CMU Shaft Height

[marinaman]

I have a project where the client wants a shaft to be built beside his existing building. The shaft will house a lift that will deliver goods from the first floor to the second floor. It is not an elevator, just a lift. The lift is self supporting, including laterally. The lift does not depend upon the shaft for lateral bracing (It is braced, but not by the shaft)

The shaft is about 10' wide and 12' deep. The shaft is about 26' tall. The question I have is, how do you guys handle a shaft like this to determine how tall it could be and be unbraced?

When I looked at this, since the lift serves the second floor, I tied the shaft to the steel framing of the second floor. Therefore, the shaft is pinned at the base, is pinned at the second floor, and then cantilevers up another 12' or so, for a total shaft height of about 26'. It has openings on one side at the first and second floor to allow for a door to access the lift.

I did review the existing building to see if I could tie the two structures together. The existing building can laterally brace the shaft at the second floor without me having any code issues with the existing building (per IBC Chapter 34)

My question arises from the CMU. If its 8" grout filled and heavily reinforced CMU, as an enclosed shaft, how tall of a cantilevered shaft could I have had an not tied it to the second floor? I tied it....but couldn't get this question out of my mind.

How do you look at the CMU? Spanning horizontally?......Spanning vertically to bond beams at 48" that then span to the perpindicular walls?....or could this have been looked at like a giant tube via a finite element analysis?

How tall could this have been unbraced? Just wondering aloud.....

No vertical loads except for self weight.

 

[marinaman]

No comments?

 

[DST148]

Your post suggests that you would like to design this new shaft as a free standing structure in reinforced masonry. Is the shaft covered at the top at a height of 26 feet?
This shaft would behave more like a channel (open) shaped flexural member. ACI 530-05, allowable stress design does not have any slenderness limits for compression members. The strength design chapter imposes a slenderness limit of 30 when factored axial stress exceeds 0.05f'm. Since the 10 feet x 12 shaft is carrying only the self weight, the slenderness should not be an issue.
Are there return walls on the door side? The capacity of the shaft to carry lateral loads effectively as an open shape would depend to a large extent on the length of the return walls on the door side. I would suggest using 12" thick return walls on the door side. Walls would have to be designed as spanning horizontally for out-of-plane loads and as shear walls for in-plane loads.

 

[focuseng]

I have the same question on a current project. New external Elevator shaft 32' tall not tied to main structure. I called IMI and have been referred to the consulting engineer for the region. Essentially my preliminary discussion is that FEA as a big tube will yield the most economical results and is a very common situation. I have another conf call on next week to get educated. Will post back more after that.

MAP

 

[marinaman]

DST148 - Exactly. I was thinking about this shaft as a free standing structure. The doors on this shaft are only 3'-0" doors, so there are masonry returns there. Also, when I designed the shaft, as you suggested, I used 12" CMU along the wall that has the doors in it. Yes, I do have a roof over this shaft that is comprised of W8 steel beams and metal roof deck. I have a good diaphragm at the roof level.

Thanks focuseng. I look forward to hearing what you discover.

In my case, the archictect was restricting me to 8" CMU on three sides, but the side with the doors, I could use 12" CMU. I really wanted to design this shaft as free standing, but I don't have any FEA software, and, I just don't like 8" CMU that tall unbraced, so, I tied the shaft to the existing building at the second floor.

When I designed the shaft, I placed a cast-in-place concrete horizotnal beam all the way around the shaft at the second floor, so that when I tied to the second floor, I knew that my CMU was spanning from the ground to my concrete "ring beam" and then cantilevered vertically another 12' or so above my "ring beam". This allowed me to break down my CMU design into something I could handle by hand.

But, I've been wondering what the results would have been had I had the software to design the shaft freestanding. I was wondering if the 8" CMU would have worked as a shaft that's 26' tall. Thus the question.

I look forward to hearing what you guys find out.

