RC Pile cap design (BS 8110): choice of effective widths and deflection check span

Question

What values should I input for the following parameters?

• b_mr = effective width for max/min area checks
• b_ccs = effective width for tension spacing checks
• L_s = span of pile cap for deflection check

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Answer

Firstly, you will appreciate that in many engineering designs the choice of appropriate values for design parameters may be a matter of engineering judgement to some degree, especially where values are not clearly or explicitly prescribed in the relevant design code(s) or accompanying design guides. 

In such cases, Tedds calculations commonly will not have a default value and you must make the choice of an appropriate value and enter it yourself.  This is the case with these parameters.

Although we may be asked the question of what value(s) to input, in general we cannot give categorical answers, since this would be doing the engineer's job for them and effectively providing engineering consultancy.

Having said this, we will where possible provide some guidance as to the rationale to use in such choices.  This is the case for this question.

Effective Widths b_mr and b_ccs

These inputs are required for 3 and 4 pile caps.  Note the information text given for these values shown in the interface - this is highlighted in the picture above.

Logically checks of min/ max tension rebar area and spacing could be made.  However, as the information text states, when detailed the tension rebar may be banded within a specific width of the cap which will determine the width used for these checks.  Hence you must specify some width to be considered, taking into account this banding width, either known or estimated.   Inputting too large a width might be unconservative if the steel is actually banded in a smaller one.

BS 8110 Pt1 Cl 3.11.4.2 for the "Truss method" - which is implemented by the calculation - provides some guidance.  It states that for widely spaced piles (spacing exceeding 3x the pile diameter) only rebar within 1.5 times the pile diameter from the centre of a pile 'should be considered to constitute a tension member of the truss'.  Logically then when detailing the pile cap, the tension reinforcement would be banded within this width.   This implies that for more closely spaced piles, a wider width could be considered, but no guidance on this is given. 

So, conservatively, we would suggest using the minimum of 2 x 1.5 x pile diameter and 2 x the distance from pile centre line to the edge of the pile cap.  However you could exercise some judgement on this where piles are more closely spaced.  Ultimately however, you must decide on and enter your own values.

Span L_s

This applies only for 2 and 4 pile caps and when the Deflection check option is deliberately enabled by the user (default is disabled).  Hence you need only consider this input when you have deliberately chosen to undertake a deflection check.

For a 2 pile cap, the span value is relatively clear and could conservatively be taken as the cap length or less conservatively the distance between pile centres. 

For a 4 pile cap the span is less clear and some judgement is required - the span could possibly be any of; the pile centre to centre distance along the edge of the cap; the centre to centre distance diagonally across the cap; the distance between the faces of the piles.  Again ultimately you must decide on and enter your own value.