Concrete Grade Beam

[Buzzer]

I am working on engineering a house for a friend of mine and unfortunately his soil report came back suggesting that he need piles. The geotechnical report is going to give us the pile capacity and size as well as driving criteria. Now, Several questions arise.
1) do we just figure the weight of the house and divide it by the pile capacity given by the pile report to figure the number of piles required?
2) The footing (or grade beam) that spans between piles, Don't we just design like a simple concrete beam or say a beam spanning several supports? This also, I am assuming could have an effect on the number of piles required.
3) Is there a rule of thumb for figuring the weight of a house based on light frame construction?
4) Is there any other considerations to be accounted for?

Please help, thanks
Buzzer

 

[SlideRuleEra]

Buzzer - You would look at the load combinations required by the building code, which would include the dead weight of the house. However typical pile capacity is usually several tons per pile. The arrangement / spacing of piles needed to keep the grade beam spans reasonable will almost certainly mean more than enough piles to carry all vertical loads for light frame residential construction.

Unless the house is at the edge of a swamp, elevated for flood protection, etc., it would be unusual for this type construction to require piles. If they can be used, oversized footings will almost certainly be more cost effective. A second geotechnical opinion may be worth the expense - pile driving / drilling is costly.

As for the weight of a house, see Chapter 3 in the "Residential Structural Design Guide: 2000 Edition" at this link (free .pdf download)

http://www.pathnet.org/sp.asp?id=1442

http://www.SlideRuleEra.net

 

[dik]

Item#1: No. Piles may require placement at corners, etc. and the number obtained by your method will be the absolute minimum. Also long beam spans between piles may result in abnormal amounts of beam reinforcing or size.
Item#2: The grade beam should be designed for the loadings. If the beam is continuous, then this should be taken into account for both loads and deflections. Also consider some fixity caused by the beam at right angles.
Item#3: Yes there are... but these are generally used by house movers. Best to determine the loads on the beam.
Item#4: I'm not sure what your discipline is, but, if you are engineering a house you might want to contact a professional of that discipline.

Dik

for others: in case some turkey red flags this response, if he's engineering something, in our environs, he must be a registered engineer.

 

[archeng59]

buzzer, your questions indicate that you have little or no experience designing foundations, which should be performed by a qualified licensed structural engineer as dik indicated. even if it is "just" a house. a poorly designed and constructed foundation can cause your friend problems in the future.

 

[Buzzer]

Thanks SlideRuleEra. yes the piles are probably going to be good for 10 to 20 tons each which would indicate that there would be more than plenty to hold up the home. I have that download and am familiar with it. Right now he is in the planning stage so we were trying to get off on the right foot. My first suggestion to him was to get a second opinion and he still may. I'll email you with some more of the specifics since there seem to be more people willing to try and discredit one's abilities and knowledge than to help.

I always thought this forum was one to be used to gain knowledge or gather information that you needed. My questions were not whether anyone thought I was qualified as a licensed structural engineer.

 

[PSlem]

For your average two story, grade beam will be about the same size as a conventional footing if piles are kept to 6-8' spacing. Reinforcement will be required at the top and bottom. Easiest to just make both continuous. Size will keep shear low wnough stirrups not requred. Residential exempt from seismic, but if a high seismic area, you may want to add stirrups. Loads will vary from about 1500 PLF for a vinyl ranch to 3000PLF for a two story brick. Don't worry about joist orientation and just treat everything like a diaphragm.

 

[UcfSE]

No one is trying to discredit your abilities. We simply can't hold hands with someone who is asking some seemingly very simple questions. There are liability and ethics issues at hand and we try to cover those bases as well. The forum is to be used to gain knowledge, insight, interpretations and opinions but it is not to be used to save a buck over hiring a knowledgeable professional.

That said, make sure you check in your local building code for any pile minimums. Sometimes there are requirements for spacing or minimum numbers of piles per cap, and so on. You will design for the reactions of the grade beam on the piles. These reactions include any dead, live, wind, snow or seismic loads and may be due to lateral as well as vertical loads. It is not just the weight of the house. You may design the grade beam as simply-supported but you will have to have steel for negative bending as well or you will get excessive cracking. You will have to then find the reactions of the grade beam as a continuous member since the reactions are higher than they would be for a simply-supported beam.

 

[Buzzer]

Thank you for your input.