Metal Building Foudnation Design

[SteelPE]

Recently I was awarded two small projects from two end users. The projects consisted of the design of some metal building foundations. In each instance the columns rested on a knee wall that was approx. 3’-0” high with a required frost depth of 4’-0”. In each instance I have approximately 15.0 kips placed at the top of the wall. I designed the foundations and sent them off to the client only to get irate phone calls back about the size of the foundations I selected. The issues at play could have easily been mitigated by making minor modifications to the building during the design but this is not what I was hired to do. I see foundation designs with similar loads (15 kips applied 8’ above the footing) and 4x4 footings being used. I know there is no possible way an engineer can justify their design.

I am beginning to realize that doing work on this end of my industry just isn’t worth the hassle (end user metal building foundation design). The clients can’t really grasp the idea of the forces at play and never really appreciate the difficulty of the problems that were solved.

 

[JStructsteel]

Welcome to the club. I have done one MB foundation and passed on many others. Most clients/contractors dont realize the crap they end up with until the building is on the ground. Usually the contractor is long gone, and all that is left to go after is the engineer. Stick to your design, or fire yourself from the job if need be.

 

[SteelPE]

I have designed these systems for 18 years averaging about 6-10 per year. I also have a mentor who was the chief engineer at a large manufacturer a number of years ago who I go to for advice. So my designs are not based upon lack of knowledge.

I usually work with contractors and rarely end users however, recently someone I used to work with became a sales rep for a metal building company and he is sending his clients my way. While I appreciate the thought and am flattered by the recommendation, I am beginning to realize that this might not be worth the effort.

 

[MotorCity]

I would say to try and consider any assumptions that the other engineer could have made to justify the 4'x4' foundation. If you can validate any of those assumptions, see if they will make a difference in your foundation size. In other words, can you "sharpen your pencil" at all or are you being overly conservative in any areas? If the answer is no, then stick to your design. Were the irate phone calls from the owner or contractor?

 

[j19]

SteelPE - You are preaching to the choir. I don't do very many metal bldg. foundation designs but almost without exception the owner will question the footing sizes when the contractor tells him that they are too big. I've gotten to where I tell the owner that if he had decided to have the building constructed with block walls then the footings all of the way around the perimeter would be bigger than the small turn-down slabs that are on metal buildings (in the south). With metal buildings the concrete is concentrated at the columns rather than spreadout around the perimeter. At least that's my story.

 

[Leftwow]

SteelPE - A 4'x4' footing seems pretty minimal to me, especially when you have to design for wind and earthquake lateral loads. I usually have to explain my case when designing rectangular footings to the project manager. You've got 2 foot to the edge from the centerline of the column. I always take my hands to show how big 2 foot is. You have 2 foot on one side, and 2 foot on another. For some reason the number in feet always sounds HUUGE, until they conceptually visualize it.

Perhaps the depth of the footing is what they are crying about? If I was the designer and they wanted an engineering stamp, then I would have to design to the frost depth, period.

 

[JedClampett]

SteelPE, if you do a search on my threads, you'll see this is a common complaint with me. It's like the PEMB designer is purposely making the foundations, and my favorite, the anchorage, impossible to design. Four anchors, spaced on a 4 inch grid, sure, what the heck! I've tried multiple times and in multiple ways to get the PEMB supplier or the GC to do the anchorage calculations, and they've slithered out of it each time. They'll give me four anchors with just enough area to meet the steel tension allowable, and no recognition of the concrete design. I have to be the bad guy, similar to your experience.
I don't know a solution for this. Maybe if everyone puts their foot down and won't design these foundations, the PEMB engineers will have to get involved to stay in business. Or owners will realize that the building needs to be on a foundation and you need to consider its price, too. But someone is always willing to take the money, put in a 4' by 4' foundation and be on their way.

 

[XR250]

@SteelPE, is the issue the gravity load or the column thrust?

I don't design these anymore either =- too much hassle for the money.

 

[BSVBD]

The last PEMB foundation I designed, with a grade beam foundation, I designed:

12' x 3' x 36" thick footings on the side walls. T.O.F. = 100'-0"
7'-0" x 7'-0" x 18" thick footings at the corners. T.O.F. = B.O. grade beam.
- 33" long anchor bolts.
- With footing centered on anchor bolts, the portion of footing outside the perimeter is below grade.
8'-6" x 8'-6" x 24" thick intermediate footings. T.O.F. = 99'-4".

These footing sizes are to resist excessive uplift.

