magnitude of a springs stiffness

[cucu101]

Greetings all!

May be this is something basic... but I feel like I'm drowning in an empty pool. My current problem is that usually when I use structural analysis software's like Risa, RAM elements, etc.
I have never had to use a spring as my support/boundary condition, and just recently I was reviewing a fellow engineer calcs package and realized that he was using a spring as his support.
My question is, when should I use a spring, and how do I determine the magnitude of the springs stiffness?

any guidance will be greatly appreciated.

Thanks!

 

[JoshPlumSE]

It's entirely a matter of engineering judgment.

If the support is pretty rigid compared to the flexibility of the beam, then I use the regular, rigid supports. If, however, the support is pretty flexible (i.e. soil) then I will use a spring.

Sometimes (with RISA), I have to use a spring if I've got multiple rigid items connected. Like rigid end offsets, boundary conditions, diaphragms... etc.

 

[BAretired]

If you don't want to use a spring, you could use a member with specified properties. A vertical spring could be represented by a member of length L, area A and modulus E, pinned top and bottom.

BA

 

[cucu101]

@JoshPlumSE, do you know of any place where I can find more info on how to understand/use/incorporate springs into my structural models.
I have tried the Risa support webpage even here at eng-tips. but honestly I'm still confused.

Thanks

 

[phamENG]

As Josh mentioned, soil is a key place for springs in models. I use them when I do grade beam analysis - I work with the geotech to determine approximate deflection of a pile under load and use it to generate a spring constant (a spring constant is just the stiffness of the support - x kips/in of deflection). When supporting walls and other rigid structures that helps to understand the true distribution of the load.

If you need to know the TOTAL deflection of a point in a frame, you can model the beam in question and approximate the stiffness of its supports with springs. If a beam spans between to other beams, the other beams will support it with a stiffness of x kips/in (apply a 1kip load to the connection point and see how much it deflects...that's your 1kip/yin...adjust accordingly to get a spring constant). When you do this, the ends of the beam will also move, so your total deflection will be max deflection of the beam + deflection at the supports.

 

[JoshPlumSE]

Cuco-

If you're replacing a column with a spring, then the stiffness of that spring would be AE/L.

If you are replacing a beam with a spring, then you can do a simple hand calculation. At a point load (P) to the beam at the location where you want to replace it with a spring. Calculate the deflection (Delta) of that beam, and you've go you stiffness (P/Delta).