Step in Concrete Beam 2

[hsp]

I am designing a 500mm deep concrete beam with a 225mm (ie soffit of beam steps down) step down at midspan. I can not find anything on this in any code nor can I find any books on the subject. Can anybody lead me in the right direction.

 

[AJUK]

I doubt such a scenario is covered by a code. However, the relative stiffness between the two sections will mean that the stiffer section (deeper) section will tend to attract more of the moment at the step. Therefore, assuming that the beam is simply supported, think of it as a propped cantilever with the built in end being the deeper section.

Detailing seems to me to be the key to solving this problem and full anchorage of the bars from the shallower section into the deeper section is essential in order to generate the bond required to resist the tensile ('pull out')force in the bottom steel.

If there are no point loads then shear will be negligible at the step and only nominal links required in accordance with normal code rules for beams.

Hope this helps.

 

[austim]

hsp - just stick with basic principles.

Make sure that the bottom reinforcement in the shallow section has a generous overlap with the deep section reo (ie well over any simple code requirement for lapped bars).

I would increase the shear reinforcement on both sides of the step (even if your calculations tell you that you don't need it). Whatever else you do, do not bend the shallow section reo downwards in the overlap region.

There will be a ready-made crack starter at the step, (that is part of my reasoning for increasing the shear reo). Some diagonal bars across the potential crack (from the top of the shallow section to the bottom of the deep section , and fully anchored at both ends) would be further cheap insurance against future problems.

Good luck with this.


Ajuk - A couple of points.

First, if the beam is simply supported, then the stiffness at any section has no effect whatsoever on the distribution of bending moment.

Unless the beam is subject to serious axial loading, then the moment in the shallow section must equal the moment in the deeper section adjacent to it, whether the beam is simply supported or not. (Otherwise how would the local imbalance in moment be resisted?)

The suggestion of zero shear at midspan is not entirely correct. The non-uniform Dead Load alone will produce a shear force at midspan. There may also be some significant live load shear (even without point loads) if there is any non-uniformity of distributed loads (eg partial distributed load over one end only).

 

[rapt]

hsp,

The two separate bottom reinforcement bars must have full developemnt past each other (similar to a lap but they are not together). To achieve this,

The deeper section bars must cog upwards and extend a full development past the intersection with the shallower section bottom bars. This will notmally require that they extend to the top of the beam and cog again to run parallel to the top reinforcement for some distance to fully develop. As this can be a difficult bar shape to produce accurately, some designers would use a Z shaped bar at the lapping to the deeper section bottom bars and extending up to the top surface and along the top face. Obviously bar sizes need to be relatively small to achieve the tight bends needed for this detail.

The bars in the shallower section should extend at least a full developemnt length past the intersection with the upturned deeper section bottom bars.

AJUK,
You can never assume zero or negligible shear at mid-span (or any point) as the uniform loading is only an idealisation and the real loading will consist of partial length loads and point loads. This will cause significant shear at all points in a member depending on the actual loading pattern at the time, The theoretical zero moment point will only have zero shear for the nominal loading used to determine that moment diagram.

 

[AJUK]

I agree with the points raised but I did state my assumptions. Of course, the shear reinforcement should be designed to resist the shear at the relevant point on the beam and the code rules will be sufficient! We are not aware of the actual loading conditions.

The point we do agree on is the need to provide good anchorage into the deeper section.