Structure with one side retaining soil pressure loads

[mathcadboy]

I am currently designing a two-story concrete structure that is going to be located in a sloping terrain. The owner doesn't want to backfill or cut the land but he wants to make it sorta look like a "basement" (since only one side of the "basement" will have have soil in it).

Anyway, here are my concerns. I would like to know what is the most economical way in designing this kind of structure. I tried letting the concrete moment frame resist the lateral earth pressure loads, but it produces very huge moments in the columns. Is it more advisable to just build a separate retaining wall to hold the soil then build the structure independent on the soil pressures?

and a follow up question, does anyone here knows how to compute the capacity of a CHB block? the owner wants me to check at what level in the wall do we start just placing CHBs. By the way, the height of soil the structure will be retaining would be 3meters (about 1 story)

Thanks in advance to those who can give their insights.

 

[Ron]

Forgive my ignorance, but what the hell is CHB?

 

[hokie66]

Look at your structure as a box. The walls which are retaining on the uphill side will span between the footing and the upper slab, the slab will distribute the load by diaphragm action to the end walls and any intermediate shear walls, then the shear wall will take those forces to the ground.

Like Ron, I don't know what CHB block is, maybe concrete block. For a 3 metre high retaining wall, best to stick to reinforced concrete full height.

 

[mathcadboy]

sorry for the shortcuts, CHB is concrete hollow block. It is what we call it here in our place. I think some call it concrete masonry unit.

This is my dilemma. If I let the moment frames take the earth pressure loads, I am getting huge moments in my columns. I dont think its going to be more economical than just say build a separate retaining wall system to take the earth pressure loads.

 

[concretemasonry]

Your "CHB" can easily be reinforced at intervals depending on the load.

Distribution the lateral loads to the rest of the structure is usually simplest for construction once you have decided on the abilities to transfer the loads to the other members that will ses it whether you distribute it in your calculations.

For a normal basement with normal strength block, a 9' high wall would require a 12" CMU wall reinforced at 4' O.C. (or 8" or 10" equivalent) and full grouting adds little and creates other problems. This is a "seat of the pants" suggestion based on many years and the frustration of trying to engineer a basement wall.

No matter what you use for the wall, you will have some over-excavation and fill required, but make sure you require drain tile since you apparently are on a slope where the subject wall intercepting the drainage. - I could be wrong, but based on the bare information it is an assumption to be addressed.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

just build a separate retaining wall to hold the soil then build the structure independent on the soil pressures?

yes!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rowingengineer]

Have a few questions because I would have thought Hokies idea was the solution. Are you not connecting the side returning walls to the slab? (Now you can answer this by words or give us a diagram showing the wall layout, some sections and who knows we might just reply by using the same diagrams marked up).

How is this acting as a moment frame, can it not be a braced frame for the last storey. You could use the concrete masonry walls for this purpose. the height you can build a RW without reinforcing is about 400mm in my opinion, so you walls are going to be reinforced something like dick suggested.

However if the basement is to be water proof to any level above zero building a separate RW is the way to go.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[mathcadboy]

I am attaching a plan and an elevation view to clarify things.

 

[msquared48]

I say againm make it a free standint (yielding) retaining wall, not a basement wall (non-yielding).

It's the only way to go here unless you use deadmen or tiebacks.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Disagree with Mike on this. I don't know where you would place a cantilevered wall. The wall which retains soil is 3.5 metres back under the building above. No reason not to make this a basement wall. Don't try to use the columns to resist the earth pressure, but rather use shear walls. You have to have something to retain the earth where it transitions at the sides, so use those walls as shear resisting elements.

 

[hokie66]

Oh, and you could indeed use reinforced masonry for the walls below grade. But unlike concretemasonry, I would never build retaining walls without filling all the cores. If you have H-blocks or knockout blocks available, use those to ensure the grouting is complete. Whichever way you go, if you want a dry basement, waterproofing the external face is essential, and remember to protect the membrane from damage when backfilling.

 

[rowingengineer]

I agree with Hokie on this one. if you need a better explain I would be happy to mark up your plans tomorrow while near a scanner. let me know.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[mathcadboy]

I was just surprised when I saw how big the moments are in the columns when I used it to resist the lateral loads from the soil. so moment frames are already an X.