This is not uncommon for me, especially with grade beam foundations.

Fortunately, I work with a very reasonable general contractor client. I do at least one PEMB foundation every month.

SteelPE... I'd also be concerned about the thrust on top of the 3' high knee wall. What was your thrust? Did your pier V-Bar account for this?

 

[SteelPE]

BSVBD

This complaint was more of a general complaint, not about a specific project. We usually check the vertical bars in the pier for the thrust. The can be an issue as we try to keep the bars a small as possible, so we will tend to use more bars vs larger bars in the peir.

Good news is that I was able to meet the client and I went through the problems with him and he seems to understand.

I don't mind doing these designs with contractors I work with. The projects are usually small and I can make decent money squeezing them in-between larger projects.

 

[dflva]

Many of us have received this over-design rant from an owner or contractor at one time or another. I find they're usually leaving a piece of the design out in their comparisons.

Examples:
Owner: "My last building having the same spans used only a 7 in. slab, yours is way too thick at 8 in." . . . but he fails to mention his 7 in. slab was post-tensioned, while the current design is conventionally reinforced . . . and he also says nothing about the design loading.
or
Contractor: "I've built many retaining walls of this height and not one of them had a base that large." . . . but he fails to mention none of them had a 1:1 sloping backfill which the current design considers.

I suspect your Metal Bldg owner is not presenting the whole picture. Perhaps his building with 4x4 footings used "hair-pins" or cross-slab connecting rods and yours does not?
Usually when owners and contractors make these accusations of over-design they're not comparing apples to apples.

 

[DaveAtkins]

The size sounds reasonable if hairpins are used. Or am I missing something?

DaveAtkins

 

[WinelandV]

Dave,

From SteelPE's description, the columns are resting on piers that have tops that are 3' above the floor slab. Thus, you've got 3*thrust (kip-ft) of moment to take down to the footing.

 

[DaveAtkins]

I am not sure I agree. If you have hairpins, you can create a resisting couple, with one force at the slab-on-grade, and the other at the footing. No moment is transferred to the footing. Or perhaps passive pressure and friction under the footing are not sufficient?

DaveAtkins

 

[SteelPE]

Dave,

While I understand the concept and I used to use hairpins before, I no longer use them (this is at the advice of some colleagues/mentors). We just find that they are too unreliable especially since they typically rely on the WWM to transfer the loads to the other side of the building. When we did use them in the past we would limit their allowable design capacity to around 10-12 kips

In the instance you give of forming a couple, the load on the hairpin will actually increase due to prying action (using passive resistance will only make the situation worse as the lever arm below the hairpin will actually be less than it is if you assume bottom of footing). In the instance I have given (3' knee wall and 4' frost depth) a 1k thrust at the top of the wall will 1.86 kips in the hairpin (assuming the hairpins are placed at the center of a 6" SOG). So it doesn't take much thrust at the top of the wall to negate the effects of a hairpin.

Just my $0.02

 

[spieng89]

@SteelPE. I agree with not using hairpins. I design PEMB foundations to resist the shear (thrust) and induced overturning moment if columns are on a pier.

 

[DaveAtkins]

And I routinely use hairpins. If the thrust gets too large for hairpins, I use continuous bars across the building (a tension tie, essentially). I follow that Butler Buildings guide from many years ago, authored by Jim Fisher.

DaveAtkins

 

[SteelPE]

Jim is great and has some good details. I had a black book a while ago about metal building design the went through their design, not sure if it is the same one you reference.

In regards to tie rods, if you have 40 kips of thrust do you just size the rods to resist this load or do you place some additional reinforcing to account for the elongating of the rods? If this load could be resisted by 1 square inch of steel(which is plausible) the elongation of the rod would be 0.0165 inches/foot of length. So for a 80' wide building you are looking at an elongation of 1.32 inches.

My colleague/mentor was the chief engineer for a very large metal building manufacturer and knows Jim very well, he still refuses to use them. Also in regards to tie rods, his theory is that by the time you are done trenching and placing the rods and encasing in concrete you might as well just make the footing bigger, then you don't have to worry about the elongation of the rods under load (which can be significant).

 

[TLHS]

Alexander Newman has a pair of books about pre-engineered metal buildings that are practical and well thought out. They talk a lot about pitfalls and how to avoid them. One is "Metal Building Systems Design and Specfications," the other is "Foundations and Anchor Design Guide for Metal Building Systems." I highly recommend them.

 

[SteelPE]

I have the anchorage book already and which talks about the pitfalls of each foundation type