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Metal Building Footer Cracks

Metal Building Footer Cracks

Metal Building Footer Cracks

(OP)
I started a new job (came from bridges and sign structures). I came into this project a bit late. This will probably be hard to understand without pictures. Contractor is building a pre-engineered metal building. He had an engineer design the footer. The engineer came up with continuous footers for the columns with a trench footer to connect each column footer. The column footers were to have #6 bars spaced @ 10" o.c. long. & transverse top and bottom. The trench was #5 bars spaced 15" o.c. on bottom. He also called for I believe #5 bar hairpins at the columns extending 20' in the slab. The column footer cages were to extend to 3" from the outer of the exterior perimeter of the trench. However, the column footer cages were installed and fell 7"-9" short of the exterior perimeter. The frame of the metal building is up but no roof or siding. We haven't poured the slab with the hairpins. We are seeing cracks in the trench footer ranging from 1/16" to up to 1/4" mostly within a foot of the columns. To me, anything over 1/8" is excessive and cannot be argued as shrinkage cracks. Also, the cracks are still growing slowly. I believe they are settlement cracks due to the footer not being continuous due to improper rebar placement and the nature of the cracks. I am worried it could be from a horizontal force since our slab is not in with hairpins. However, I was thinking this force should be small due to no roof or siding. In addition, the contractor wants to skip the hairpins. This in my opinion is a no no since there will be no alternative to resist the horizontal loads. Thoughts on settlement?

RE: Metal Building Footer Cracks

Pictures would be helpful. Soil settlement is not likely unless you have very poor soil conditions (and if so, you might want to re-think putting a building on there). The soil is not heavily loaded at this point.

You can get cracking from settlement of the concrete around top rebar (this is actually a form of segregation of the concrete rather than true settlement), particularly if the cover is not good.

There are numerous reasons for the cracks to occur. Would need more info to help. What were the temperatures at the time of concrete placement? What were the wind conditions? What was the slump of the concrete? Was the concrete vibrated?

DO NOT ALLOW THE CONTRACTOR TO CHANGE THE STRUCTURAL DESIGN BY DELETING THE HAIRPINS. There will be lateral thrust that needs to be accommodated and it is apparent that the structural engineer chose that method to do so.

Also, please refer to the "footers" as footings or foundations. "Footer" is a contractor's misnomer.

RE: Metal Building Footer Cracks

(OP)
I will try to draw something up tomorrow. Soil is a tad clayey but was tested at borrow b. I was told it was not tamped with a jumping Jack before the footing was poured.

The temperature was in the 80s and wind around 10mph. Nothing out of the normal. Concrete was tested and was fine. It was vibrated.

The cracks are mostly in the trench footing. There is no top bar. Just longitudinal and transverse bar which I refer to as a ladder. It is 3" off the bottom of the trench which is 1' wide and 1.5' deep.

I am not allowing the contractor to disregard the hairpins. The client has already agreed to disregard before I came on the job. However, things are on hold with these cracks in the footings and since the rebar was not installed according to the engineers plans we are discussing our options.

RE: Metal Building Footer Cracks

Unless you poured onto jello I also doubt you're seeing settlement as you only have partial dead load on the footings.

Pictures would be highly useful.

Do the cracks run perpendicular to the flexural rebar of the footing or are they randomly oriented? Do the cracks start at discontinuities (typically reentrant corners)? How thick are the footings where the cracking is occurring?

Footer is what you have at the bottom of a document.
Footing is what you place your foundation piers on.
Continuous or strip footing is what you place foundation walls on.

Maine EIT, Civil/Structural.

RE: Metal Building Footer Cracks

(OP)
The cracks are perpendicular to the flexural rebar. They almost mirror one another on opposite walls. All are close to the columns at maybe 1' away. I'm going out tomorrow to map everything. The footings are 1' wide by 1.5' deep where cracks are. It's very close to where they didn't carry their rebar cages out enough.

There is a block wall going up on the outside of the steel structure (non structural). The first two course were started 2 months after the footing was poured. The large crack (1/4" wide) has caused the mortar to separate above the crack. I assume the block was leveled with mortar on top of the footing. One side is level and the other is a hair out of level. That is the reason I was leaning towards settlement.

RE: Metal Building Footer Cracks

Let us know the spacing of the cracks and whether they are short, discontinuous cracks or whether they extend all the way across the footing in relatively straight crack. How long after the concrete was placed did the cracks appear.

Also, the client (unless he is the structural engineer of record) has no authority to modify the structural requirements of the project. I will assume you are in the US....if so, to deviate from the record plans is a building code violation unless such deviation is submitted and approved by the building department. Such a submittal would have to be signed and sealed by a licensed profession engineer.

