Why is the lapping kept staggered in a column ?

A Lap is required when two pieces of reinforcing bar (rebar) are overlapped to produce a constant line of rebar. The length of the lap fluctuates according to the concrete strength, the rebar grade, size, and spacing.

The objective of lap is to transmit load from one bar to another bar as well as retain continuity. The positioning of lap is dependent on the bending moment of the column.

Stress generation is more at lapped portion of bar as compared to the place where the bar is continued.

If all bars will be lapped at a single location then due to stress concentration at a point column may be fail in buckling, crippling or in compression as the case may be i.e. for lap in center of column it will fail in buckling and for lap near end points it will fail in crippling.

Best zone to provide lapping in columns :-

Rebars in column should be lapped at around 1 meter from the story. The most important reason behind this is workability.

We know that during lateral forces and because of interstory drifts columns experience maximum moments at the top and bottom of the story. Now we cannot lap the bars into the joint as those are the locations of maximum stresses. So the best possible location is around 1 meter from story height. This way, when the floor below is constructed the workers can easily stand on the floor and work on the column without worrying about the formwork and heights. Thus the construction process is accelerated.

Lapping should be avoided in tensile zone of construction members. For column bending moment at mid point is zero so try to lap at mid point. Lapping should be staggered and alternate. Lap should not be given at same point because buckling may accur. Due to maximum stress we can’t lap at column /beam/ slab joint. Lapping should not be given at L/4 distance from top and bottom of the slab.

For beam lapping in top bar avoided L/3 distance from both end .for top bar lapping should be at mid span.

For bottom bar lap should be column junction or L/4 distance from column face but not in mid length