CMU Wall Design, Unusual condition

[bbookz]

I'm somewhat stumped with a CMU wall design. I'm designing a gym, mostly one story but has a partial mezzanine level. The walls are 12"CMU + cavity, insulation and brick (1'-6 3/4") total thickness.

There is a clerestory window essentially wrapping around the entire building. The window sill elevation/ top of wall is 20'-6".

On one side of the 150'x70'building I'm retaining 10'-8" max of soil. The other side is not retaining.

Because the wall is not connected to a diaphragm I decided it could be designed two ways as a cantilever or as pinned at the top by a girt. When I initially designed the project (It has been on hold 3 yrs) I determined that the cantilevered wall would not work for deflection. A rough calc., but I'm getting 1.29" wall tilt based on 100pci soil spring and .75" based on the wall deflection.

I placed 24"SQ concrete piers @ 17' o/c. The concrete piers support steel posts a few feet high which carry the roof framing. A steel channel girt frames into the bottom of the steel column which is anchored into the top of the pier. Essentially the concrete piers support the gravity load of the clear span roof, support the walls laterally and are the entire lateral system for the building. Deflection in the concrete piers based only on the lateral loading (no soil spring) is 0.75" max. I didn't take tilt into account (probably should). The pier is cantilevered with 284 k-ft max at the base. I have notes on the drawing for bracing and back-filling requirements.

The project came back to life and looking back at the concrete piers the load path just seems convoluted. The approach doesn't seem incorrect, but it just seems like it would be so much better if I could get the wall to work as all masonry. Any suggestions would be appreciated. I've attached an elevation.

 

[JAE]

If you embed the steel columns into the concrete columns, to create a contiguous vertical element from ground to roof diaphragm, then the lateral load from the walls could be transferred into these columns which would span as vertical simple spans.

Then the lateral load on the diaphragm would only have to travel through the steel columns (either cantilevered or moment frame from roof beams) down to the top of the CMU walls which could act as shearwalls - taking the lateral load to the footing.

 

[bbookz]

JAE,

Thanks for the input. I thought it might be a challenge to get the diaphragm load all the way to the side walls through the five columns on the end wall, but I could check this. If I can make it work then I wouldn't have to worry about the pier deflection (pin-pin would be negligible); and if I've already designed the pier footings for the lateral overturning moments then I would have some redundancy in the system which would make me feel better. Do you agree?

 

[bbookz]

JAE,

I was designed all of the steel posts with base plates and anchors embedded in the concrete piers. I didn't think to actually embed the steel column in the pier without a base plate. Is this what you're suggesting and is it practical? Are there codes for this type of design? Gravity loads would be easy enough to check, but checking for lateral forces doesn't seem straight forward.

Thanks,

 

[JAE]

We've had pipe columns extending out of footings - with headed studs welded up the sides - embedded about 3 to 4 feet to ensure complete moment capacity of the pipe - wrapped the pipe column with a lot of column ties to bind it in.

I don't believe there is a direct "code" method for determining how deep the embedment, how many ties, etc. It is sort of a physical problem with bending/bearing etc.

The steel pipe columns could also be full length - taking all the load with the concrete used as a cover/fireproofing.

 

[Den32]

Instead of embedding the columns in concrete, why not just design Tube Columns to span vertically? They may be hefty tubes, but forming tall concrete columns won't be an easy task, plus the steel tubes probably can be smalled than 24" square which will save floor space.

Add a girt to top of CMU which spans to the tube columns, and design the tube columns to span vertically.

I would limit deflection on the girt and column since you are backing masonry.

Is Architect worried about losing interior space due to these 'bump-outs' at the columns?

 

[bbookz]

The bump outs are incorporated into architectural, they interrupt and are inset into the CMU so the projection into the space isn't intolerable. 20+ tall columns with the wall girt load might be difficult to make work, but I'll take a look. Thanks.

 

[JD2]

As relates to embedding the columns into the footing: In Chapter 5, Section 5.12 (P.5-18)in the 2nd edition (red book) of the PCI Handbook for Precast and Prestressed Concrete(its been a while since I left the industry) there are 2 design examples for structural steel haunches - steel "W" and Tube shapes embedded into concrete with rebar confinement that will handle shear and moment - which is essentially what you are looking at. I would guess that a newer edition would have similar design examples.

 

[JAE]

That's a good idea JD2.

 

[a2mfk]

A quick elevation sketch of your total system would help, I am a bit lost.

Using concrete tie beams and columns within a CMU wall assembly is common and perfectly acceptable if the CMU alone will not work. I have used it before on plenty of projects. It helps to just do a nice elevation of the whole wall with tie beam and tie columns and intermediate CMU reinforcing all called out, sort of the way you would detail a tile wall... Keeping the trades to CMU and concrete also can help in the sequencing and coordination.

 

[bbookz]

I finally came decided on the design approach. The CMU walls braced by girts will be designed as fixed-pinned. I'm providing additional earth face dowels at the base of the walls and increasing the footing sizes slightly for the walls. The net effect is that the load to the girt at top of wall is greatly reduced, so the cumulative wall load getting to the top of the concrete fixed-free piers is also reduced. Overall this will control deflections in the wall/pier system as well as provide a little redundancy/ stability to the wall.

I'm not relying on the roof diaphragm or return walls for shear, but I know it will contribute to stiffness. I'm being conservative, but I don't think I'll be increasing overall cost greatly with the updated design.

 

[a2mfk]

THere are plenty of ways to skin this cat.

My last idea I will throw at you that I have had to use before for lateral wind but not soil is very similar, but you use horizontal conc tie beams to transfer the load to vert concrete tie columns or CMU piers, and you use the end walls also. So the horizontal beams span between the end walls and interior piers. Thus he CMU is simple spanning vertically between the tie beams, spaced as needed to make the CMU and tie beams work. You can play around with the spacing and size of each element for economy and to meet arch demands.

You can still end up with some pretty serious foundations at the interior tie columns if you are modeling as fixed-free, but if you use the roof to carry shear as fixed-pin, then its not too bad.

If you are using steel girts or diaphragm to brace CMU/concrete just keep deflection compatibility in mind.

 

[csd72]

Why do you need any exposed piers? Why not use part height piers on the soil side where they cannot be seen and support the roof directly over the top of the 12 inch wall?

That way you will have the stiffness and strength where you need it (at the fixed base) with none of the visual complications that architects hate.

I would also suggest you use a wider footing to reduce the rotation as necessary.

 

[bbookz]

The clerestory strip windows shown on the elevations conflict with bearing the roof directly on the wall. The piers are not visible from the outside as they align with the face of CMU. Thanks for all the input. I have a design I'm going to stick with for the time being.

 

[csd72]

Yes, then you could embed steel in the cavities to span this last section as recommended in the precast text mentioned above.