Maximum Moment at Construction Joint 1

[civeng80]

This is a question Ive always had in my mind with Reinforced concrete design. Sometimes the maximum frame moment accurs at a construction joint simply because of construction technique e.g. footing to retaining wall joint, column to slab joint, or beam to column joint.

Is the joint especially designed taking in account the weakness inherent in the joint? or designed as if it cast monothically. Incidently how about the design of connections in RC concrete.

I'd love to hear any comments/ thoughts amongst designing engineers.

Thanks !

 

[JAE]

It depends on what forces are going through the joint. But generally, joints in concrete don't matter that much. With reinforcing going through the joint, usually you have enough strength to deal with any shear or orthogonal stresses.

 

[civeng80]

Thanks JAE

If a contractor asked your permission to form a construction joint in a suspended slab close to the maximum moment would you permit him ?

Thanks !

 

[JAE]

Yes - do so all the time. I avoid CJ's at points of high shear and try to place them near midspan....or within the middle third of the span.

 

[hokie66]

I disagree with JAE somewhat on this, or at least with my interpretation of his statement that "generally, joints in concrete don't matter that much". I think that direct shear on a joint should always be considered. For instance, you would not cast a column above a floor slab soffit before casting the slab unless there were a seat on the column for the slab.

Civeng80, construction joints in suspended slabs are best made where the moment is large and the shear is small. So about centre span is best.

 

[JAE]

yep - good point.

 

[civeng80]

Thanks

especially hokie66 always very accurate comments!