Reinforced Masonry to footing design 1

[Sparkes1992]

Hi guys,

i was hoping someone could help me get my head around the connection from a masonry wall to a strip footing. I've noticed that its quite typical just to extend the masonry reinforcement down into the footing and to cog it by 200mm or so. My question is, as this reinforcing bar is typically in the centre of the masonry ( acting like a pin connection?), won't the masonry essentially want to open up on each side of the reinforcement before the moment is transferred to the footing?

 

[hokie66]

What moment? If it is designed as a pin, it doesn't need to resist moment, and strip footings are normally not designed to resist overturning.

On the other hand, if this is a moment connection, as in a cantilever retaining wall, the effective depth must be correctly maintained.

 

[Sparkes1992]

Sorry my mistake... Its a cantilevering wall. Im trying to say doesnt the wall to the footing need to be a moment connection in order to transfer the moment?

 

[hokie66]

Yes, it certainly does. And unless there is a very small moment, the bars should be set closer to the tension face. No different in that respect from a cast in place concrete retaining wall. And again, unless there is a very small moment, a 200 cog on the bar is not likely enough. There are many threads on this site about retaining wall to footing connection.

 

[Sparkes1992]

Ok thanks Hokie. Well what ive got is basically a brick fence only designing it for wind pressure. Isn't the cogged length purely just to get the desired development length of the reo?

 

[hokie66]

I assume you mean a double brick wall, so your effective depth would be perhaps 120, unless you have a wider grouted core. Yes, you have to develop the bars, and you also have to get the moment into the footing, all the while checking for overturning and sliding as with an earth retaining wall.

 

[Sparkes1992]

Yes, i do need to get moment into the footing. But my question is; will a single reo bar in the center of the wall do that? Thats the standard detail my office uses for these walls but i just dont understand how this is transferring moment...

 

[hokie66]

The moment is transferred by the couple of the bar tension and the compression block. I assume you understand the importance of effective depth, "d", in the moment calculation. With the bar located centrally, the "d" will be the same with the wind blowing from either direction. You are correct that the masonry will tend to open up (crack) slightly on the tension face.

 

[Sparkes1992]

Thanks for that Hokie. Yes, that's what i was thinking. I was wondering how i could prevent this cracking as the only way to reinforce this type of brickwork is centrally. Just in terms of checking the strength of the actual connection, is it as simple as calculating the shear capacity of the bars going into the footing? I know that here in Australia, in the masonry code, when checking the shearing capacity of reinforced Masonry, the reinforcement strength is always reduced to 17.5 mPa. I've got no idea why this is and was hoping someone had some idea.

 

[hokie66]

I don't think you can totally prevent this type crack forming, unless you keep the stress due to bending to less than the stress due to applied vertical force. Some folks have tried to do this by post-tensioning. But in practice, the cracks usually don't matter, as the alkalinity of the grout protects the bars. If in doubt, you could seal the bottom of the wall.

Not sure about the 17.5 MPa. I haven't used AS3700 intensively in some years, so may have forgotten. I will venture a guess that this is the dowel component of the equation, and is an indirect way of restricting the dowel contribution which is actually controlled by masonry crushing.

You have limited your response from those who might give better answers by your choice of forum. Posting in the AS/NZS Code Issues forum may get some better answers.

 

[Sparkes1992]

Thanks hokie, appreciate the response mate.