Torsional Moment in Reinf. Concrete Walls

[wutiger]

StaadPro output gives Mxy or torsional moment for a given structure. However, most walls, etc. are designed only for Mx and My output, and Shear (i.e. Sx in Staadpro) is also checked. Should I be concerned with this output? I have seen a PCA publication (Rectangular Concrete Tanks) that suggests direct addition of Mx or My to the absolute value of Mxy for worst case. ????

 

[tld23]

wutiger

I am not familiar with STAAD, nor am I a structural engineer so my responses are based on 'common' sense engineering.

Obviously if the torsional moment is great enough the structure WILL Fail. Concrete generally cracks due to direct tensile; the shear may increase the tensile stresses and so its effect should at least be considered. If you are using a stress or strain based design criteria use the maximum principal stress or strain for concrete failure. Use von Mises for steel reenforcement.

I would have thought that reinforced concrete structures should be built with most of the material in compression and as little tension as possible. Generally reinforcement is not designed to take torsional loads. Should the design be revised to eliminate the torsion?
TERRY

 

[wutiger]

Torsional moment applicability to beams as show in American Concrete Institute 318 Sect.11.6 (i.e a beam with a point load off to one side causing a twisting of the beam.)This could naturally be done by hand, for example (10k point load 3 in. off center of the beam causing a torsional moment of 2.5 ft kip. along the beam length. This is not as simple in a wall panel with say, a uniform wind pressure or fluid load. If the design were done by hand, certainly one would not calculate a torsional moment, you would simply calculate a moment based on support conditions (i.e a cantilever for a long fluid containing tank) and reinforcement would be designed for the vertical direction with applicable minimum horiz. reinforcement. Shear would also be checked. No torsional moment? It's also hard to get a feel of a torsional moment in a wall panel. As it normally maxes out where bending moment is a minumum and vice versa. ???

 

[abhio]

I recall using the French code of practice BAEL (beton armee etat limite), and adding half of Mxy to the absolute values of Mx and My. Abhijeet Oundhakar
Design Engineer
Rashid Al Owais Engg. & Consulting
Sharjah, UAE

 

[dollarbulldog]

Hi,
I think that you did model this structure like plate in bending (that why you got Mxy). In finite element code, we normally convert torsional moment Mxy as effective shear force when applying the torsional clamped boundary condition as allowed by St's Venant Principle. In other words, you can also interpret Mxy as shear force and certainly you should not ignore it (normally unit force per length we have).

Note: Total qx:=qx+Mxy,y , and
Total qy:=qy+Mxy,x

where qx:=Shear force per unit length on face perpendicular to local x axis and vice versa
Mxy,y = d(Mxy)/dy and vice versa where Mxy = Torsional moment per unit length at edge of element which
Mxy = integrate(Txy*z)dz , Txy = shear stress, z is indirection of plate thickness

Please differentiate the physical meaning of shear flow in this case, the Mxy is not a kind of shear flow like thin walled structure in pure torsion but imaginary equivalent shear force on that edge. The relationship between Mx, My, and Mxy could be derived from the system of PDE for plate bending like

Mx,x+Mxy,y-qx = 0,
Myx,y+My,y-qy = 0 and
qx,x+qy,y+p = 0 where p = distributed load on plate

Please excuse, I don't know why we could sum up Mx+My = Mxy??? it makes non sense to do in that way; however, code stated for that, if anyone know the reason please tell me. Thanks

 

[wutiger]

I mean Mx + abs(Mxy) = Mx design or My + abs(Mxy) = My design. Notice wording in the first thread. Thanks for the insight, could you reiterate the understanding of the last set of equations, Mx, x+Mxy, y-qx = 0 .....are the commas an operator? Divide/? Thanks again!

 

[kags]

The value of Mxy is generally considered in to design by wood armer equations. You can refer to this.