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Safety and reinforcement in foundation 1

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bojoka4052

bojoka4052

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I did calculations according to EuroCode to find the degree of utilization of the reinforcement in my foundation. It came at just over 1, or 100%. It was 1.02 or 102% utilization. I was told its fine since its precast concrete and we use safety factors.

I am not sure how to justify this in text; should I just write: "As the degree of utilization is only slightly above 1, it is considered to be acceptable as the foundation will consist of precast concrete and we have used safety factors"? I think I get the precast concrete argument, as it will be cast under more controlled circumstances, but what about safety factors, isn't the degree of utilization supposed to be under 1 with the safety factors?
 
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Or you could just sharpen your pencil on your applied loads slightly to bring them down to just under 100% utilization. Most times that's why I'm conservative on my loading, so when I come up against something like that, I can just reduce the loading slightly and no changes to the design or drawings are required.
 
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Putting the safety factors applied to the loads and capacities to one side, there are probably several other conservative assumptions in your analysis that haven't been considered. A few I can think of:

*The actual characteristic yield strength of reinforcement probably being somewhat greater than the nominal characteristic strength assumed in design
*The benefit of load redistribution in the entire structure after cracking and localised yielding

My understanding has always been that the nominal characteristic strength (i.e. the specified reinforcement grade) is still somewhat less than the true characteristic strength. This probably comes down to tight quality control and risk avoidance by the manufacturer.

In Australia for example, Grade 500N reinforcement is required to have a characteristic (i.e. 5th percentile) yield strength of 500 MPa. The average yield strength when tested normally comes out at least around 550 MPa, with little variance for a particular heat. My gut feeling is that the true characteristic strength must be up around the 530-540 MPa mark. Obviously, none of this reserve capacity is guaranteed or required, but it is almost certainly there.

This is always at the back of my mind when a design comes in at 101% or 102% utilisation. I wouldn't worry about it.
 
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It was actually 103% or 1.03, but I guess the same goes for this utilization aswell?
 
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If you need to put something in writing, you can't just say "Good because of safety factors". Although we as engineer's think that and would let it go when we've quickly reviewed something, if it needs to be put into writing, we need to ensure we can prove that it is below utilization. Whether that proof is by being more diligent on your loading and analysis, or using the in-place strengths of materials as opposed to conservative numbers, depends on the situation. But you still must be able to show on paper it works.
 
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If your calculations say 1.03 or 103% utilisation, then it is not code compliant if your code requires you to design as per limit state approach.

If something goes wrong with the element you designed (and its not your fault but a force of nature or a builder mistake), a lawyer will have a field day with simply saying that the design was not code compliant. You will get inadvertently pulled into the finger pointing discussion.

In my opinion, its important to remember that Structural Engineers duty is to uphold the requirements of the code.

That being said, as mentioned above, I'm sure you can probably find that extra 3% in an "above board" way such as:

- Have you used all the live load reduction you can?
- Are your cover assumptions conservative? Your not casting directly against ground so you may be able to relax your cover, increase your effective depth and reduce demand on reinforcement
- Is your peak design moment location accurate? (face of column or better depending on the code you are designing to)

For the latest insights into the art of Structural Engineering
 
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If I have to I’ll live with up to 5% over, since I know if I do sharpen my pencil I’ll get it in under the line if I absolutely need to.
 
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No lawyer is going to successfully pick you apart for a few percent over. Allowing a few percent over is commonplace in the industry. We're not building a watch here folks and the attorney on your side will point this out.

If you are confident in your loading and analysis, then don't sweat it. If you (or others) made unconservative assumptions or your analysis is not conservative, then a few percent over is compounded by other factors.

And not all elements are the same. If you are a few percent over, for example, in deck welds where those welds are the only wall attachment for a building's endwall to the roof diaphragm, then you might not want to stress things right up to the limit. This is where engineering judgment and experience comes into play.

Our work is not black and white - at least I prefer not to think of it that way. There are many, many unknowns. There is a certain art to it. There is no way to know within a few percent where your design really stands.
 
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When faced with that situation, when I check the design load vs. capacity, My conclusion in the design states "Say OK", rather than just "Ok" for one where the load is lower.

If you do want to get it down that 3%, in addition to the suggestions above, consider increasing your design concrete strength, if it can be met economically by local production facilities.

Moving the reinforcing a fraction of an inch would likely be the easiest way to boost the capacity. A more detailed method of calculating the distribution of concentrated loads could also help.

Depending on the configuration of the foundation and what is failing, an increase in the soil bearing capacity could reduce the design loading. Our geologists are usually amenable to taking a closer look at the foundation material properties (or just the elevation of the footing, if their initial assumptions were different than the final bearing depth) and giving us the ok to use a higher capacity. (Once, the geologist doubled the allowable bearing capacity during a 2 minute phone conversation, which kinda made me a little nervous - he should have at least pretended to review the data.)

Rod Smith, P.E., The artist formerly known as HotRod10
 
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Just a caution... with foundations, I generally don't like a 'tight' design. In my experience foundation costs are pretty insensitive. Look to Millennium Towers for all that money that was saved due to a reduced foundation cost.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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