Discover Advance Design FEA for Advanced Supports

written by Evangelia Palkanoglou

In the context of structural engineering, advanced supports refer to supports or connections between different structural elements that exhibit nonlinear behavior. This type of support mimics a variety of realistic conditions taking place in structural systems, such as the behavior of bridge metallic/rubber bearings, piles or soil. In this technical article, modeling using nonlinear supports is investigated and compared to modeling without any nonlinearity occurring at supports. For this purpose, Advance Design will be used as a finite element analysis (FEA) software, and the obtained results will be useful for structural engineers.

Advanced Supports

Advanced supports are often incorporated in modeling of structures. They help consider the behavior of different systems used to enhance performance under extreme loading conditions, such as earthquakes, hurricanes and high winds. These supports are designed to provide damping, energy dissipation and stability to the structure, particularly during large deformations.

Advance Design provides a wide range of advanced supports. They facilitate the creation of supports with distinctive characteristics in different directions and also introduce a whole new set of possibilities for defining nonlinear relationships –especially important for some types of structures requiring a more complex definition of supports. Advanced supports are available for point, linear and planar support.

Figure 1: Selecting advanced supports within the interface of Advance Design.

1. Gap supports

Gap supports allow displacements/rotations within specified limits. Once these limits are reached, the supports are activated. In addition, it can be determined whether the relationship is elastic after activation and whether the relationship is symmetric (i.e., the limit is in both directions of action).

Bridge bearings fall into this support category. Bearings are designed to provide damping and energy dissipation during seismic events or other extreme loading conditions. They provide excellent absorption and transfer of external forces from the superstructure to the foundations. Meanwhile, they allow for stable rotations in any axis and horizontal sliding movements to reduce the forces upon bridges and buildings.

Bridge bearings are not only used in bridge construction but also can be applied to highways, roads, docks, airports, buildings, and other applications where large and heavy objects need to operate safely.

Bridge bearings can easily be modeled in Advance Design by selecting the gap support option provided within the software interface. In Figure 2, a typical gap support is shown. The response after the activation can be elastic, symmetric or non-symmetric (Figure 3). Since gap supports express a nonlinear behavior, the user needs to establish nonlinear analysis load case in Advance Design, allowing the solver to account for this response in the analysis (Figure 4).

Figure 2: Typical gap support with limit in distance.

Figure 3: Characteristic curves for gap support (2 cm gap) are defined as symmetric without elasticity (left) or elastic (right).

Figure 4: Establishing nonlinear analysis load cases in Advance Design.

 

Figure 5 shows the supports’ vertical reaction on a steel footbridge modeled in Advance Design assuming two different types of supports. On model (a), all the translational degrees of freedom were considered fixed throughout the entire analysis. On model (b), symmetric gap supports were implemented on these degrees of freedom, with a limit displacement of 5 mm. The rotations around the x and y axis remained enabled, whereas the one around the z axis was fixed in both models.

It is evident that the reaction forces changed when gap supports were introduced into the modeling scheme. It is also obvious that the vertical displacement in the first case remained zero (Figure 6 (a)), whereas in the first case reached the limit of 5 mm (Figure 6 (b)).

Figure 5: Support reactions Fz on a steel footbridge, where gap supports were (a) not considered and (b) considered.

 

Figure 6: Vertical displacement Dz at supports on a steel footbridge, where gap supports were (a) not considered and (b) considered.

2. Hardening supports

Hardening supports are nonlinear supports that provide resistance up to a certain force level. Once this threshold force is exceeded, they maintain their constant reaction; however they fail to offer further support, enabling any movement thereafter.

The structural response of a rigid pile can be represented effectively by a hardening support. Normally, a rigid pile offers a vertical reaction, disabling any movement. Once its capacity is reached, the pile starts moving vertically, sinking deep into the soil, causing vertical displacements in the structure. But it still provides its vertical reaction, which is equal to its capacity.

The performance of a flexible pile is quite similar to that of a rigid pile. The only difference is that a flexible pile allows some deformation instead of totally disabling it. Both behaviors can be easily modeled in Advance Design by selecting the hardening support option provided within the software interface (Figure 7).

Especially for a flexible pile, the user needs to specify a force-displacement diagram in order to allow the solver to identify the initial stiffness of a flexible pile (Figure 8). The same principles apply regarding the type of analysis required for this type of support.

Figure 7: Typical hardening support with limit in force.

Figure 8: Typical hardening support with limit in force established with force-displacement diagram.

 

In Figure 9, the horizontal reaction force Fx at supports is depicted for two different multi-story frames. Hardening support with a force limit of 10 kN was introduced to the x translational degree of freedom for both of them. A load 2.5 times higher was applied to the frame (b) in order to activate the hardening support.

It is obvious that, as the load remained low, the hardening support on the frame (a) provided resistance not allowing any movement (Figure 10 (a)). As the load increased, the support was still providing resistance; however, the force required to keep the node undeformed exceeded the threshold of 10 kN. Therefore, the hardening support could not provide any further resistance, resulting in horizontal movement of the node by 3 mm (Figure 10 (b)). A maximum reaction force of 10 kN was still provided by the support.

Figure 9: Horizontal reaction force Fx at supports of multi-story frames where hardening supports were introduced. The red circle indicates the hardening support.

 

Figure 10: Horizontal displacement Dx at supports on a multi-story frame where hardening supports were introduced.

 

Supports active only in tension/compression

Nonlinear supports active only in tension/compression are structural supports that provide resistance and stability to a structure specifically in either tension or compression, depending on the direction of the applied forces. These supports are designed to effectively transmit and distribute forces in one direction, while offering minimal resistance or movement in the opposite direction.

A typical example of a support that is active only in one loading condition is a footing foundation. Soil exhibits a nonlinear behavior: it can provide resistance when compressive forces are applied to it, but it is unable to resist in tensile forces.

This type of support can be easily implemented in a modeling scheme within the Advance Design interface. It can also be defined as having a different relation for each direction, for instance, compression-only relation for vertical translation and tension-only relation for horizontal translation (Figure 11).

Figure 11: Advanced support assignment for each degree of freedom.

 

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