Learning Forum

[pv00170]

I am analysing the reaction of a bridge under self-weight (Y direction). The bridge is halved, and a symmetry plane is applied to reduce computational time.

When I check the reactions, I get:

• X axis = correct (0 N)
• Y axis = correct (1.46 e6 N, which is the same as the self-weight)
• Z axis = ?? (5.12 e5 N)

I have some questions regarding the Z axis:

• Are these forces existent (even though no Z-direction forces are applied) because of the symmetry plane?
• Does this imply that if I run the model but symmetrically I should get the same Z but with the opposite sign?

[pv00170]

For the whole bridge

I have compared some results. I have used Solution > Insert > Probe > Force reaction, and then I select Location method > Boundary condition and then Fixed support.

I have 16 supports, 8 on the left-hand side and 8 on the right-hand side. The following table shows the results of the reactions:

Whole bridge	X (N)	Y (N)	Z (N)
Left (8 supports)	-3.83E+05	5.44E+06	1.24E+06
Right (8 supports)	-3.82E+05	5.44E+06	-1.24E+06
Sum (by hand)	-7.65E+05	1.09E+07	100

But then, if I consider the 16 supports altogether (rather than sepparatly) I get a different value:

	X (N)	Y (N)	Z (N)
Both (16 supports)	-6.67E+05	9.41E+06	-1

 

For the symmetry bridge

I get the reactions of the 8 supports which multiplied by two, coincides with the previous one. 

Symmetry bridge	X (N)	Y (N)	Z (N)
Left (8 supports)	-3.33E+05	4.70E+06	1.66E+06
2 time left (by hand)	-6.66E+05	9.41E+06	? 3.32E+06

So my question also is: how come the addition of the 8 + 8 supports is not the same as the 16 supports?