@ Hokie, When you say basement wall, what does it mean? What would be the wall made of? any details on that and can you point me to any reference of structural calculations for this?

@ rowingengineer, yes i would definitely appreciate it.

I am attaching another photo of what would the separate RW I think would look like...

 

[msquared48]

Hokie:

I understand the shearwall solution, and is a standard one. However, it's use depends on developing the floor diaphragm to pass the shears to the shearwalls. These would be permanent, one directional shears, and the seismic/winds forces be on top of these lateral soil loads. This restricts the use of the shearwall and diaphragm materials pretty much to concrete.

The use of the cantilever wall at the interior basement wall location shown in the OP's diagram just gives more construction material options. To me, it is very do-able.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[chicopee]

Don't forget to insert vertical rebar and concrete fill the hollow block wall otherwise it will displace. Three houses down from my house, the owner found out the hard way when he began to backfill his newly build basement wall.

 

[concretemasonry]

Based on general masonry designing principals, I hate to see all cells fully grouted. Just grout the cells that contain reinforcement. There are enough block shapes and the "double H" with two webs 8" on centers provide a clean core from top to bottom that can also be fully bedded for controlled grout placement and vibration. Matching bond beams are also available to provide continuity both vertically and horizontally for for reinforced masonry "web" within the wall. - Jim Amhrein, legendary masonry engineer from California, always only count on what you can see and verify.

Full grouting improper units results in sloppy construction and the grout really adds little to the strength of a properly designed wall, since the controlling factor is the actual block strength, with controls the the f'm (based on a hollow masonry prism). Full grouting is a "belt and suspenders" good walk-away feeling.

The "double H" block is logical, modular and works ideally for details, especially with 8" thick walls. For 6" and 12" walls the corner details are a little different. I have projects with 6" "double H" block used for 15 story loadbearing buildings because of the economy, weight and labor savings. They just used different block strengths for different levels of the buildings, but used the same mortar and grout for the project because it was produced on site for a number of buildings

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

mathcadboy,
The "basement wall" is the wall in question, which as I said is braced bottom and top by the concrete slabs. It can be built in either reinforced concrete or reinforced masonry. If you don't know how to design it, suggest you seek advice from your mentor or a structural engineer who does. The building in question is quite a small structure, but nevertheless deserves attention to detail by an experienced engineer.

Mike,
The OP did say this is a concrete structure.

chicopee,
Of course.

concretemasonry,
I disagree completely. Guess we'll have to leave it at that. My approach, and the approach of most engineers in Queensland where reinforced masonry construction is the norm, is essentially to treat the block in reinforced masonry as permanent structural formwork. Amhrein's methods may be the bible in California, but not here.

 

[concretemasonry]

hokie -

I just think of masonry in conjunction with a concrete framed structures as a structural element to accept the load and distribute it depending on the geometry of the panel dimensions. Usually for smaller residential walls, most of the load goes to the top and bottom of the panel since the vertical dimension is usually smaller than the length.

I am aware of the usual construction in Queensland and visited sites with Jim. Fortunately, Boral and others provided a long list of structures (finished and under construction) to look at. I don't understand why the producers there do not take advantage of the available manufacturing technology for strength and the shapes available like they do in South America. It might be due to the existing codes and standards.

The use of conventional masonry in your area is very good and represents value in design and construction and is certainly sound. Too bad no one has 3000 or 4500psi f'm prisms available.

Great place and good examples of construction, but we also traveled with a Kiwi.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[mathcadboy]

Thanks for all the inputs!

Anyway, I thought of an alternative and tell me what you guys think. I will just make the slab a suspended slab then distribute the retaining wall in two locations making it less than 1.5m walls. and also, the volume of backfill that would be required will be lesser, though suspended slabs + beams to support the slab would cost more. Im attaching my sketch.

 

[concretemasonry]

That is one solution that could work (without running the numbers) and satisfy the owner (who pays the bills) and wants to avoid a massive excavation and backfill and access problems. 1.5m is pretty stout and possibly more rigid than the concrete frame structure.

Engineer and international traveler interested in construction techniques, problems and proper design.