Requirements For Transverse Shear Reinforcement - Minimum Shear Reinforcement

[structbureau]

It seems that the expectation for all foundations designed in flexure that are greater than >750mm deep requires minimum shear reinforcement in accordance with clause 8.2.1.6 (21.3.1 > 9.3.2(a) > 8.2.1.6 > 8.3.2).

My understanding is that it is due to the irregular shear crack spacing and large crack widths which significantly reduce the shear capacity of deep sections and alignment with MCFT and the Canadian Code which also has this minimum requirement.

Has anybody had an issue with this because it is not industry practice to use shear reinforcement in such members (core raft slabs, crane bases, pad footings). Previously you would size the foundation for Vuc to negate the requirement for shear reinforcement.

Obviously an outcome would be to design to section 12 - Non flexural members and design as a fan based strut system but this does add complexity to the design model.

Like to hear peoples thoughts and whether or not all of Sydney's raft foundations and pad footings are now failing in shear...

Also the draft amendment still upholds this minimum requirement for foundations.

 

[Agent666]

I see in proposed amendment number 2 they proposed to revise this clause. Not sure if this is the latest but:-

But states now that depth requirement is for beams and one way slabs. A pad footing for say a crane base is 2 way slab. Unsure why there would be a distinction but there you have it.

The originally worded clause was actually under the beams section, but there is no equivalent section in the slabs section.

In NZ for comparison deeper footings would be sized for the old rule of shear demand is less than 0.5 times concrete shear strength means no shear reinforcement required. Over that and you require shear reinforcement. Yes this seems to be generally complied with. For deeper footings with larger plan size and smaller bars I prefer to have some shear reinforcement as it also aids to support the cages from a practical standpoint. But can be a pain to assemble on site. Luckily these types of cages lend themselves to prefabrication off site.

 

[Reo-Checka]

Obviously an outcome would be to design to section 12 - Non flexural members and design as a fan based strut system but this does add complexity to the design model.]

This is how I've done it. As Sydney sits on sandstone the length to depth ratio of foundations indicates the governing behaviour should be classified as non-flexural and designed to section 12.

Like to hear peoples thoughts and whether or not all of Sydney's raft foundations and pad footings are now failing in shear...

In the context of flexural elements, shear failure doesn't apply as the stress fields are non uniform. Whether they could fail under strut bursting forces is another question but because of the greyness you describe most foundations are designed more conservatively than other elements.


ReoChecka

https://reochecka.com.au/

 

[bugbus]

My understanding is that it is due to the irregular shear crack spacing and large crack widths which significantly reduce the shear capacity of deep sections...

It is to do with the size effect observed in deep members with little or no shear reinforcement. There are some very interesting papers by Bentz, Collins, etc. that talk about the risk of under-designing very thick slabs and foundations.

...it is not industry practice to use shear reinforcement in such members...

This should not matter whatsoever. If the numbers say that shear reinforcement is needed then it has to be included.

Obviously an outcome would be to design to section 12 - Non flexural members and design as a fan based strut system but this does add complexity to the design model

Agree that if the STM can get it to work with only fan-shaped struts then there would be no need for shear reinforcement.

 

[structbureau]

Hi All,

Thank you for your comments.

@Agent666 Beam shear is a failure mode of two-way slab design, so it still must be considered under the proposed code provision - even though the revised text seems more ambiguous.

@Reo-Checka Agree for discrete foundations pads/strips can be sized as non-flexural fan struts not requiring bursting reinforcement. However, I disagree of course shear applies to flexural elements, this is 101 beam theory.

@Gusmurr This issue that I am raising is, for raft foundations you may not be able to abide by the limits of non-flexural members in section 12 so you need to design the rafts as flexural elements, particularly through the mid-spans between wall elements. The high shear loads are generally within the non-flexural regions (not requiring shear reinforcement because designed as fan S&T system) but as soon as you move from the D to B region you need to provide minimum shear reinforcement (even for 1 kN of shear force). Just doesn't make sense how can you not require any shear reinforcement in the highly loaded area and as soon as you move into an area of low load intensity minimum shear reinforcement is triggered.

Previously there was a waiver 0.5Vuc before the need for shear reinforcement.

 

[rapt]

As Agent666 pointed out, the 750 depth limitation has been changed in the next amendment to not apply to flat slabs or two way action. So it does not apply any longer for mat foundations, or isolated pad footings

It now only applies for beams and one way slabs.

In earlier codes it only applied to beams. But it was still there. One way slabs has been added. I gave up arguing against the change, basically because there are not a lot of one way slabs deeper than 750mm!

But at least it no longer applies to two way mat foundations, transfer slabs and pad footings.

 

[structbureau]

ok thanks RAPT.