Shear Wall Transfer Beam 3

SteynvW · Feb 1, 2016

I currently have a shear wall that has to transfer the load to the columns.
I have looked at systems that use a beam (in the same plane as the wall) to
transfer the loads from the wall to the column. The problem that I do a
finite element analysis it shows that the bottom part of my wall or beam if
I had one will be in tension. What advantage will I gain from using a beam
and not just put my wall down on the columns with column heads?

SteynvW · Feb 1, 2016

I currently have a shear wall that has to transfer the load to the columns.
I have looked at systems that use a beam (in the same plane as the wall) to
transfer the loads from the wall to the column. The problem that I do a
finite element analysis it shows that the bottom part of my wall or beam if
I had one will be in tension. What advantage will I gain from using a beam
and not just put my wall down on the columns with column heads?

KootK · Feb 1, 2016

The advantages certainly are fewer if your walls happen to terminate above columns at the ends. There may still be some however:

1) Your wall flexural compression block may not fall entirely within the columns. Column heads may well be able to deal with this.

2) If the slab is thin it may well be difficult to make the shear friction between wall and slab work. A beam/thickening helps with that.

If possible, I prefer to run the columns up to overlap with the wall by a story. There can also be all kinds of seismic implications to this if that is a big concern for the project.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

SteynvW · Feb 2, 2016

Thank you kootk. It is a 16 storey building, so there is quite a load
coming down in the wall. The load arcs to the columns and creates
a triangle tension zone above the spanning wall with the maximum tension
in the bottom and gradually decreasing as you move up the wall, it should
therefore be scheduled appropriately. But I am looking a maximum tensile
stress of 12 MPa that has to be carried by the reinforcement. This maximum tensile
stress is basically in the slab.

1. Can I just increase my reinforcement in my slab to handle the maximum stress?

2. How many stories of the wall do you use as a "deap beam"?

Kind regards

cliff234 · Feb 2, 2016

How thick is the wall and how wide are the columns? From a formwork perspective it would be ideal if the wall thickness matched the column width. You have a strut and tie condition here. Your computer analysis showing a big tension force at the bottom of the wall is correct. That tension force is the tie. I am assuming that the columns are under the footprint of the wall. If the wall is thinner than the width of the columns then I agree with KootK's suggestion to extend the column up one story into the wall (overlap wall with column) to better transition the load into the columns. If the column is the same thickness as the wall, you need to detail the column reinforcing to do the same thing - extend the column vertical steel one floor up into the wall and transition it with the chord reinforcing steel.

rapt · Feb 2, 2016

We do not have a lot of detail on what it looks like but from the sound of it, the wall will be far stiffer than any beam you provide so the wall is going to transfer the
load in any case.

Sounds like the wall is acting as a Deep beam and should be designed by Strut-tie action. You will need to make sure there is a mechanism to transfer the load from the wall to the columns, but all a beam will be doing is give you some extra concrete to house the tension tie reinforcement at the bottom.

KootK · Feb 2, 2016

You're most welcome StynvW.

Q1) You bet. That's just what I'd do.

Q2) As others have mentioned, it's probably better to think of this in strut and tie terms as apposed to deep beam. That said, for gravity loads, I've seen recommendations similar to below. That essentially establishes an effective beam depth which can be used to proportion your tie. A quite different model would apply under lateral load, of course, and would depend on whether or not you have net overturning etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

KootK · Feb 2, 2016

Slight revision:

Q1R) I believe it better to place your tie in the bottom of your wall rather than in your slab. That way you don't wind up with a cold joint crossing your diagonal struts at an oblique angle. I'm not sure how critical this is but it's what I'd do.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Trenno · Feb 2, 2016

Make sure to properly anchor the tension tie.

rapt · Feb 2, 2016

I would get the reinforcement as low as possible, so in the bottom of the slab, or it may even be necessary to extend the wall below the slab (like a beam) to provide adequate concrete to transfer the diagonal strut load to the column in compression at the nodes, depending on the relative shapes of the wall and the columns and the overall arrangement. The compression stresses in that node joint will control the overall dimensions of the connection.

The cold joint can easily be handled by the vertical reinforcement your should be supplying in the wall and fully tying around the tension tie reinforcement so that it is fully developed by the time it reaches the cold joint.

KootK · Feb 2, 2016

rapt said:
The cold joint can easily be handled by the vertical reinforcement your should be supplying in the wall and fully tying around the tension tie reinforcement so that it is fully developed by the time it reaches the cold joint.

Correct me if I'm wrong but it sounds as though you are thinking of using the entire cold joint connecting the wall to the slab. The cold joint that I'm concerned with occurs over only distance X in the sketch below. It's certainly not impossible to make it work via shear friction but it's no slam dunk. It warrants careful designer attention in my opinion. And, if you wind up needing extra localized roughening or shear friction reinforcing, that's just one more thing that requires additional attention and inspection on site.

rapt said:
I would get the reinforcement as low as possible, so in the bottom of the slab, or it may even be necessary to extend the wall below the slab

Why? In something that is many times taller than it is wide, it seems to me that you'll end up with exactly the same quantity of tie reinforcing regardless of whether you place it low or really low. And nearly identical performance.

@SteynvW: I found something in print. See the clip below. Everyone prefers a reference over some stranger's advice, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

rapt · Feb 2, 2016

Kootk,

And how is the whole connection going to work with the tension tie above the top of the slab?

I did suggest that there should be a downturn which would have the compression strut coming into the side of the column! There is no way I would detail it as you have shown. Why, because it feels better to me. But that is just my opinion.

rapt · Feb 2, 2016

Kootk,

And how is the whole connection going to work with the tension tie above the top of the slab?

I did suggest that there should be a downturn which would have the compression strut coming into the side of the column! There is no way I would detail it as you have shown. Why, because it feels better to me. But that is just my opinion.

Teguci · Feb 3, 2016

Just want to note that any lateral loading attracted to this stiff shear wall will need to be dumped into the floor diaphragm at the transition. Any overturning moment will probably require tension reinforcement into the supporting columns and will probably make moot any issue with shear transfer at the cold joint above the columns. A shear wall to column configuration like this may cause a "weak story" situation that will require additional detailing and consideration.

SteynvW · Feb 3, 2016

@Kootk

If I understand you correctly, if I do install a beam right under the slab with a increased width, I will have my highest zone inside my beam. In bending this will not have a advantage, but if I use the tie and stud method and I have to use shear to transfer the tension to the reinforcement an increase in width wil be an advantage?

Kind regards

KootK · Feb 3, 2016

It's probably time to consider posting a couple of dimensioned sketches of your situation (plan/elevation). Much depends on your particular configuration and applied loads. There are situations where a beam element can improve the compression transfer from wall to column. It's not always a sure fire fix however. Additionally, one must consider compression capacity where the strut in the wall meets the top of the beam and strut depth remains constrained.

Not having a beam obviously simplifies formwork. My first stop would be to see if it can be made to works without the beam.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

SteynvW · Feb 3, 2016

This is a simplified version of what I more or less have.
My load is more or less 1000kN per meter.

Kind regards

KootK · Feb 3, 2016

Thanks. What country?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

KootK · Feb 3, 2016

Step 1: does plain old bearing work where wall meets column (170 x 1200)?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

SteynvW · Feb 3, 2016

South Africa.

It does not work with 40MPa concrete, but with a slight widining
in terms of a column head and a bit of steel it works.

Thank you very much for you time. This type of system has only been used a couple of times in South Africa so the experience and literature is quite scarce.

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