Residential basement wall footing design

[kingpenn3]

We are designing a new home with an 8 foot tall basement retaining wall. Currently, we do not have a soil report and the client does not plan to obtain one. We designed the wall for an assumed soil bearing capacity of 1500 psf and the footings are approx. 60" wide. Needless to say, the client wants smaller, more normal footings.....approx. 24" wide.

Should we lessen the footing width by assuming a higher soil pressure of 2500 psi and state this was assumed on the drawings?
Or should we keep the current design to really cover ourselves?

 

[jdgengineer]

Could you design the basement wall as pin pin rather than cantilevered from the base? Our office personally doesn't commonly do this but in large areas of the world it's very common and will reduce the footing requirements at the base.i imagine stability is largely governing the width of the footing and increasing the bearing pressure may only have a small impact on foundation size.

Keep in mind that if you do design as pin pin the floor has to be constructed prior to backfilling or the wall has to be braced until the floor is in place.

 

[kipfoot]

With the information you've given, no engineer on an internet forum would tell you what allowable soil pressure to use.

I will say that I think you should check your loading. Your 5' wide strip footing suggests that you have a 7,500 lb/ft load on the soil. Or maybe you said 'basement retaining wall' and it's really a cantilevered retaining wall?

Anyway, in my opinion you should always state what soil bearing capacity was used on the drawings.

 

[Canuck65]

It must be either a very tall house or have very large spans. A typical 2 storey wood framed house, with below grade basement, supporting dead loads, live loads and snow loads can normally be supported on 20" to 24" wide footings.

 

[kingpenn3]

Actually our design is a restrained (at top) wall and not a cantilever. We do specify to backfill after floor joist have been fully installed, so this is a pin, pin design. With a restrained wall and sliding prevented by the basement slab, does bearing pressure becomes more a factor?

 

[oldestguy]

Basement walls carry their lateral loads by combinations of several factors, not usually known very well, but in combination, they "work" without any calculations. They are: bending resistance in the long direction, bending resistance in the vertical direction and vertical compression from the wall and building loads. Any calculation usually assumes things for one of these factors and ignores the benefit of the others. Follow a suitable building code and forget the calculations.

 

[jayrod12]

Generally for residential construction where the walls are designed as pin-pin assuming the basement slab and the main floor diaphragm transfer the soil loading from one side to the other, the footing is only designed for the gravity loads and any shear wall loading from the above grade structure.

Around here, 30" wide is the typical for a 2 storey wood framed house. As kipfoot mentioned, your size calculated means you have 7.5k/ft of vertical load coming down which seems quite unlikely in residential construction.

 

[oldestguy]

OK guys: Here is a question as to "designing basement walls". In the past and still in some areas it is common that basement walls are some form of block, such as concrete or vitrified clay, no reinforcing. Sure some have failed, as saturated earth fill or frost is too much load, but designed as pin and pin? no way. Not even designed. Yet they worked.

 

[rdubs]

The IRC contains prescriptive minimum dimensions for residential footings, including 1500 psf soil conditions... TABLE R403.1 MINIMUM WIDTH OF CONCRETE, PRECAST OR MASONRY FOOTINGS. Is there a reason this code section would not be applicable?

 

[jayrod12]

Not even designed. Yet they worked.

You are also right there. Around here for typical residential construction, it's laid out clearly in the building code how tall it can be, what it can support in terms of storeys above.

However we don't do block walls here, ever. For which I am grateful. Old houses are rubblestone foundations, the rest are concrete. Some (very few but still some) are PWF wood foundations.

 

[kingpenn3]

Problem found...We did not check the box for "the stem base is pinned", creating a pin, pin condition. This solved the problem. Also, we can and really should have used the tables in the IRC.

 

[dik]

As Canuck notes... typically strip footings are 2' max width... any reason it is so wide? What is the tributary width of loading, how many storeys, and what is the wall construction over? A load of 7.5 klf seems excessive.

Dik

 

[oldestguy]

kingpen: It goes to show using cook book methods of designing needs common sense in addition.

 

[kingpenn3]

oldestguy: Agreed...

 

[dhengr]

And, why is common sense so uncommon? Once again, it has nothing to do with having a vaguest understanding of the fundamentals of the actual structural problem, but rather, the damn computer program couldn’t read their mind about what they wanted, and wasn’t cooperating.
[pre][/pre]

 

[SLTA]

Wow, dhengr - grumpy much? and you've never ever made any mistakes or had to ask for help?

kingpenn3, glad you figured it out. My state code also allows 2000psf before a geotech has to get involved. You should check what your state code says (assuming you're in the US).

Please remember: we're not all guys!