Beam ligatures at angle 1

[oranda]

How would placing beam ligatures at an angle (on plan) affect the beam shear capacity provided that the number of legs are satisfied? Sometimes the steel scheduler makes mistake by making the ligatures wider than the beam width and the steel fixers decide to place them at angle to the beam width.

 

[jeg]

The shear Force in a beam varies along the length according to the position of the loads and supports.

The vertical shear force causes the beam to fracture on a diagonal slope. If the fracture plane can be prevented from opening then the interock provided by the aggregate in the concrete will prevent shear on the vertical plane.

The diameter of the links is calculated to provide an area of steel in the two vertical legs (or 4 legs in the case of double links) sufficient to hold the concrete together across the diagonal fracture plane.

For a sloping fracture plane, the depth of the beam will determine the number of legs at a given spacing that cut across the fracture plane and hold it together. This number of legs and the value of the shear force will determine the diameter of the rebar used.

If ALL the links are oversized they will all angle over by the same amount. This will result in the first link at a support point having one leg displaced and the last link at the other support point having an extra leg. This is not good at the first support point as this is a point of max shear but is excellent at the other support.

Practical thoughts:

1 - beam rebar commences well before the point of max shear (at the junction of beam and support) and the links generally start well within the support area. Therefore, the loss of one leg within this area is generally not critical.
2 - if the links are angled it will be difficult to build the beam rebar correctly, often the top bars will be poorly positioned and the cage unstable - not good.
3 - the factors of safety in the design process do cover workmanship but should not be squandered knowingly.
4 - no designer should produce a beam dessign that is so tight that the loss of one leg of the shear links causes concern.
5 - and finally, after this long dissertation, everyone does it !!!

 

[ishvaaag]

As long as the stirrups are well anchored atop and bottom (which can be by being a closed loop anchored in the core concrete), it is the stirrups reinforcement ratio (total section of stirrups per unit length) what determines the strength.

Anyway this detail means miscare in execution (anyone sees distorts the Mörsch analogy in which the reinforcement by stirrups is based) and is better order to remove what badly fabricated in order to establish proper practice.

Skewed stirrups mean bad practice. In fact I never have seen them in the works I have participated in, so it maybe the fabricator had some surplus, more than miscare.