21. Worked example #7: Calculating doubly reinforced moment capacity

Questions and discussion for this lecture live here. Fire away by hitting Reply below :fire:

Hello Sean,

I have three questions regarding the calculations of moment capacity for the doubly reinforced rectangular sections.

  1. In the singly reinforced section there was a check for the ratio of z/d and limit it to 0.95. Does it apply also to doubly reinforced sections?
  2. Is it acceptable to not subtract the area of concrete where compression steel is placed?
  3. The max area of steel of 0.04 is defined for both compression and tension steel or just tension steel?

Thank you in advance,

Hi Ahmed,

  1. This limit wouldn’t be relevant for a doubly reinforced section since we would only ever have such a section (i.e. provide compression steel) if the compression block was fully utilised - as such we would expect z to be relatively far from the limiting max value of 0.95d that applies to singly reinforced sections.
  2. Yes this is acceptable - the influence of double counting this area is negligible - especially by comparison to other potential sources of inaccuracy.
  3. This limit applied to for all longitudinal steel - so yea, tension and compression.