Bending moment capacity of a circular RCC section

[ZShan]

Our team hasnt designed piles before but we came accross a request from Geo Tech to design a pile of 1.5m diameter with 28 bars of 40mm dia.

Quite a massive pile to resist a bending moment of 5000 kN.m

As I said, I am not an experienced engineer and never had a chance to design piles but the Geo Techs were interested in the design which ignored the axial load effects to make the bending moment worst. So I approached the design with generic solution.

One of my colleague designed the section on SAM Leap program and found out the bending capacity to be 7800 kN.m

I found the capacity to be 9000 kN.m by my section analysis. For the past 4 days I have been trying to figure out if I have done it wrong but cannt find anywhere anything wrong about my approach.

here is what I have done:

-Assumed the max strain in concrete to be no more than 0.003
-BS 5400-4 fig 1 and 2: 3 modes of reinforcement strain. Elastic, partial plastic and full plastic.
-Drawn the strain diagram with assumed depth of Neutral Axis (adjusted NA later on to equalize compressive force against tensile force).
-Calculated the strain in each reinforcement bar at different levels
-Ajdusted the values of strain found in the bars to max plastic strain, so that no bar is having more than max plastic strain.
-Calculated the stresses (compressive and tensile) based on this adjusted stress.
-Calculated the forces in each bar based on this stress.
-Calculated the moment, each force in the bar will induce at the Neutral axis.
-Calculated the area of concrete above the neutral axis and adjusted it for the reduced stress block.
-Calculated the moment of this area with Neutral Axis.
-Added both these moments (from reinforcement and concrete) which came out to be 9000 kN.m

Can anyone tell me whats wrong with my approach to the solution? (considering all calculating errors are elimnited).

Thanks.

 

[rscassar]

Zshan,

It is not uncommon for different design packages to give you different answers because of the different assumptions they are programed for. For the design of reinforced concrete columns, the shape of the concrete compressive stress block affects the output. Some packages use an idealised rectangular stress block, others use a parabolic stress-strain distribution.

I think the source of your problem may be the capacity reduction factor. Either you haven't included one or your peer has included an additional strength reduction factor for a cast-in-place pile because of the uncertainty with the compaction etc. It is a requirement of some piling codes that an additional material factor be included as well as the material reduction factor from the concrete code.

There was nothing wrong with the calculation procedure you included. I would not say the strain is modified because the strain is real. Plane sections remain plane after bending. It is the bar stress which gets limited to the yield stress.

By the way, I wouldn't be taking structural design advice from a geotechnical engineer.

 

[rapt]

Zshan,

It depends on cover and concrete strength, but I get between 8000 (25MPa) and 9000KNm (65MPa) for BS8110 for this pile.

Rectangular stress block is not very good for circular sections. It is really based on rectangular sections. EC2 would reduce the compression force for a circle by 10%. Try this and see what happens.

 

[tumbleleaves]

Nothing beats having the the wherewithal to do the supporting calculation by hand with minimal design aids (to back up the output).

Looks like you'll have a lot of fun analyzing this cool massive column. If you can justify the cost or volunteer the time, take the time to run your hand calc's, draw the interaction diagrams, assume an equivalent rectangular section, calculate using working stress, calculate it using LFD, calculate the length slenderness becomes relevant, calculate it every way to Sunday.