Anchorage Length of Rebars (Design of Beams)

Updated: Feb 15

Adequate anchorage length of rebars is a primary requirement for the safety of a member. especially a beam.Anchorage length of rebar is a topic not completely understood by most of the young civil engineers.Suggest you read this blog fully and then see the VIDEO at the end.

What do you mean by design of a beam?

As always,any member design involves ensuring strength,stability and servicebility.

In a beam, most students understands that ensuring the right amount of rebars at tension zones and ensuring adequate size (amount of concrete/depth),grade or compressive rebars is important for ensuring the strength of the beam.

However inspite of ensuring this,if the anchorage length is not sufficient, the beam can fail. The rebars at the top supports can fail in bond due to the tensile force.

As an example,Consider a blanket which is fully stretched and held tight by a person at each end. When a heavy load like a stone is put in the centre of the blanket, it will sag. What if one of the person leaves his grip at the end. The blanket will not tear but still the system fails since the person drops his hold at the end and the blanket falls down. This is very similar to lack of bond for the rebar.

How is Development length calculated?

Development length is calculated as per IS 456 Cl. 26.2.2

Consider we are using a HYSD bar of strength 415N/mm2 (Fe 415)

Then the full stress is 0.87fy = 0.87x415 = 361.05

Considering M20 concrete, Cl.

Bond sress in above table is for plain bars. Plain bars are not used practically. An increase of 60% bond stress needs to be applied if we use deformed bars as per IS 456 Cl

Bond stress = 1.2x1.6 = 1.92

Anchorage = 48 x dia Approximately for Fe 415 deformed bars & M20 concrete

For more clarity, see the Fig below.

The anchorage length = 48x16 = 768mm (Practically, this can be rounded off to 50xdia =800)

The important point to note is that if the column dimension is large enough, the rebar need not bent down.It can be a straight bar as in figure.

If the column size is less and if it is not possible to provide a straight bar, then the bars need to be bent as in the figure. When a bar is bent, it will have additional anchorage because the bend provides more of it. Hence 48d anchorage is not needed.

As per IS456 Cl.,a 45 degree bent provides an anchorage of 4 x dia. In this case, it is a 90 degree bend and 8 x dia anchorage is available. Anchorage needed here is (48 - 8) x dia = 40 x dia. When you calculate the straight length, account for cover and column rebars. In the above case anchorage needed is 40 x dia = 640mm. 450 mm is the column size. However take only 375 mm length for bar, accounting for 40mm cover of column rebars and a 25 to 30mm tolerance assuming at site , the bars may not extend up to the end due to various reasons of placement. Vertical length needed = 640 - 375 = 265mm.

In a case when this vertical length needed is more or If the beam depth is lesser, the vertical leg might have to be extended inside the column. This might become an inconvinience during construction as a dowel bar will have to be provided from column. Note that the columns are concreted up to beam bottom.Beams are cast at a later date. To avoid this, a U bar can be provided at the support as in the sketch below.

When you do this anchorage also increases since there is one more 90 degree bent. So the benefit becomes 16 x dia.There fore the straight length needed can be 48-8-8 = 32 times dia. However for seismic combinations, there might be stress reversal and it