Safety and reinforcement in foundation
Safety and reinforcement in foundation
(OP)
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?
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?
RE: Safety and reinforcement in foundation
RE: Safety and reinforcement in foundation
*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.
RE: Safety and reinforcement in foundation
RE: Safety and reinforcement in foundation
RE: Safety and reinforcement in foundation
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)
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RE: Safety and reinforcement in foundation
RE: Safety and reinforcement in foundation
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.
RE: Safety and reinforcement in foundation
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
RE: Safety and reinforcement in foundation
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