General Rules For Lapping In Column Reinforcement

 General Rules For Lapping In Column Reinforcement

In this the article, we will explain about general rules used while calculating lapping reinforcement bars.

                   LAP LENGTH DIAMETER             

If you extend the top bar of a column from a lower (bottom) bar of unequal diameter, the lap length is calculated for the smaller diameter bar.

 

Example Calculation:

If the Top bar of 12mm dia. Is lapped with a lower (bottom) bar of 16mm dia and if the lap length is taken as 50d, then

Lap length 50d = 50 × 12mm = 600mm. ← Correct

Lap length 50d = 50 × 16mm = 800mm ← Incorrect

                     LAPPING ZONE                           

When we provide lapping in a column, all reinforcements must overlap in zone B-Zone as shown in the drawing. Therefore, the upper and lower part of the column should be avoided in A-Zone (length L / 4) as there will be a maximum moment in this area due to the lateral forces acting on the post.

If the length of the column is L, then the length L / 4 from the top and bottom of the column is classified as tension  A-zone and the central length L / 2 of the column (B-Zone) is considered a safe area for the purpose of lapping. 

                   STAGGERED BAR                        

 All bars that are overlapped should be offset in B-zone as shown in the drawing above. In any case, no more than 50% of the rebar should be lapped to the same level. If all bars in the column are overlapped at the same level, it will cause column failure.

                    STIRRUP SPACING                      

 The spacing of the stirrups in the zone of lapping should be minimal compared to the regular spacing of the stirrups in the c/c column.

 

If the regular spacing designed for the column stirrups is 175 c/c, then the spacing of the stirrups in the zone of lapping can be 150 c/c or 125 c/c depending on the column design.

                     WELDING THE REBAR               

According to the IS code, if the bar diameter is 36mm or greater, you should consider (welding) soldering these bars instead of lapping. If welding is not possible, you can overlap these bars with an additional 6mm bar spiral at the splice length.

JOGGLE LENGTH

 

For rebar larger than 12mm in diameter, you may consider providing a 1 in 6 joggle. This means if the bar dia. is 16 mm, then the length of the jogged part of the bar can be calculated as

 

J = 6 ×Diameter of rebar

 

= 6 × 16 mm = 96mm.

REBAR PLACING

When we tie the reinforcement in the lapping area, the joggled part of the bar must be placed on the inner surface of the lower bar (as shown in the picture) to get an even transparent coating from the concrete surface.

If you anchor the joggled portion of rebar to the outer surface of the bottom rebar, the lapping area will invade the concrete coverage area provided for the rebars.