Design Values at Support 2

[normm]

I wonder if any of you can throw some light on a question :

In British Standard for Reinforced Concrete Design, the Code asks for the continuous beams to be designed for bending moment values at centre line at support. Although there is guidance on the redistribution of support moment, it is the value at the centre line of support that has to be allowed for in the beam design.

But the section on design of bases says that the critical section for bases is at the face of support.

Why should there be a difference in which value to take for design? I guess that the ACI and Eurocode are similar in their reccommendations, but will appreciate if you have any comments.

 

[hokie66]

I don't know if I can answer your question, as I am not familiar with the British Standard, but the principles should be the same.

For a continuous beam, the maximum bending moments occur at the centre line. But the design moment which controls how much reinforcement you need can depend on the type support you have. If the support is a concrete column, I would use the moment at the face of the column (or at least close to the face), and design the reinforcement using the beam depth. This reinforcement will be adequate for the centre line moment, because the depth is much greater within the column width.

If the beam support is a steel column, steel beam, hanger, masonry wall, or similar, you would use the centre line moment, so the reinforcement would be greater.

For a concrete footing, the same reasoning applies as for the beam over the column. Use the moment at or near the face for reinforcement design.

 

[csd72]

normm,

I too am not yet familiar with british codes (but I soon will need to be. )

This provision seems unnecessarily stringent as the moment occuring at these points will actually be much lower.

The reason for this is that your analysis aa pinned support whereas your column or support has a width over which the support is provided. The result is that the maximum moment gets rounded off over the column cutting much of the sharp peak.

hokie66 is also correct in that the effective depth is increased over the column, but this doesnt happen instantaneously.

The australian code allows you to take the analysis moment at 70% of the width of the column which seems like a realistic approximation.

 

[normm]

Thank you hokie and csd.

The British Standard allows you to reduce peak support moment, but to take advantage of that you have to increase the span moment by corrosponding value. Here, I am not talking about that.

What hokie and csd have stated about using the value at the face of suppport is obviously reasonable. But I found that in the UK, only the engineers working for contractors on fixed-price contracts try to design to a value at the face of support the way you have indicated. Generally, designers working for consultants work to peak value at the centreline.

I am hoping some others will chip in with their response.

Thanks.

Normm