Mxy moments in concrete floor design - v2 1

[apsix]

From this previous thread; thread744-266307

"mux = mx + k|mxy|

muy = my + k-1|mxy|

where k is typically taken as k = 1. Here mux and muy include both the normal and torsional components of the moments."

Does the above mean that muy = my ? (as k-1 = 0)
Is there a reason why mxy is added to mx, could it be added to my, or 0.5mxy to both mx & my?

Currently I'm being conservative and adding the full mxy to both mx & my.

 

[rapt]

apsix,

You should get a full description of the application of the Wood Armer solution.

 

[rowingengineer]

I use the below spreadsheet to do some quick hand calcs to check my designs.

I have also attached PDF for woodarmer equations broken down into a more useable form.

http://www.childs-ceng.demon.co.uk/proforms/proformas.html

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[apsix]

Thanks RE

RAPT; I certainly need to study the W-A equations, but will that explain the previous advice that
muy = my + k-1|mxy| = 0,
as k = 1 typically?

 

[rapt]

apsix,

The previous posting that you are refering to was a response from the AS3600 committee explaining why you have to include Mxy effects in your design moments. It was not written by me. It was not a full description of the Wood Armer solution.

I am currently on holidays and not in a position to study it further but it would appear to not be the complete solution.

 

[rowingengineer]

Apsix,
wood suggests a different solution for when you get less than 0 as the result also note the multiply would come before the subtraction.

Be aware the wood armer solution is a conservative solution and really is meant for thick plate situations, most finite element packages would use Nodal Reactive Moments Method hence they will report a different answer to your hand calcs.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[apsix]

Thanks RAPT & RE

It's a bit disconcerting to realise how poorly understood this is. I've got calcs for review in front of me now where Mxy has just been ignored.

A definitive guidance note from the CCAA or similar would be good.

 

[rapt]

apsix,

So would education of engineers. I know that UQ emphasise the need to include Mxy in their civil courses.

Commercial Software developers who follow the rules would be nice too! Not ones who push things like ignoring Mxy moments in design. At least 2 companies were using it as a selling point as it resulted in cheaper designs than 2D methods and FEM software that does include the effect automatically. Unfortunately designers often use software without understanding what it is doing internally (including RAPT) or believe the developers when they say you can ignore certain things in design.

 

[sdz]

It took me a while to find a reference to confirm my thoughts but generally
Mux = Mx + Mxy; Muy = My + Mxy; where all are the same sign.
So the design moment is always increased in magnitude by Mxy

The Mxy can be distributed in different proportions in which case
Mux = Mx + k.Mxy; Muy = My + (2-k).Mxy; where typically k=1

See App D of

http://www.housingauthority.gov.hk/tc/common/pdf/business-partnerships/resources/concrete.pdf

But you should get a good reference to make sure you understand the method because there can be limits on the distribution factor k.