very long shearwall

[Lion06]

I have a 160' shearwall at the basement of a building. We're providing control joints in the wall to control cracking, and our typical details call for interrupting 50% of the bars crossing control joints.

My question is would you consider this multiple shear walls from control joint to control joint or one long shearwall?

Also, the shearwall sits on a wall footing which bears on weathered rock (6ksf), and the columns sit on caissons. The wall supports 1 level, while the columns support 11.

Is it reasonable to have the shearwall on a wall footing for this scenario, or would you consider adding caissons below the wall footing?

 

[ishvaaag]

The matter of if caissons need to support the walls will be dictated by if the behaviour on not doing so is tolerable. The settlement will be bigger on loads at the center and on lesser stiffness under the walls so somehow compensate; at the proportions of your building if you are envisaging that simply a footing may be enough it is likely differential settlement shouldn't be a problem; yet some kind of estimate is of order.

It is on the other hand uncommon to make "control" joints in basement walls. Normally they are either entirely cut or not; waterstops and waterproofing in general deal with watertightness whilst allowing for some degree of movement. Anyway, if the practice in your place was as is here (1ft thick and over), basement walls are reinforced horizontally as much as to only show minor hairline cracks out of minimum reinforcement; also, because of being underground, small longitudinal movement is expected along the year; so one could also choose not to cut any joint.

Respect the length of the shearwall, seeing the proportions, the wall being considered one or some number, smaller, may be of scarce consequence; if you cut one side of the rebar then account it at the actual reinforcement anywhere, yet could count the whole length on its actual properties. And then there is the matter of joints in the structure supported on the walls, everything needs some kind of consistency in a model.

 

[slickdeals]

I think I would consider this as individual wall segments unless you provide for vertical force transfer at the control joints.

This is no different than a tilt-up wall building where each panel is individual.

 

[cntw1953]

1. Crack not always obey the designer's wish - fall on control joint. The control joint links adjacent wall segments which will deform (laterally) as an unit.
(I have always wondered what is the proper shear distribution on a long wall, and believe the shear flow dimishes with distance it has travelled.)

2. I would try wider, thicker wall footing to minimize stress on the weathered rock before resorting to cassion.

 

[hokie66]

A control joint is only a straight, defined crack. Unless you have flexible material in the joints, I think you can use the entire wall, or longer segments than just between the joints, for shear calculations. It is not like a tilt wall unless it has soft joints.

As ishvaaag said, the columns should settle more than the lightly loaded walls, so I see no benefit in founding the walls more deeply.

 

[southard2]

I agree with the last poster. In tilt-up construction there isn't any rebar going through the joints unless one is splicing the rebar at the top to form a chord. If overturning is a problem in tilt-up wall construction there are two lines of thought.

1) To prevent tipping you connect two panels together with embed plates spaced say at 4 to 6 feet on center. The plates are designed essentially for the shear walls chord force. Or the force required to hold down that side of the panel.

2) Another line of thought is that a connector at the top of the panel (an embed, drag strut, ledger angle. etc) will drag over the access shear load to the adjacent panel the second it starts to move. A tipping panel moves vertically and horizontal. In your case if the shear is already uniformily distributed this this example won't help you because the panels will all be tipping at the same time. In that case option 1 in your only option. Or you can install hold-downs.

If this is cast in place concrete you may also want to consider the foundation weight.

Another way of looking at this problem would be to pretend the shear wall segments are a built-up cantelevered beam. If the rebar crossing the joint can resist the shear flow then yes they can be considered a single segment. This where the type of control joint matters the most. If its a joint like you see in tilt-up then it is unlikely the rebar can do this. If it is a joint like a 1" deep sawcut joint in slabs on grade where concrete is still in full contact then you can use shear friction calculations which should yield lots of strength to resist the shear flow.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[Lion06]

Thanks guys. I was mostly concerned about 50% of the rebar being discontinued across the crack.

I think I will consider it as one wall. I'll look at the crack with the 50% rebar as a shear friction problem and I doubt I'll have a problem making it work.