All rebars comes with a stock length. For this reason, we will need to lap these bars at sites to achieve the required length. There will be a lot of planning and measuring needed to achieve the right lengths to achieve economy. Preparing a Barbending schedule to strict tolerence can ensure accuracy and economy.
Placement of bar and curtailment of bars at right place and lapping it in the right places are all a design requirement.Many times the design intent drawings do not cover these and engineers are ill informed about the requirement.
In this blog, will discuss a few lesser known points or often compromised points about lapping of rebars during structural RC detailing. Before getting in to that let me quickly state some important points too.
1) All beam bottom rebars shall NOT be lapped at the span.
Most knows this rule that the moments at bottom are more at the spans and hence the bottom reinforcement shall not be lapped at the span.Then where do we lap the bottom rebars in beams?
Most think that the best place to lap bottom rebar is at the support. Even SP34 indicates this in one of the sketches (Fig 8.15 in SP34). However this is true only for gravity load cases. As most of India is a seismic zone, it is not great to lap the bars at the support as the lateral thrust is at the column beam junctions and this opens up the rebar laps.
IS 13920 : 2016 has this covered. It says tha lap splices shall not be provided
a) with in a joint
b) within a distance of 2d from the face of the column
c) with in a quarter length of the beam adjoining the location where flexural yielding may occur under seismic effects.
d) Not more than 50% of bars shall be lapped at same location
This makes it a bit tricky to choose the lap location for the bottom bars.
2) All beam Top bars shall NOT be lapped near support
This is also a known rule that the moments (the hogging moments) are at the supports and hence need to avoid the lapping of top bars at the top. SP 34 in the same Fig 8.15 shows the curtailment at 0.25 time span from the face of column which by itself explains that we shall not lap the bars in that zone.
What many people miss is the note under that figure which set rules for that thumb rule to work.
The beam shall be singly reinforced, the spans shall be similar,the loads shall be fairly uniform and so on.
You can see a discussion on this topic in civilera forum here. Please discuss your structural detailing related doubts there.
3) The more compromised rules!
Clause 26.2.5.1 of IS 456 has rules that are as below
a) Lap splices shall not be used if the dia of bar is more than 36mm
b) Lap splices shall be considered as staggered if the centre to centre distance of the splices is not less than 1.3 times the lap length calculated.
c) For direct tension lap shall be 2Ld or 30dia whichever is greater. You can discuss any questions about this lap condition here.
d) Where lap occurs for a tension bar located at
(i) top of a section as cast and the minimum cover is less than twice the diameter of the lapped bar,the lap length shallbe increased by a factor of 1.4
(ii) corner of a section and the minimum cover to either face is less than twice the diameter of the lapped bar or where the clear distance between adjacent laps is less than 75 mm or 6 times the diameter of lapped bar ,whichever is greater, the lap length should be increased by a factor of 1.4
Summary
The desired design performance can be achieved if the detailing is carried out to suit as it is the right blend of concrete and rebars that makes a concrete structure. The right place of lapping is atmost important to ensure desired performance and undesired accidents.
There are lot many more rules that a fresh civil engineer need to be aware of and you can read an additional blog on anchorage length in one of our previous blogs.
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