RE: Metal Building Footer Cracks

(OP)
The cracks extend the full width of the trench footing and are straight. They also extend the full depth to the bottom of the footer. I do not know when the cracks first appeared. I am coming into this 6 months down the road.

The client is exempt from being overseen by the local building department. This is why we are in this issue in the first place. The local inspector would have never allowed the rebar to be placed the way it was. However, there wasn't really any inspector on site. I agree the engineer plans should never be deviated from unless the design engineer was consulted.
This is why I am also apprehensive of posting pictures rather than sketches.

RE: Metal Building Footer Cracks

Your should sketch us a plan view of the column showing the locations of the cracks. I think that there's a serious problem here. Based on the details that you have provided, I think that the columns rest upon eccentric footings which have been incorrectly designed. As a result of poor detailing, the columns are trying -- in vain -- to develop moment connections with the footings. This may be resulting in a wedge of concrete being pried out from the exterior edge of the footing.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Metal Building Footer Cracks

With no top bars in the strip footings, there is nothing to stop crack propagation at or near the column footings. How do you know that the cracks extend all the way to the bottom...is the side of the footing exposed or did you excavate to find out?

With a block wall going up before the slab is cast, how will you now install the hairpins? I'm not a fan of hairpins in general, but in the situation which you now have, there is no other load path for the thrust at the bottom of the column.

RE: Metal Building Footer Cracks

(OP)
Hokie, your thoughts are mine exactly. What is considered to be "shop drawings" really are conceptual drawings. I've never seen such lack of detail in rebar drawings. No plan view. No hairpin details. No calcs.

I would have liked to see a grade beam for the strip footings. I believe that is our crack issues near the column footings. I will post a plan view of the crack locations.

Apparently the footing was excavated before I was on the project. I'm taking others word that the cracks extend to the bottom.

There is still room to fit hairpins in with the block walls. If the blocks need to come out it isn't a big deal. They only have two courses on and they are the half blocks so about 8" in height. I think they were thinking to use those as their forms for the slab (contractor had form issues and had burp outs on the side of footing). That is another issue altogether. I'd rather see the block on top of the slab.

RE: Metal Building Footer Cracks

Just a thought: Are the cracks coincident with any buried utilities, like pipes or conduits? Sometimes they backfill the trenches with real garbage and don't compact it at all. That stuff would settle, you'd lose support and get a crack.

RE: Metal Building Footer Cracks

Just looking at the crack pattern: Was everything poured into earthen forms, or were they formed/cast and then backfilled? If concrete was poured in trenches and such I would say those are shrinkage cracks. Those column footings are locked into the earth and are not going to move. The shrinkage has to come from somewhere, and thus the skinny bits with virtually no rebar are a great place for that to happen. They seem wider than I would expect, but not unrealistically so. There is no top steel to restrain them so they would get quite large.

RE: Metal Building Footer Cracks

I thought that the cracks were 1' away from the columns? Here's what you've probably got:

1) The strip footing thing is hanging up at hard spots like the edges of the column footings.
2) The "hanging up" is resulting in negative, hogging moments in the strip footings.
3) Because your strip footings have no top reinforcement, they're developing cracks to relieve the negative moment.

It's poor design but not a terribly big deal.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Metal Building Footer Cracks

And/or shrinkage. I like that too.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Metal Building Footer Cracks

Most of them look like shrinkage cracks as they occur roughly halfway between the column pads. It is likely that they are all shrinkage but the couple that are close to the column pads could be explained by KootK's negative moment theory.

RE: Metal Building Footer Cracks

Occam's Razor

RE: Metal Building Footer Cracks

Could be a combination shrinkage crack combined with the negative flexure of the continuous strip footing (assuming there is negative flexure).

Maine EIT, Civil/Structural.

RE: Metal Building Footer Cracks

(OP)
I knew we couldn't make the argument that these weren't shrinkage cracks being under 1/8". Then I came to the 1/4" crack which has grown. Then I was thinking the temperature change could help that to crack the mortar in the block. It has been cold here. I think going forward we are going to require the builder to have an analysis done on the footing the way it was built. We will work from there.

RE: Metal Building Footer Cracks

As these strip footings are essentially unreinforced, they are not really structural elements. Why does it matter that they are cracked? As for the masonry supported on the slab edge, I would make sure that it is well articulated. I assume that the masonry is a veneer which is supported laterally by the building structure.

RE: Metal Building Footer Cracks

(OP)
The only thing I can assume the strip footings are for is to make things continuous so things were the settle uniformly. However, that is now gone as none of the rebar connects with the columns or in the corners. The main concern is no rebar under part of the column footings. I was only questioning a 1/4" crack coming from bridges that is not good but bridges are easier to diagnose as to the cause.

RE: Metal Building Footer Cracks

Agree with dcarr and jayrod....these are likely just shrinkage cracks. No structural detriment for the members and locations.

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