How much does a residential house weigh?

[joenorm]

My engineer designed my foundation for 1500 psf soils because there was no geotech report and it's my understanding that it's standard practice to design down to a conservative number.

My question is what kind of force does a typical two story, wood framed house exert on the soil? How much does it weigh?

thanks

 

[JAE]

In the midwest USA....roughly:

Roof Dead Load - 15 psf
Roof Live Load - 20 psf (or snow load depending on location - 20 to 50 psf or more)
Second floor Dead Load - 15 psf
Second floor Live Load - 40 psf (or 30 psf for sleeping rooms)
Main floor Dead Load - 15 psf
Main floor Live Load - 40 psf
Exterior walls - 10 psf for stud/wood siding - add 40 psf for brick veneer
Basement walls - depends on type - solid concrete about 100 psf for 8" wall. Unreinforced block - 40 to 50 psf. Reinforced block - 60 to 70 psf
Include footing weight


Sometimes floor dead loads can be larger with tile or an additional layer of hardwood floors
Roof can be heavier with tile roofing or concrete shingles.

 

[joenorm]

So is the total PSF just all those things added up? So your example would have a total PSF of about 250?

 

[Okiryu]

I expect that your designer may know at least the type of soils and chose the correspondent presumptive soil bearing value indicated in the IBC (if you are in the US). 1500psf is in the lower bound for allowable bearing pressures and it the presumptive value for clayey/silty soils.

The adequacy of the selected 1500 psf is also depending on the type of foundation system. If it is a mat foundation, the 1500 psf may not that bad but it will affect deeper soils. Anyway, if you are worried about this and if your area is known for having soft soils/foundation problems, I would suggest to contact a geotechnical engineer.

 

[EireChch]

Firstly I would say is you are looking for trouble doing design without a geotech report. I know it is done for thousands (or prob millions) of houses around the world and most turn out fine but if youre the unlucky one whos building on fill, peat, soft clay etc, eventually you will have to pay 20x the cost of a geotech report to fix differential settlement, cracks, doors not closing etc. If had no geotech report I would at least rent a man and a backhoe JCB for the day and dig a test pit. Any engineer designing foundations should have some idea of soil type at least which is a start.

Anyways, I think on average you are looking at 5kPa for your slab and max 50kPa for perimeter and internal strip footings. Ive never had to work it out but that would be my best guess. The structys are better placed to advise.

 

[joenorm]

Of course having a GEOtech report would be optimal. But it's rarely done for residential around here, and where it's worked once, it will work again, right?

I'm not so much worried as have just got much more curious about how the calcs are done and what it's all based on.

There are so many forces at play when designing a foundation, it's interesting.

Are there any very introductory books on the subject? Geotech and foundation engineering for dummies?

 

[JAE]

No you do not simply add up the values I posted.

If you don’t know that then I’d suggest you ask your engineer who can show you how he derived the footing Sizes.

Using 1500 psf allowable soil pressure is very typical and allowed by most adopted building codes including the IRC in the US.

Would I get a Geotechnical report if I was building a new house? Yes but I’m a nerd engineer. Is it really needed? Not unless there’s evidence at the site of past large grading operations or if the locale has a history of things like EireChch mentioned.

 

[BridgeSmith]

So is the total PSF just all those things added up? So your example would have a total PSF of about 250?

Well, that's the first step. After you add it all up, then you multiply it by the tributary area for each of the supporting components (exterior walls, interior supporting columns, carry it down through those components, and apply it to the effective area of the foundation. This is where the geotech's expertise comes in. Once the effective area of the foundation is established, the structural engineer designs the footing to be adequate to distribute the load over that effective area.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JAE]

No you don’t first add it all up. You first apply the tributary areas based on span direction and proper load path. The you begin to add them up in accordance with proscribed load combinations.

 

[BridgeSmith]

Yeah JAE, that would be the better way. I suppose that even for a house, there are multiple applicable load combinations. Mostly, I was just trying to demonstrate that the adequacy of the foundation for a house takes alot more than just addition.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Did anybody mentioned wind and seismic effects?

 

[r13]

TheRick,

I've no clue.

 

[msquared48]

Wind and seismic can very well play into the foundation in the Northwest, depending on a lot of different factors as you all well know. Seldom have to use use more than 1500 except in higher end residential structures.

As for the average building soil pressure for two story structures, I have seen 800 to 1500 psf on the exterior and interior strip footings, and closer to 1500 psf, by design, for interior spread footings.

I have no problem with the 1500 psf value and doubt that a higher value from a Geotech report would save you much money. Piece of mind, maybe, but not money.

Mike McCann, PE, SE (WA, HI)

 

[phamENG]

It's also dangerous to run down the loads and decide "I only have 1350plf, so my foundation only needs to be 11 inches wide." There are a lot of factors apart from just the directly applied loads. You have to consider stability of the foundation, constructability of the wall above and the interplay of construction tolerances, as well as preventing certain failure modes from manifesting themselves in the soil that are more severe than the settlement we typically target as the "controlling failure."

 

[chicopee]

Was there any indication about the type of house being built? If it is a one story ranch, then I would go to the town engineer who would more familiar about the site on which the house is being built on.

 

[dauwerda]

While geotechnical reports have not historically been used/required in the past for residential in the US, there are now areas that are starting to require them. I know that most jurisdictions along the Front Range in Colorado are now requiring them due to the highly expansive clay soils in the area. Many of the new builds in that area have basement walls that are supported on drilled piers with a wood framed floor above a "crawl space" (not a slab on grade). Many of the older homes (built in 90's and 2000's) that aren't built in this fashion have serious movement problems, as well as associated lawsuits and settlements.

My question is what kind of force does a typical two story, wood framed house exert on the soil?

I expect that a "typical two story, wood framed house" usually doesn't have its footing size determined by calculating this out, rather the table provided in the IRC is used - Table R403.1.2 (IRC 2012) shows that a conventional light-framed 2-story house with 1500 psf soil requires a minimum footing width of 15". See table below:

 

[phamENG]

Related question I've always wondered about. When it says "masonry footing" is it refering to the old school tapered masonry (starts at, say, 16" wide but then steps up to be 8" wide at the base of the wall)? If so, could you actually get away with compacting a layer of gravel, setting in CMU longways, grouting it, and then building a CMU wall on top of it? As a modern structural engineer the thought makes my skin crawl, but then again I have seen 200+ year old structures sitting on just that kind of foundation with nary a crack. (But also several with cracks.)

 

[geotechengineer1]

Who knows! In Canada, structural engineers told me ~50 kN/m on exterior footings. In New Zealand they tell me it's 10 kPa per floor on the footings, regardless of the footing width or the area supported by the footing.