To be continued >

Lap-splices in columns rebars

In multi-storey buildings it would be ideal if each of the column’s longitudinal rebars could be placed as one single piece throughout the structure’s entire height. This however is impossible for practical reasons and therefore, the longitudinal rebars are cut in one single piece to cover the height of each storey.
To ensure continuity between the rebars of a storey with those of the stories above and below we could weld them together. This method however presents certain practical difficulties and is applied only in specific cases. The practice usually followed is the rebar lap-splicing i.e. simple rebar overlapping.

Examples for the necessary lap lengths in cm

Bent rebars in the conjunction area

The bent rebars can be placed in contact with the straight ones in any direction as shown at the figures.
When seismic design is required, as it is for the columns referred to in this book, it is preferred to place rebars only inside the corners of the stirrups thus ensuring that no buckling will occur. Therefore, it is better to use fewer bars of larger diameter.

Bent rebars in a square 400x400 section

In a square 400x400 section with the stirrups placed in a rhombic layout (having 4+4 corners), 3000 mm2 total area of required reinforcing steel and use of longitudinal rebars up to Ø20, the usual reinforcement is 4Ø20+12Ø14. If using longitudinal rebars up to Ø25 the ideal choice is 4Ø25+4Ø20..

The mass of columns

The earthquake resistant columns have a large mass. A common 400/400 column with Ø10/100 stirrups placed in a rhombic layout and 8Ø20 longitudinal reinforcement, weights 150 kg and the also usual 500/500 column, with stirrups placed in a cross layout and 12Ø20 longitudinal reinforcement, weights 230 kg.