 

[ron9876]

Don't see where FEA is needed. The walls will try to span in the 10'-12' direction since the height is much larger. So I would design a series of bond beams spanning horizontal to resist local forces. I would design 10' and 12' shearwalls to resist global forces. I also don't think that slenderness would be an issue in this condition if you have returns on the ends of the walls.

 

[marinaman]

Thanks for your comments Ron9876.

Ive attached a quick sketch of what we're discussing.

I looked at it this way......

Looking at the plan, stability left to right is provided by the shaft being tied to the existing building.

Stability up and down the sheet is provided on the right side of the shaft by the tie to the building. It is provided on the left side of the shaft by the 26' tall shearwall.

If I understand you right Ron, slenderness effects due to unbraced length would not have to be considered on the shearwall, because the shearwall, locally, is spanning horiztonally via short vertical spans to horizontally spanning bond beams, and, the entire height of both ends of the shearwall are continuously braced by perpindicular walls.....so....if looking at the shearwall as a cantilevered element, with b = 7.625" and d = 120" - 4" and a length of 26' (height of cantilevered wall).....the compressive stress in the masonry comes out to be about 90 psi, and since we're not worried about slenderness due to continuous bracing of the wall edges, we could compare that 90 psi back to (0.33)(f'm) = (0.33)(1,500psi) = 500 psi

In which case, 90 psi < 500 psi, so the shearwall is good.

Left and right, I see your point Ron. Up and down the sheet, it seems that the introduction of openings in the wall fouls this idea, causing the shaft to have to be tied to the building at the right....but we could still use the shearwall at the left, as stated above.

 

[DST148]

@marinaman: I don't see any thing unusual about using 8" thick masonry on three sides so long as it is reinforced horizontally and vertically. Make sure the horizontal reinforcing is continuous at the four corners.
I would avoid bringing in another trade like concrete. But if you decide to use concrete, then you may also consider providing a concrete ring beam at the roof level. On the door side you would have two masonry piers connected above doors by this concrete beam which would increase the lateral load carrying capacity of the wall with the openings.
If you have commercially available software, use it. If not, you could use approximations for distributing the lateral forces.
For simplicity and a reasonably good approximation, you could distribute lateral loads based on elastic rigidity of uncracked or cracked walls. For a cantilever wall subject to horizontal load at the top, the deflection due to bending and shear is given by
def =( V x h^3 / 3 x Em x I) + (1.2 x V x h/ Gm x A)
For a small structure, you may even assign 100% of the lateral load in a given direction to each of the walls and be done with the lateral distribution. (Don't get bogged down simply because of unavailability of FEA software)

 

[DST148]

@marinaman - Sorry. It seems our posts crossed...

 

[ron9876]

Like DST148 said about the side with the openings. The loads should be relatively small so a conservative assumption shouldn't kill the design.

When you design the shearwalls you need to design for tension also right. And yes that is the method that I would use to design the walls locally.

 

[focuseng]

Follow up:
So I finally had my consultation with IMI engineer consultant. It was brief but informative. Essentially he modeled the shaft in RAM-advance to show stresses. But then he checked the different elements again in planar design. The result is that the horizontal span controlled and the side walls just became large shear walls but didn't have the same stresses as stand alone shearwalls because of the front and back walls are nice anchors. I was surprised at the lack of tension reinforcement needed for chord forces.

My structure in plan was about 11'x9' with 32' height. Free standing, no intermediate floors. 8" block with vert bars at 64" oc would have worked. Horizontal reinforcing controlled the out of plane forces. Slenderness was not an issue despite 8" thick walls 32' high.

In summary it reinforced the discussion already played out here and it seems there could be some economizing which could be had by FEA modeling it, but in a pinch, good old simple breakdown will give a reasonably cost effective and conservative solution.

No Worries

MAP

 

[BAretired]

The shaft will act like a tube with a couple of openings in one wall. The height to wall thickness has no bearing on the problem.

BA