Bearing simulation

[Shz713]

Hi all,

I need to model bearing at abutments and bent using SAP2000 or CSI Bridge. What possible scenarios I can use?
I mean use two joints link support for instance or joint constraint.
I appreciate if someone shed some insights regarding this matter

 

[graybeach]

What type of analysis you are doing - gravity or lateral loads? What type of structure, e.g. multi girders, box girders, truss, etc.?

 

[Shz713]

Thanks for your response.

I'm modelling RC box girder bridge built in 1971. Bridge has two spans each having length of 25.3m. At each abutment the deck is supported by 4 neoprene bearing pads which sit on bearing pedestals founded on top of a RC headstock. The abutment headstock are each supported on four piles.
The central pier support consists of a rectangular RC column stem. The base of column stem is cast into RC pile cap which in turn is supported on 2 rows of six piles. Toward the base of column stem there is a rocker joint consisting of three Tetron bearings aligned on a common axis.

I have completed the modelling using CSIBridge and right now I'm doing model updating to match the results we obtained by dynamic testing on bridge (such as fundamental frequency). I noticed that the fixity of support play a major role in the output. I'm not sure how to connect the deck to sit on abutment (simulation of pad bearing); and for headstock which sits on three piles. For central bent which is integrated to deck with no bearing ( it has rocker joints at the connection with the pile cap). I'm not sure either how to model the bent to act integrally with deck.

As of now, I need some guidance regarding what type of connection to use to simulate the bearing at abutments, pile for headstock and connection between girder and bent. Here's my thoughts:

1_ to use joint restraint along the headstock to simulate the connection with pile (how about line spring/joint spring)?

2_to use two joints links for bearing between abutment and headstock (I don't know should I change the stiffness values or not)?

3_to use two joints link with 6 dof free and no stiffness and zero length( similarly I don't know how to do that)

Pardon me for writing such a lengthy descrption, just wnated to make sure you have full idea. This modelling is very crucial for me as I'm doing strucutural health monioring and need to update my model for selecting optimum sensor and laoding positions.

Peace

 

[JJIako]

Hi Mohandes2018,

I`m also in Model updating, time-series analysis and modal identification of bridges.
In CSI bridge you can simulate the integration by going to the bridge bent data (in components menu) and checking the 'Girder support condition Integral'. As for the seating of the deck in bearing, the assumption made for rigid (unyielding) support is by modeling an elastic spring introducing very high stiffness in the translation of the vertical-axis (UZ). The latter elastic stiffness is obtained by using K=[5*A*G*(S^2)/tq]+5*G*tq*(S^2), where A describes the area of the bearing (B*L), G is the shear modulus, tq is total thickness of the elastomeric (without including the outside layers)and S is the geometry factor given by S= (B*L)/[2*(B+L)*ti], where ti is the thickness of the one layer of elastomeric, B is the breadth and L is the length. The horizontal stiffness in UX used is obtained by Kn= A*G/tq. In addition, it a common practice to introduce rotational stiffness in horizontal and vertical axis. In particular, the rotational stiffness in horizontal axis (RX) and vertical axis (RY) can be both obtained by Kr= [(B^5)*L*G]/(n*75*(ti^3)), where n is number of the layers composing the elastomeric (excluding the top and bottom safety steel layers). The difference between RX and RY is the determination of B and L. The B is always the dimension perpendicular to the axis of rotation, while the L is the dimension parallel to the axis of rotation. I think the introduction of UZ, RX and RY are adequate assuming that the UX, UY and RZ are free.

The boundary conditions influence the exact values of the eigenfrequencies and the order that the eigenmodes are being obtained. The rule of thumb is that higher eigen-frequencies means that the system needs more energy in order to achieve the eigenmode. Restraining the system subsequently means that the system is forced to a specific movement pattern. If it is ok with you, I would like to recommend the use of ArteMis modal analysis, for modal identification. The concept of elastic spring stiffnesses introduced in UZ, RX and RY can be employed also in order to simulate the elasticity of the ground (different calculations).

 

[Shz713]

Dear JJIako,

I acknowledge your detailed explanation. The main issue is that I have only very limited HAND DRAWINGS of bridge and hence no details about bearing. So, in my case in order to make bent integrated (monolithic) with girder, you suggest to use elastic spring;
- Would you elaborate a bit more in terms how to define and utilize this spring as means of connection?
-Is it ok to increase the stiffness by 100 times (as written in bridge engineering handbook) to simulate rigid connection in UZ, RX and RY and make other DOF free?

Plus, for the abutment bearing that is neoprene, is it reasonable to use Rubber isolator?
I consider the length of link to be length of bearing at abutment, is this correct assumption (variation of links on output)?

regarding Artemis, my university got license but as of now I need to manually update the model. Bridge I'm working on was tested in 2013 by Department of Transportation and Main Roads Queensland Australia; so I need to match my model to their model and then start modeling various scenarios and analysing using ARTeMIS

Cheers mate

 

[JJIako]

I would give a closer look tomorrow . I think that it would be helpful if you can send me one or two of hand-drawings of the bridge relative to the details-position of bearings, in order to give you a more detailed answer. See contact details in attached.

 

[Shz713]

Thanks a lot, I have emailed you the details of bearing.

 

[Shz713]

Dear JJIako,

Would you tell me the formulas you provided are based on what design Code? Because they differ than that of AS 5100.4 (Australian Code)

Since you have seen the neoprene bearing details for the bridge I'm working, if I want to introduce partial fixity (not fully fixed) using stiffness in UZ, RX and RY --> Is it okay to assume followings:

Shear Modulus (G): It is not mentioned, but code proposed two values, viz. 0.69 MPa and 0.90 MPa (based on Durometer hardness which I have no idea what is it)

Bearing Details (12" * 1" * 24"): So I reckon Area of bearing is (12*24)=288 in^2

Total Thickness (tq): There's only one layer of neoprene which sits on 2 layers of bearing pedestal with dimension of 2'1.5" * 1'2.75" * 1". Should I include the bearing pedestal thickness (total 2 inches) as well?

Shape factor (S): Ti will be 1 inch which is thickness of neoprene bearing excluding pedestals, right?

number of layers (N): For this bearing is 1 layer, correct?

Would you show me for this bearing the rotational stiffness in X and Y directions? I obtained compression stiffness for UZ to be 508.8 MN/m using AS and 405 using formula you provided.

Again heaps of thanks