Min reinforcement steel in foundation for self erecting crane

[TWfea]

Hi all, I have been asked to approve a raft foundation for a self erecting crane, and am running into an issue regarding the minimum steel requirement (As,min). The foundation has already been poured and is designed in accordance with the suppliers design recommendations.

The raft is 7m x 7m x 0.5m C35 concrete with a layer of A393 mesh top and bottom.

• Based on the design bending moment, the as-built steel is fine (unity check = 0.9).
• The min requirement As,min = 0,26(fctm/fyk)btd ≥ 0,001 3btd (Eurocode 2 - 9.2.1.1 - Beams) gives a value that is 1.8x greater than the specified steel.

I understand the Eurocode is for permanent structures, and this is only a temporary structure (1yr). So I was wondering

• If there were any allowances when dealing with As,min for temporary foundations
• Is the strict application of the As,min design critera for beams necessary

You feedback, insights, and expereince would be very welcome!

 

[hemiv]

I worked for a large heavy civil contractor, and we would, as a rule, take advantage of the temporary nature of our projects. We made use of ASCE 37. I am not familiar with Eurocode.

If the foundation works for the loads, and if the As min is just there for crack control, and if you can live with the cracks for the life of the structure, then it may be okay.

 

[human909]

This was discussed in a recent thread. Minimum flexural steel requirement are for more than just crack control. They are there to stop brittle failure. There are ways around this in AS3600 but those ways probably couldn't be justified for a tower crane where flexure is at 0.9 of ultimate strength.

(The way around it in AS3600 is that flexural failure at that point will not result in collapse.)

 

[HTURKAK]

I understand the Eurocode is for permanent structures, and this is only a temporary structure (1yr). So I was wondering

• If there were any allowances when dealing with As,min for temporary foundations
• Is the strict application of the As,min design critera for beams necessary

My points are ;
- For temporary structures the coefficients at loading combination changes . Refer to EN 1990
- The assumption of two way slab and providing the sagging rein. As,min =0.0035 is reasonable approach. I guess in your case , supplier design recommendation based on structurally required thickness rather than the chosen thickness. The minimum structurally required thickness is the allowable thick. for shear / punching.
- I would check the punching thickness requirement and compare the min .reinf. ratio..
- I am not in a position to defense the relax of the code requirements but that was the old engineers' approach to circumvent the code requirements.

 

[GeorgeTheCivilEngineer]

- I am not in a position to defense the relax of the code requirements but that was the old engineers' approach to circumvent the code requirements.

You could make the argument in this case.

The OP says it's a raft, so assuming there isn't full support from the ground and the raft does need to span, then minimum steel requirements has relevance.

- But is ensuring ductility a significant concern? While it's preferable to a brittle failure, because it is at ground level a brittle failure may not be any significantly worse than a ductile failure. It isn't like having to escape from a failing building.
- Are there any possible unforeseen bending moments? Unlikely in a raft on well prepared ground and the loads should be well understood by the crane supplier
- Creep and cracking is unlikely to be an issue given the temporary nature
- The strength requirements is covered by design


I can't think of another reason to concerned over min steel area.

 

[HTURKAK]

I can't think of another reason to concerned over min steel area.

I think in this case The devil is in detail. The OPs statement '' .....The foundation has already been poured and is designed in accordance with the suppliers design recommendations...''
I can not prove but i bet , this is a manufacturer's typical detail and they are using since more than 50 years without questioning.
Nowadays this design recommendations may not justify valid standards but may be thousands of self erecting crane foundation constructed with that recommendation.

 

[TWfea]

Hi All, thanks for some good inputs.

• The punching shear, shear, bearing pressure all pass with Unity factor from 0.25 - 0.1. So there are no concerns there.
• Generally speaking the loads are quite low relative to the foundation size, and there is no expectation of uneven settlement (the raft type foundation is only required for soft ground, but the contractor has installed on good ground 150MPa bearing pressure, with a compact hardcore base)

All in all, I cant see any reason why the foundation can't be approved, although I will specify regular monitoring for cracks and uneven settlement as a precaution.

Thanks again!