Before discussing about the staggering of laps in rebars, let us quickly discuss the need of lapping in rebars for the benefit of entry level civil & structural engineers.
What is lapping of rebars?
When two pieces of rebars are placed in an overlapping manner to achieve continuity, it is called a lap. The needed lap for the rebar,ie the lap length varies based on various aspects like grade of concrete, grade of rebars, cover to rebars, size of the rebars, spacing and its position.
Why do we need laps in rebars?
All rebars comes with a stock length. You may have site conditions where the length of members is more than the stock length of rebars. The only solution is to lap the reinforcement to meet the length requirement.
Sometimes the site will end up having short rebars after cutting and bending and all of this cannot be scrapped since it is uneconomic. Therefore, to use the short bars, laps may be needed
Very rarely even to control the weight of the rebars for ease of handling and transportation the length of the rebars is limited. This can also be a reason that we need a lap.
Is it all right to have a lap
Generally, codes of practice allow lapping of reinforcement. However, there are various rules that needs to be adhered to. These rules are related to the length of the lap, location and amount.
We all know that in RCC flexural members like beams and slabs, the compression due to bending (unless it is doubly reinforced)is absorbed by concrete and the Tension by rebars.
When this is the case, it is obvious that the lap shall not be provided where it is a tension zone.
For gravity load, in a beam or slab, tension occurs at bottom midspan as well as at top support zones. Therefore, it is recommended to avoid lapping at these locations.
During seismic conditions, there will be stress reversal and there can be tension in the bottom zone near supports. Therefore, here too lapping is not safe. In a separate blog on anchorage length, this point, I have clarified with a sketch.
Why do we stagger laps and when do we call it staggered?
Even though lap is unavoidable in some cases, lapping all the bars at same location is not permitted by codes. If we lap all the rebars at same location, it creates a weak section. Therefore, the codes suggest to stagger or separate laps. For example, if it is a slab and if the slab has 50rebars, we should maximum lap 25 numbers at one location and the remaining 25 at another location away from the other lap location. Now the question is how much away? How much separation is needed to call the laps staggered?
As per IS 456 Cl.126.96.36.199. b, Lap splices shall be considered as staggered if the centre-to-centre distance of the splices is not less than 1.3timesthe lap length calculated.
You can read more about rebar lap rules in one of our previous blogs
Laps may be unavoidable. However, it needs to be provided carefully avoiding the tension zones.
Also, care shall be taken to adhere to all code provisions for lapping and staggering.
Additionally, there are more rebar lap related requirement to be followed if the building is in seismic zone 3 or above where ductile detailing is mandatory. We will discuss about that later in another blog
Take a look at Revit structures training here which covers all rules of detailing.