not placing any top bars (or nominal top bars), that should stop the moment transfer.

 

[ishvaaag]

Anything showing stiffness (lateral in this case) in a model is contributing to resist solicitations. Hence If you are to have a linear member and do not want moment strength contribution to stiffness you need model it as hinged, and also build it as hinged. This for steel structures (to build it) may be less a problem since we can detail the connection (or local system) to show enough ductility under the calculated structural excursions without significant structural damage, or just maybe some small plastic deformation. Contrarily, is more difficult to design concrete members and joints able to sustain earthquake excursions without showing cracks that are not only permanent damage to the structure but may impair severely the capacity for shear transmission to the supports, hence constituting an identified source of structural ruin; I always remember the advice of an structural designer of Hawai (that sustains sitnificant earthquake action) that recommended including in the design of slabs and beams inclined rebars, preferably the same main rebar bent at ends like inclined shear rebar, in order to that, when the earthquake distroys the ability to pass moment and the joint is severely impaired to transmit forces in shear, at least through hanging "catenary" or "cable" action the weight of the slab can still be suspended hanging from steel dowels passing from the slab side of the crack to that of the column. So having as severe cracks in shear means a ruined connection to pass weight and this consideration must be in mind with designing anything that can see this kind and level of action.

Also, rotational ductility diminishes with the structural depth of the standing member. This is easily understood: atop a steel flange or top rebar, for whatever limit strain of the ductile steel behaviour (upper strain of the yield plateau), the rotated angle at the joint is inversely proportional to the depth of the member or slab. Hence by just using the smaller depth compatible with the wanted structural behaviour, you are enhancing the rotational ductility. Again, this may have more value for steel structures and connections that are required to undergo at full strength the complete earthquake rotational excursions, but for concrete structures it is seen that allowing for such extreme ductility is inconsistent with the convenient design and behaviour of the structures and the contrary path of limiting the lateral excursion under earthquake force is taken. This way we design for concrete structures having higher response to earthquake, more stiff and showing less lateral displacement, requiring less ductility at connections, and, out of the lesser displacement allowed, more likely to sustain with the required integrity the earthquake event. So if you have actually designed stiff shearwalls, your earthquake excursion must be controlled and likely you will be able to design the smaller depth members you can that, through confining action of close closed stirrups or hoops, may pass efficiently your forces from your floor to the shearwall.