Minimum shear reinforcements in footings

[nathan7]

Hi all,

For the design of pad footings or rafts I use the equivalent beam approach for bending and shear design (and I check punching shear separately).

Using the beam approach i follow chapter 8 from AS3600.

Clause 8.2.5 (a) which require minimum shear reinforcement for sections deeper or equal to 750mm. I've never seen shear reinforcement on a pad footings even when deeper than 750mm. Is this because normally the shear at D from the face of the support (V*) is always less than phi*Vuc as per (ii)?

Perhaps this is an easy question but I just like to make sure that my understanding is correct.

Thanks

 

[asixth]

I've never had shear reinforcement in a pad footing because they are usually dimensioned to avoid it. I have used shear reinforcement in a raft, particularly when they require large outstands to stabilize lateral forces. For raft slabs I provide shear reo when V*>phi*Vuc as you mentioned.

Trenno also posted something similar not too long back that would be worth a glance thru:

thread507-375602

 

[nathan7]

Hi, perhaps my question was not clear. I normally design to avoid shear steel as well, however the code require minimum shear steel for section deeper than 750mm no matter what the load is.

I'm not sure if this can be completely waived.

 

[asixth]

Nah, seen plenty of pad footings and raft footings deeper than 750mm without shear reinforcement in them.

I would be using 8.2.5.ii. Minimum shear reinforcement requirements may be waived for slabs if V*<phi*Vuc

 

[nathan7]

I Agree with you. In all my career I've never put Any shear reo in footings and always design deep enought to satisfy punching and beam shear. Probably because of phiVuc

However there is always shear ligs in crane base even if the principle is the same.

Can't really explain why

 

[asixth]

Here's a crane base that the temp works engineer submitted the other day. For a 6-storey development, typical tower crane. I forget what the overturning was but the ultimate pile reaction was about 2,500kN. The reinforcement might be "standees" and they are placed in there to help secure the top and bottom mat in place.

 

[nathan7]

When the section is deep and there is top reinforcement some form of steel must be used to support the top mat, however, in my opinion, it is not the engineer responsibility to specify this type of steel. The builder normally used big reinforcement steel chairs to support the top mat.

In my experiences with crane base design the shear force is often less than phiVuc, as such no shear reinforcement is required but still specified.

I think that crane engineers are used to specify ligs on the crane base but in a lot of cases there is no real need for it.