Converting FEA moment contours to reinforcement contours

[NTCONLINE]

Dear colleagues,

I am having a very quick question and I would like to have your opinions on this.

I use a FEM software, and I am able to plot the moment contours such as Mx and My (and Mxy).

Let say at every point on the floor slab, there is Mx, My, and Mxy. After I convert
those 3 moments to the design moments Mx* and My* (taking into account the Wood armer effect),
can I directly convert these moments into reinforcement contours ?
Since the calculated Mx* and My* are the design moments per unit width, and I know the slab thickness,
so I would just use the code (such as Eurocode 2 or AS3600) to calculate the required area of reinforcement
to satisfy those Mx* and My* - I do this procedure for every single point on the slab and therefor I can plot the contours of reinforcements
per x and y axis.

My question is:

is the procedure of converting the moment contour to reinforcement contour I describe
above correct ?

Or is there anything else I am missing ?

I am looking toward to knowing your opinions
NTC

 

[rb1957]

so long as you rerun your reinforced structure to confirm the reinforcements haven't changed loadpaths.

another day in paradise, or is paradise one day closer ?

 

[SethGuthrie]

I don't think you want to be designing the reinforcement for the peak stress or bending moment values, assuming you have some point supports where spikes occur. Most methods for designing reinforced concrete slabs based on FEA results average the moments over some discrete strip width in order to come up with reasonable reinforcement levels. For example see:

http://www.post-tensioning.org/Uploads/Technote11.pdf

 

[JAE]

I agree with SethGuthrie - you can certainly create a contour for rebar (using Mx + Mxy and My + Mxy) but FEA models create very high peak values at some discrete locations and you need a way to average down the peaks over some rational width - we sometimes use 4 to 6 times the concrete thickness...so a 10 inch thick slab would be designed for the average moment over 40 to 60 inches of width.

Also take care with sign convention as your Mx values and Mxy values might be additive or subtractive. Usually Mxy is quite small and we therefore will simply add it as a like-sign to the Mx or My just to be conservative.

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[IDS]

so long as you rerun your reinforced structure to confirm the reinforcements haven't changed loadpaths.

Assuming the original analysis was elastic with section properties based on concrete dimensions, how would the reinforcement change the load paths?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/