Elevator Shaft 2

[haynewp]

We are using an 8" CMU elevator core to laterally brace a 2 story building. One of the walls was struck by a Lull. This caused a 15' long vertical crack at the face where it was hit (inside and outside shells cracked, towards middle of wall), separation at the corner walls at each of the sides of the impacted wall, and also separation at one corner at the rear of the shaft. I am not yet sure if they installed the ladder type horizontal reinforcing specified, and if they did, they probably did not lap it at the wall corners (hence the large separation that is seen).
Therefore, the core would no longer be acting as a unit, instead as 4 independent shear walls. When we find out what exactly was done,
1) If there is no horizontal reinforcing, besides shear cracking, what other potential problems and any suggested solutions?
2) If there is horizontal reinforcing but it was not lapped at the corners, what problems might arise under lateral load? (Thinking about bolting on TS brackets on outside of wall to remedy this).
3)In addition, they may have left out the bond beam at the mid floor level. The guy I talked to wasn't sure. He said it could be vertical reinf. spanning the entire height. All cores are solid.

 

[dik]

An impact as described could have compromised many/all mortar joints, including those not obviously damaged. If the stress level in the wall is high, this could necessitate complete reconstruction of the damaged portion of the wall.

If you wanted to have the shaft act as a unit so that the walls would act as flanges and not just as webs, then the intersecting masonry walls should be 'toothed', one course overlapping the adjacent one, rather than butting into it. Drawings/specs should describe the connection required and any supplemental reinforcing.

With regards to the horizontal reinf, it's common practice for the inner wire to be cut and the reinforcing bent at a right angle so that the outer wire is continuous.

A portion of the corner can be easily cut out (assuming it's not concrete filled) and the condition and method of construction reviewed and reconstructed.

It could be possible to cut out all damaged mortar joints and repoint/reconstruct depending on confidence in the contractor and the level of stress in the wall, but judging from the impact, the entire shaft may have to be reconstructed or reconstructed within the damaged storey.

 

[haynewp]

They are going to begin tearing down the impacted wall today, down to about the next story. We will be able to see what kind of horizontal reinforcing was placed. How much weaker is the shear wall if there is only verticals?

I didn't design this building/shaft, the first time I saw the project was on the inspection yesterday. I am not sure if the engineer designed the walls to act as a unit or not, he has left the company. Perhaps not, I will be investigating.

I am still concerned about the bond beam that may have been left out at mid level. Another problem is, they left the bond beam off the very top. The reason was because they didn't know to poke holes (U shaped block) in the bottom shell of the U block so the verticals could run into the bond beam.(Although this was shown on our drawings)

If the bond beam (was) placed at mid height, did they run the verticals through the bond beam or did they discontinue the verticals and just sit the bond beam on top. If so, obviously there is a weak plane I am going to have to deal with at the story level.
We will be talking to the guy who was in charge during the time of erection today, although he has not been much help so far.

 

[ishvaaag]

Well, it looks you are more than conscious enough to do what required: restore functionality anywhere, and mainly at the stabilizing core. Whilst doing that remember the structure is weakened, and some complementary bracing during the works may be required. Also don't fail to check as dik points how any joints anywhere stand...just in case, for as also pointed, the impact really got to the core, so also went elsewhere.

Respect analysis, since reinforcement will be weak, you can analyze the thing in FEM assuming low mechanical properties, to see what is the behaviour for the code loadcases, and also in any construction stages. Even simpler would be if instead of plates you assume the core be a line element, then you would have shear as always for a beam and see what then happens: do it stand without stirrup-like reinforcement what required? In any case, assume for this check a very low shear strength for the c.m. wall, since both depth, joints and your description indicate such needs be our assumption.

 

[JAE]

Are you in a major seismic zone? If not, I'm not sure I'd get too concerned about the horizontal reinforcing if you've got a well constructed vertical reinforced wall. However, with a seismic zone, there are usually minimum amounts of horizontal reinforcing that is required.

Agree with dik that you need to study the calcs carefully to determine if the stair core was analyzed as a "tube" with the perpendicular walls acting as flanges. If not, then the intersection between walls is not critical to your wall performance other than cracks appearing there.

For a two story building, I wouldn't imagine a CMU stair core taking that much load.

 

[haynewp]

Thanks for responses, and no I am not in a high seismic area. 80 mph wind controls.

 

[dik]

JAE:

For a 2 storey building, depending on what stair cores (if any), I've often found that the elevator core can provide a substantial part of the lateral resistance. For small buildings I often select a few elements to use for lateral resistance and don't consider the entire building as a system. The core is likely stiff compared to other components and there's not likely any other interaction. It's a matter of doing the sums...

As you note, horizontal reinforcing is not likely an issue, in seismic zones, for short walls (as in elevator shafts) the horizontal reinf can add strength to resist forces normal to the wall and help hold things together. As normal practice, for exterior walls and bearing walls, I usually run it ea second course and for interior partitions, usually each 3rd course, needed or not.

Was curious about the bond beams at mid height and it's possible that the elevator supplier may require anchorage for brakes at these points or something.

 

You could have the walls grouted solid (with cleanouts at bottom) to increase the shear resistance and mass (if they aren't already). Did you call for special inspection of the wall reinforcing in the documents? We had a 4 story storage building on the east coast with two cmu stair towers designed as tubes. The local building inspector realized after the towers were complete that all the vertical wall reinforcing had been omitted. We had to X-brace the exterior of the building. Lucky there were no windows. We started requiring special inspections after this one.