In the below picture I want to design B4 and B6 but I am not sure which one does support the other and how the detailing of the area of intersection between these two beams?
Is it better that B6 supports B4 because B6 have the shortest span?
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1$\begingroup$ Wait, what exactly is your question? Is it how to determine which beam supports the other? How to detail the steel in such an intersection? Both? $\endgroup$– WasabiCommented May 9, 2017 at 14:05
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$\begingroup$ yes both but actually I want the detailing in the intersection because I think it is related to which beam is supporting the other (if B4 supports B6 that means the longitude reinforcement of beam B4 must be below beam B6) that is why I related the question to each other, Plus I want to know if we put any stirrups in this area. $\endgroup$– J.DaouCommented May 9, 2017 at 15:13
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1 Answer
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You decide which beam supports which by how you detail the steel reinforcement at the intersection.
Usually, one chooses to make the stiffest beam support the other. In your case, that means that yes, B6 would support B4. However, that decision isn't taken merely by looking at the beams' spans, but also at their $EI$. Since they seem to be of same (or very similar) cross-section, then the difference in span becomes the only relevant variable. However, if B6 were actually half as wide (or tall, which would be even worse), then it would probably be far less stiff than B4, in which case one'd define that B4 supports B6.
If you prefer, you can also let an analytical model make that decision for you. Look at the shear diagrams of both beams at the intersection. It should be clear that one "lost" shear force and the other "gained" it, meaning the latter is supporting the former.
Now, once you've defined which beam supports which (in this case, B6 supports B4), we need to inform our decision to the universe by detailing that intersection accordingly.
It is not merely a matter of which beam's rebar goes beneath the other's. After all, it is common to assume that reinforced concrete transmits its load from the bottom of the beam, at the end of the compression strut:
This means that, as far as the supporting beam is concerned, it just received a concentrated load near its bottom fiber. Therefore, additional suspension reinforcement in the form of stirrups is necessary to "raise" this load, allowing the full height of the supporting beam to work. This reinforcement is found by obtaining the support reaction between the beams and calculating
$$A_s = \frac{R}{f_{yd}}$$
(when the supported beam's bottom fiber is above the supporting beam's bottom fiber, this reinforcement can be reduced somewhat). This reinforcement should be added to the normal stirrups and doesn't need to be exclusively within the intersection volume. Item 9.2.5 of Eurocode 2 allows the suspension stirrups to be placed in the following region:
