Foundation Design for Prefabricated metal Building

[KHTODD]

I'm a General contractor that has recently ventured into the prefabricated metal building market. I live in rual NC and have found that Agriculture can be a large part of my prefabricated building market. In NC Farm operation building projects do not require building permits. The building manufactuer provides engineered drawings as well as column reactions on the prefabricated structure itself, but no foundation design. That being said we all know its about the bottom line as well as liability exposure. In order to be more competitive in this market, how can I take the reactions provided by the manufacturer and calculate a suitable foundation design that will will be adaquate as well as cost saving? Comprimising the integrity of the finished product is not an option.

 

[Ron]

KHTODD...first of all, you are to be commended for asking the question. As a contractor, I'm sure you know that many of your competitors wouldn't ask...they would put in what they think would be necessary...either way over designed (wasteful, but safe) or way underdesigned (potentially unsafe and prone to property damage).

Given that, I would suggest that you engage a local structural engineer to help you. The reactions can range from low to high and affect the soil bearing and the footing design. Even though a building permit might be waived for agricultural buildings, you still need to follow the building codes (NC State Bldg Code).

 

[DHKpeWI]

I have designed the foundations for several buildings just like this. Insist that the metal building designer provide the individual load types (Dead, Live, Wind, Seismic, etc.) to your engineer. Often the metal building engineers provide only load combinations. The footing engineer will sometimes combine the worst uplift/vertical force with the worst horizontal shear regardless of whether these loads are acting simultaneously. This can result in footings much larger than necessary.

I know engineers in my state who are registered in NC.

 

[CPENG78]

KHTODD,
I have also designed foundations for such type buildings and like Ron mentions, if the question is not asked the building ends up with an extreme of either over or underdesigned components. Even though the building permit may be waived for this type of construction, the building codes such as the NC State Building Code, International Building Code, local code amendments or any combination of the previous still need to be followed.

In my experience, either the purchaser of the building, the building manufacturer or the contractor would come to us to complete a structural analysis report. This report would cover analysis from roof to foundation loads. The reports would get a little more involved when people began to want to use these pre fab buildings for other types of uses and loading requirements would change accordingly.

Even when a building manufaturer provides system performance data, plan checkers may still require structural analysis reports like the ones that I have produced in the past.

The foundation design would certainly be directly tied to the practical use of the building. For example based on lateral analysis (wind, seismic) and actual building use, the structural analysis report would include a series of foundation options that would be suitable for that specific building, thereby giving the client and contractor the freedom to choose on the most cost effective option that meets the loading requirements.

I would also either engage a local structural engineer or I also know engineers in my state that are registered in NC.

 

[BlastResistant]

If you want the foundation design done properly, you really should engage a local structural engineer. Even if you are erecting the same exact PEMB at 20 different locations all within an hour's drive of each other, each location can have vastly different foundation designs due to the varying topography, soil conditions, and wind parameters. Other engineers may disagree, but as a structural engineer I would not design a PEMB foundation without a soils report either. If I did, you would probably end up with a super conservative design that will cost a lot more extra money to build than the few thousand bucks that a soils report will cost. The best thing to do would be to partner up with a local structural engineer who has a good relationship with a reliable geotech engineer and have him do all your foundation designs. The more work you do with him and the better you get to know each other, the better his pricing will become!

 

[weab]

Agree with Ron - great question. ASCE offered a webinar on this very subject today. I just assumed that most people use footings but apparently not. There are significant issues to consider. A structural engineer is a must - this said by a structural engineer.

 

[MikeE55]

Usually wind uplift controls on this type of building, and for an agricultural structure, very often there is no floor slab, so thrust loads are also a major concern. The footing design is complicated, and should be done by a structural engineer as everyone has stated.