Spring support in box culvert using Staad

[Mickeyin]

Hi All,
I am modelling box culvert in Staad pro. I have tried 2 ways in the modelling. The first one is providing pinned and roller support beneath the box culvert. Second method is by assigning vertical compression only spring support and perform repeat load case.
They are suppose to get the same maximum bending moment result, however, the base slab bending moment method 2 is lesser. Both method have the exact same load case, except that method 2 using repeat load case. Can anyone assist me on this?

Method 1: Pinned support
- a pin support at one side and a roller support on the other side
- to apply an equal but opposite upwards force on the bottom slab to model the reaction of the downwards forces
- Use load combination method

Method 2: Model vertical spring
- place soil springs underneath the bottom slab as the support conditions
- apply no external upwards reaction force
- Define compression only spring
- Use repeat load case method instead of load combination method
- Below is the way how i input in the command file:
[
LOAD 12 LOADTYPE Soil TITLE RIGHT LATERAL EARTH PRESSURE
MEMBER LOAD
3 TRAP GX 0 -28.3
PERFORM ANALYSIS
CHANGE
SPRING COMPRESSION
1 2 5 TO 8 KFX KFY
LOAD 101 COMB LC1_MAXH-MAXV_SLS_EMPTY
REPEAT LOAD
1 1.0 2 1.0 3 1.0 4 0.6 5 0.5 6 1.0 7 1.0
]

 

[IDS]

Why would you expect them to give similar results?

The support conditions for the base are completely different.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Mickeyin]

Hi IDS,
The reason I am expecting similar results is that, for the model with pinned and roller support, I have applied an equal but opposite upward force on the bottom slab (see green loading in the diagram below). This force is assumed to be soil reactions where the vertical loading will be distributed evenly along the base slab. This equal but opposite upward force results in a net zero change at the support, hence there is no reaction.

In my understanding that, the model with soil spring should behave similarly to the model with 2 supports which have equal but opposite force applying at the bottom slab of the culvert......

 

[IDS]

The load on the base in the spring model, and in the actual structure, will be a long way from uniform. The pressure will reduce towards the centre line, which will significantly reduce the bending moments in the base.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/