I have modeled the following bridge as a rigid frame system using beam element. I want to simulate the effect that soil imposes to the external walls.
I decided to use ''combin14'' to simulate the soil behaviour. The spring element is only allowed to move in longitudinal x direction. and I want to place them at a distance equal to the wall thickness. Is it sensible to do in this way?
Also, I am unsure about the stiffness value for the springs. How do I get a generic stiffness value without knowing the soil properties behind the wall? and should the stiffness of spring in deeper location be greater than those close to the ground?
(In the attached pictures are the bridge and a model I created with only one spring on each side)
Thanks everyone.
Dear Yu
I suppose to make this interaction using finite difference method and depending on the soil reaction force which can be considered the nonlinear behavior and you are only one displacement in the x direction. So, it is easy to use the differential equation to include the soil force with the wall.
I think your modeling idea is quite reasonable. I would, however, take the length of the springs to be equal to half the thickness of the wall. For modeling soil springs, a common and practical method being utilized in structural consulting practice is to use the equation for subgrade modulus: ks = 40*SF*Qa (see Foundation Analysis and Design by Bowles), where ks is the subgrade modulus (soil spring in kN/m3), SF is the safety factor and Qa is the allowable stress. The 40 value is based on the assumed allowable deflection of 25 mm. If N-values from a geotechnical report are available, then, the soil stiffness can vary along depth depending on the N-values. If soil report is unavailable, then, use of uniform soil spring stiffness may be a reasonable assumption.
If you must set the spring constant, why is its length important? I think you can choose any length if you set the right stiffness.
Mr. Luis is correct in saying that the length of the spring would not affect its load transfer mechanism when the spring stiffness is provided as an input. However, by taking the full thickness of wall would mean lesser clear span and therefore lesser stresses (based on how I interpret your provided picture.)
The interaction between soil and wall can be modeled in different ways : 1) representing the soil as lateral springs connected to the wall element which you have shown in your present figure 2) Zero thickness interface element can be provided at the soil-wall interface which will capture both sliding and gaping at the interface. These interface elements could be node to node or surface to node.
Stiffness of spring may vary with depth depending upon whether soil is clay or sand. In case of clayey soil stiffness may be constant through out the depth. However, in case of sandy soil/granular soil stiffness may increase with depth.
To begin with, you can take Young's modulus (E) of the soil at that depth as the spring stiffness k.
In case of interface elements you will have to perform convergence study to check that the sliding at the interface is converging to a constant value and inter-node penetration is within limit.
To bring thickness of wall into picture you will have to define wall as 2-D (i.e.,area) element. In present case, you have defined wall as 1-D beam element ( i.e., a line element). So you can not show thickness in it. For further details on interface modelling you can refer the linked article.
http://www.sciencedirect.com/science/article/pii/S0267726117306528
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