Where is it preferable to locate construction joint in reinforced concrete slabs and beams and why are these specific lacations
This is a design-specific issue as the optimum location for such joints is the one minimizing the straining action within your structure. Hence, the answer to your question will vary from one project to the other.
Dear Hossam,
As you can see in the attachments, ACI & ECP preferred to locate the construction joint in the middle third of the span, while BS and EN didn't specified a certain location, but they required a special design and detailing for the construction joint to ensure that shear forces will be safely transferred.
Technical wise, locating the joint at mid span (or point of zero shear) eliminates the need for special calculation and details for shear transfer and the moment will transfer as compression in concrete (by bearing) and tension in steel.
Construction wise, locating the joint in the middle of span may not be an optimum choice specially in long spans because contractor can't remove the form-work of this span, on other hand, locating the joint somewhere near the support ( at 0.2 span) may be more favorable because this short span can stand without form-work as cantilever which give the contractor more flexibility in managing his form-work. besides that, the moment at 0.2 span is almost zero (for contentious spans) it means that flexural reinforcement bars at this section are not stressed by moment and can be used as shear dowels, because of that, usually, no additional shear dowels are required.
So, as you see, both choices are allowed and applicable, but since construction joints are related to pouring capacity, and the shop-drawings are usually prepared by the contractor, the joints are usually located at 0.2 span unless the designer specified otherwise.
best regards
The optimum location is where the shear force and bending Moments are not at their maximum values. For continuous members, the maximum moments are at the supports and mid span, while maximum shear occur at the face of supports. So in this case you have to compromise the location, i.e., between the face of support and mid span sections where neither shear or moment are not at their maximum values.
Dear Hossam,
The best place according to my experience is at locations of minimum shear. The construction joint has minimal effect on bending moment capacity. Please refer to my papers on the issue. I hope that they can help you.
Remember, that minimum shear commonly means maximum bending. I agree with Samer, that construction joint has minimal effect on bending capacity, but there is also the question of cracking resistance - limited adherence between old and new concrete may cause the apperance of single and wide crack and it could affect the durability of structure. That is why better are compromise locations (middle third etc).
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