2 STORY OFFICE BUILDING 1

[joemarch]

I have a 2 story office building (approximately 60'x220') with the floor framing being precast plank and the roof framed with bar joists. The grade along one side (the 220'dimension) is all the way up to the precast. So basically its a walk out basement.

What is the easiest way to account for the unbalanced load from the soil? Would it be the easiest to treat the back wall as a cantilevered retaining wall? Or, try to take the loading out in masonry shear walls? I have limited cmu walls,so this might be tough to do.

Thanks for all your help!

 

[jike]

I have done similar buildings with a cantilevered retaining wall.

 

[FalsePrecision]

Tiebacks - you might consider those. Of course, only your Geotech can give the req'd recommendations. I did tiebacks with cast-in place tension struts to a cast-in-place "deadman". There is the other method, where you drill back into the existing or already backfilled grade. Much more involved than cantilever basement wall, but sometimes necesssary.

 

[joemarch]

Thanks for your help.

I'm probably going to use a cantilevered retaining wall. Which leads to my next question..

If I treat it as a cantilevered wall does it really act as one? I have cmu walls throughout the building. So is the cantilevered wall going to rotate enough to act as one or will it also act as a simple supported wall? Should I also put reinforcing along the inside face?

 

[FalsePrecision]

Another idea - exterior "buttress/shear" walls, on the backfilled side (they get buried). Space every so many feet as req'd.

 

[UcfSE]

It sounds like the hollowcore is your floor diaphragm. If that is the case you probably will not get enough rotation at the top of the retaining wall for it to be cantilevered or to use active earth pressure. You can tell by actually checking the deflection of the top of the wall. I would handle it as though the wall were supported at the hollow core level and design the wall as braced at the top with at-rest pressure. You will then get a reaction at the top that goes into your diaphragm load and so on.

 

[joemarch]

UsfSE, normally thats the route I would go, but I have very few cmu shear walls. So I don't think I can get this design to work.

I guess thats why I ask the question if I can treat it as a cantilever system or a buttress system knowing that in order for it to act in that fashion rotation needs to exist? Or could I treat it as a cantilever but yet reinforce the inside face of the wall knowing it will possible see a partial simple support load until rotation exists?

I hope this makes sense. Thanks for all the help!!!!

 

[FalsePrecision]

The corners of the building have retaining walls coming into them, so that the restrained earth acts only on the long "back" elevation, right? Therefore the 2 short walls are not retaining any soil?

 

[joemarch]

That is correct FalsePrecision. Only the back has earth against it. The earth slopes from full height down to the finished floor along the short walls. So they are retaining earth from full height at the back to nothing at the front. These forces cancel since both sides of the short walls have the same amount of soil against them.

 

[FalsePrecision]

OK, I understand - the hollowcore can act as a rigid diaphragm per the UcfSe post above. The short walls can act as shear walls. However, the floor diaphragm is very long, and the short walls can take only so much shear + overturn moment. There is a concept where the back wall can act as a "hybrid" cantilever/restrained wall. I have used it in the past, but is walking on thin ice legal-wise.

 

[joemarch]

You understand it correctly FalsePrecision. Why would you say this design is walking on thin ice? Is it because of the potential for the movement of the building causing cracking? What were some of the issues you had using this "hybrid" design?

Thanks again for your help

 

[FalsePrecision]

The wall itself- the bending moments go thru large reversals during the construction sequence. Wall shoring during construction needs to be defined carefully. You are hoping the Builder does this exactly correct. I only did the "hybrid" design as a favor to a builder, to save him money.

 

[joemarch]

I'll make sure the wall is braced correctly throughout construction. Is that about all I should watch out for?

How come it always seems we stick out our necks to save someone else a buck!!! I won't get started on that topic.

Thanks again. The info was greatly appreciated.

 

[LPPE]

Design the back wall as simply supported. The reaction from the wall at the top will not be very large compared to the bottom (soil loading diagram). The two short endwalls will take a portion of the load introduced into the floor diaphragm, and then beef up what masonry walls you have to account for the rest.
Put reinforcing steel for your back wall on the inside face, and make sure you put on your drawings everywhere "DO NOT BACKFILL UNTIL FIRST FLOOR FRAMING IS IN PLACE AND SECURE". Therefore, no load reversals. And, depending on the thickness of your walls, you may need reinforcing on both faces. If so, design the reinforcing on the inside face to act as a simple beam, and the reinforcing on the outside face to act as the cantilevered wall. Now you have no problems if the contractor "accidentally" backfills before the first floor is in place.

 

[FalsePrecision]

LPPE,
Careful- you have 110 linear feet of load along the top of wall. Could be a very significant shear/l.f. along the connection of hollowcore to the short side walls. Very lage accumlation of total shear.

 

[joemarch]

Your correct FalsePrecisioin. I have 12'0" high walls. Plus I'm using 60psf/ft of soil pressure. Treating the back wall as simple supported and taking this load to the end shear walls would be very tough to do. I have some other intermediate cmu walls but with this sort of load I think that this design is still very far from working.

 

[neffers]

I have ran into this issue in the past. How about if you try designing the back wall as fixed at the base and pinned at the top. This method greatly reduces the lateral load reaction @ the top of the wall. You will get a slightly larger footing than a simple span wall but not as big as if it were a cantilevered retaining wall. Just my $.02

 

[jjeng2]

I read your post and quickle scanned through the replies. I may have missed a lot of info but in general when I do these...
If the wall will be braced at the top or if you can design the precast to act as a diaghphragm, I would design it as a simply supported wall and require it to be shored until conc. has reached 85% of design strength and first floor is completely built. If you cannot get the first floor to fully brace the top of the wall then I would design it as a cantilevered retaining wall. That will generally be your worst case scenario anyway. Usually, if it is a 12" wall, I reinforce both sides anyway and #5@12" each way should take care of any bending on the other side if it develops any simply supported action for a typical basement wall.Also keep in mind the movement of the wall if it is a cantilevered retaining wall. Normally, I would build the wall with a tilt into the soil so it is straight when it is loaded but you need to really analyze this in this situation because your construction above the wall needs to line up with the wall.

 

[miecz]

We once reviewed (3rd party) a building with a deep (20 foot) foundation, where the driving force on the foundation exceeded the sliding resistance for the building. The designer needed to key the foundation to develop passive resistance.

 

[samdamon]

I suggest designing the wall for two conditions. First as a classical cantilever with a wide footing, because its very likely the conractor will need to backfill the wall to near the top before the second floor or perhaps even first floor sog is in place or finished, and you will thus need both the stand-alone sliding and bending resistance. Then also design also as fixed at the bottom and pinned at the top as a basement wall. You'll thus be designing for a bending stress reversal, and in essence will have tension steel on both faces of the wall- you'll be covered no matter how the structure is erected or how it actually behaves during its service life. Don't think there is much extra expense in this approach since for a 11' high wall you are probably looking at 10" to 12" thick or so already with a double curtain of bars for temperature and shrinkage anyway.

Having the big cantilever footing also helps if at some time in the future a substantial portion of the first floor sog needs to be removed for trenches or other reasons. It gives the owner flexibility. Regards.