Spring Restraints for Temperature Load Cases (Expansion / Contraction)

[KJKoh]

Hello,

I am modelling the spring restraints at the two intermediate piers for a 3-span steel-concrete composite bridge. At each of the pier, there will be 2 uni-directional spring restraints (with different spring stiffness) acting in opposite directions. Depending on the temperature load case, the "active" spring restraint changes. For example, in an expansion load case, the springs in blue will be "active". Whereas in a contraction load case, the springs in green will be "active".

However, I have noticed that Autodesk Structural Bridge Design (ASBD) does not account for the directions of the springs. I have tried to put a negative spring stiffness value, but ASBD does not allow me to do that.

Does ASBD assume that the springs are bi-directional, i.e., if springs are applied in the X-direction, ASBD assumes that they are applied in both +X and -X directions? If this is the case, does that mean that I will need an ASBD model for the expansion load case where I apply the spring stiffness for the blue springs, and another separate ASBD model for the contraction load case where I apply the spring stiffness for the green springs?

Many thanks,

Khai

[dave_geeves]

Hi,

For uni-axial supports I believe that the support type you need is "Limit- Elastic" where you can define a sprung support which has a limit to the reaction force - either maximum, minimum or both.

As you can see,  node 1 in my example is sprung with a given stiffness and has a positive limit of 10000kN in a positive direction but has a minimum value of 0.0 so if it turns negative it will remain at 0.0.  I could just tick the lower value box as 0.0 if I didn't want to impose a max positive limit.

You cannot specify two values of stiffness, one for positive reactions and another for negative so if your "green springs" have different stiffnesses to your "Blue Springs" then you would need to either define two models with different support conditions for either the positive temperature case and the negative temperature case or you could introduce extra nodes in the lower regions of your sub-structure where the extra nodes can have different support conditions to the original.  However , the extra nodes must be at least 100mm from the orignal as the program will automatically assume they are at the same location if they are closer.  This is an advisable condition as having very short and then long beam  elements connected together may cause ill conditioning of the stiffness matrix.

I hope this helps, but if not, then please attach a zipped up copy of your model to a reply and I will try to explain further on your model.  If this has answered your question then please mark my reply as a solution.  Thanks

Kind regards

Dave Geeves

[dave_geeves]

Hi,

I was just wondering if my reply had answered your query or whether you needed more help.  If it has then it is useful to other users if you mark my reply as a solution as it is easier to find an answer to problems if your filter on just solutions.  If not, is there anything more I can add to help.

Kind regards

Dave Geeves

[KJKoh]

Hi Dave,

Apologies for the delay in response.

I have a further question on the "Limit-Elastic" spring support. In your example above, Node 1 was sprung with a given stiffness and has an "Upper" limit of 10000kN (I assume this is in the positive X direction) but has a "Lower" limit of 0kN (I assume this is in the negative X direction). Does this mean this spring will only resist a positive force of up to 10000kN in the positive direction and does not resist any force in the negative direction, or does it mean this spring will only take a positive reaction force (equal but opposite direction to the actual force) of up to 10000kN?

In the screenshot above, the blue springs represent backfill behind the piers and the green springs represent the soil underneath the road level. Using the screenshot above as an example, I would like to set up the blue springs such that they will resist forces (and hence generate reactions in equal but opposite direction to the forces) in the expansion case (when the piers are pushing against the blue springs), but will not resist any forces in the contraction case (when the piers are moving away from the blue springs). To do this, I will need to set the blue springs at Pier 1 to have a "Lower" value of 0kN and set the blue springs at Pier 2 to have an "Upper" value of 0kN, is that correct? Or is it the other way round where I will need to set the blue springs at Pier 1 to have a "Upper" value of 0kN and set the blue springs at Pier 2 to have an "Lower" value of 0kN?

Many thanks,

Khai