Updated: Apr 1
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.
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.
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.184.108.40.206
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 220.127.116.11
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.18.104.22.168,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 is not a good idea to lap bars close to support.
Cl.22.214.171.124 (b) (2) says that the maximum bent anchorage allowable is 16 x dia. See sketch below.
As per Cl.126.96.36.199 (b) (2) IS 456 , the third bend (Leg C) shall not be taken as a benefit for anchorage.Only 2 bends 16 x dia shall be accounted for anchorage.
To conclude, let us discuss another important point. See the sketch below.
Is the anchorage 600 mm adequate?
As per the earlier discussions, we will need
48 x dia = 48x16 = 768mm.
However provided is only 600mm.
An imprtant point to note is that the in the calculation of development length as per Cl.26.2.2, the stress in bar
is considered to be 0.87xfy = 0.87 x 415
This is for a fully stressed bar. However in this case the provided steel is 2-T16 = 400mm2 and the required steel in the data is 200mm2. So the bar is only stressed by 50% and hence the anchorage length also can be lesser. 768mm will not be needed.
All the above points mentioned in IS 456 is important and a clear understanding of the clauses is needed for a proper structural detailing. When analysis including seismic analysis, structural design, manual design etc is important, proper detailing of a structure is equally important.
Drawing conveys the design intent!