Foundation design for Pre-Engineered Building 3

[Chewi00]

I am doing a foundation design for a 50' x100' pre engineered metal building and it's tricky because I am designing the foundation before the building is designed (custom design). My question is , can anyone give me a general idea of the maximum column spacing that would be used for a building of this size?

Thank You

 

[JAE]

Most Pre-engineered steel buildings in the US use either a 20 ft. or 25 ft. bay module as this is what standard 8" Z purlins can span for roof loads and for wall girts.

Dead loads range between 10 and 15 psf for the roof.

The difficult thing to estimate is the lateral reactions of the moment frame bents as the different sizes, tapers, and relative rigidities vary the horizontal thrust at the columns.

 

[steve1]

With all due respect I think you're asking for trouble.

Foundations for preengineered buildings typically consist of footings, piers, and grade beams along with a slab on grade. The footings and wall piers are cenetered under the columns, and the grade beam is set up to carry the bottom of the metal stud wall. All of this geometry is a function of many factors that are outside your control. How wll you ever be sure that the columns will line up with your footings. It is typically done the other way around.

That having been said, if you can preorder the building you may be able to get a preliminary, not for construction, set of plans from the manufacturer. You would then have to write into your spec that the building would have to conform to the preliminary layout.

I would not recommend designing the foundation without any building plans.

 

[Chewi00]

Yes, I realize the difficulty with this case and that it is usually done the other way around. I was asked to come forth with a preliminary design due to the fact that the building is a custom build and will not be available in the preliminary design. I know...I know....all the things that can be said about the way this is going but if I could just get some general knowledge on the column spacing for this 50' x 100' building, it would help me a lot.

Thank You

 

[jheidt2543]

In what part of the country will the building be located in?

A "typical" PRELIMINARY foundation design for a metal building in the Midwest, subject to snow loads, with a frost penitration of about 4', that would allow a general contractor to price the foundation system, based on allowable soil bearing of 3,000 psf, would be:

Perimeter frost wall footings: 24" wide x 10" deep
w/ 3 - #5 cont. and #5
transverse at 30"

Perimeter frost wall: 10" wide x 36" deep
w/ 2 - #5 T & B and
#5 vert. @ 4' o.c.

Spot footings - Interior: 6' x 6' x 12" thick, no pier
w/ 6 - #6 each way

- Exterior: 6' x 6' x 12" thick, w/ pier
w/ 6 - #6 each way

Column Piers: 16" x 18" x 36" deep
w/ 6 - #6 vert. and
#3 ties at 12" and
#6 hair pins 10' long each and
(4) anchor bolts at each col.

Floor slab: 6" thick with 6x6-6/6 mesh

IF the building is 50' wide clear span, then there would be no interior spot footings and bays would be at 25'.

IF the building is 100' wide, then there would be a row of spot footings and bays could still be at 25'.

The above also assumes both end walls are expandable. In either case, there would be wind columns in the end walls that would sit on the frost wall.

REMEMBER, this would be for preliminary pricing only and the final design must be based on the loads provided by the building manufacturer on the final, approved shop drawings.

 

[JAE]

There isn't any trouble involved with this if you follow these steps:

1. Review two or three Pre-engineered bldg company manuals to get a concept of how these buildings are laid out.

2. You can dictate on your design/bid plans what roof loads and max. dead loads your foundation is designed to carry. Many times we estimate the dead loads a bit high to allow for variations in the actual loads.

3. Set column locations to accommodate the different companies guidelines. - but most of these are as I said, on a 20 foot to 25 foot bay. 30 foot bays are also done.

4. Indicate on your plans what column loads you are basing your design on and indicate that if the actual column loads that result from the actual building are higher, then the contractor/building manufacturer must notify you to adjust the foundation design (your design loads getting exceeded is rare).

5. Require an engineer's seal on the building design.

 

[Cthornton]

I've designed quite a few of these foundations. Here are a few important concrete details that I have often seen over looked on Pre-Fab building plans:

1. Overall slab dimensions will typically be larger than 50'x100'. Typical slabs will be 50'-3" x 100'- 3" because of a 1 1/2" sheet ledge permiter around the entire foundation. The sheet ledge may vary depending on building mfgr. Most are 1-1/2".
2. Sheeting ledge should be stopped at all walk in doors.
3. If overhead roll up doors are used a detail should be shown to slope concrete away from the door. Otherwise water can accumulate around roll up doors.
4. Plan for concrete to be pumped.
5. If you know who the building mfgr. is I suggest contacting them.

 

[pmkPE]

Do you know the parameters for the pre-engineered building? Low eave height, roof slope, spans, column spacings, minimum clearance below steel, etc. I have designed many foundations for pre-engineered buildings before the building was designed. And I have approached the problems in different ways: 1) I do the analysis myself, 2) I review loads/reactions from standard manufacturers published literature (Butler being one that comes to mind) and 3) I call a rep. for the pre-engineered building under consideration and they usually generate the loads based on the criteria that I give them for the geographic area that the building is to be built. They all have computer software that quickly generates the reactions, the base plate dimensions and number of anchor bolts and their spacings in a matter of minutes. They fax me the answers which can often be as many as twenty sheets for each and all of the load conditions; then I do the combinations to meet the local code, (if their software did not do it).

As the foundation designer I eventually see the final loads/reactions and can make modifications as required. However, if you have not done many of these you should notify your client in advance that your design is preliminary and is to be only used for developing budgetary costs, as the design may change if they the owner does not commit to the building parameters up front.

 

[dicksewerrat]

My question would be'What's so special about this bldg?' If there is a special use, it may add to the foundation. also believe and follow advice listed above.

 

[J1D]

pmkPE,
I don't know in what circumstances you can get the info from a manufacturer of pre-engineered building if it is not a vendor (or not yet). You can get that for free?

 

[pmkPE]

J1D,

I often times, on behalf of the owner, will select a building manufacturer (depending on what I have liked about their construction or where the job is located in the past). Then I contact their represenative and get them in touch with the owner so that they have a good feeling about getting the assignment. Often times they know that they are to get the work (I do a lot of industrial jobs where our clients are not locked in to picking the lowest price, but can pick a prime contractor based on performance (what a novel idea)).

Other times I run out the calculations by hand and can readily size up the worse case loadings; I will then do the foundation design and then after the job is awarded I have the building manufacturer send me their loadings. I review against my original design loads - I can not think of a time where there was any major changes other than revising bolt sizing and/or spacing to match their preferred orientations.

 

[J1D]

Achwlidog,
The preceding messages gave pretty good ideas of the procedure. We designed a lot of pre-engineered building foundations as well before the buildings were designed. We typically issued the architecture dwgs with design live loads to the bldg manufacturer, to whom contract was awarded, and reviewed their design dwgs later. Before this time, the fdn was designed. In this case, we designed the anchor bolts and asked the bldg designer to check.

If you are working on behalf of the owner or the bldg manufacturer is your subcontractor, you can get more help from them as described by pmkPE. But in any case, you need to have the design experience of this type of building.