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FEA Simulation of Construction/Assembly Processes

Slip Form Segmental Bridge Construction | FEA Consulting
October 2, 2015 By: Peter Barrett

Most finite element simulations assume that all the elements begin in a stress, strain and deflection free state.  However, in reality, most structures, especially those that involve construction, will have residual stresses and strains caused by the assembly sequence. The construction of a concrete slip-formed segmental bridge (Figure 1, above), where the forms for the next pour are cantilevered off the completed bridge section, is an example where quantifying the sequential construction sequence is a must.

However, residual construction stresses are often equally important in less obvious scenarios. Anytime a repair is made to an existing structure, it would be in the owner's best interest to evaluate the impact of the repair on changed load paths in the final structural state.

Path dependent sequential construction FEA is a necessity in developing an efficient construction process to replace steam generators in a post-tensioned concrete containment building where an excavation hole was necessary. The construction plan for de-tensioning and subsequent re-tensioning of the tendons around the hole in order to avoid pre- and post-construction cracks is designed using analysis.  When the hole in the building is cut, the vertical force load path is forever changed by load redistribution. To compensate for this, the repair requires increased tendon loads over the original design to overcome the lost compressive stress in the hole repair region. Figure 2 illustrates the change in the vertical stress state in the building prior to repair, when the hole is present, and after the completed repair and retensioning. Other examples where FEA-based sequence modeling is important include analyzing excavations, evaluating the changing residual stresses throughout complex welding processes, and material forming operations during 3-d printing. With today's computational horsepower, it is feasible to explicitly model these construction processes using a path dependent, incremental large deflection nonlinear analysis.

Figure 2: Vertical Stress (psi) in a Post-Tensioned Containment: Prior, During and after Repair

In ANSYS, a construction/assembly process is simulated with the element birth and death tools. Element birth and death commands identify elements to be activated and/or deactivated respectively based on their locations and/or stress or strain state.

To achieve the "element death" effect, the program does not actually remove "killed" elements. Instead, it deactivates them by multiplying their stiffness (or conductivity, or other analogous quantity) by a very small factor. Any element loads applied to deactivated elements are zeroed out. Similarly, mass, damping, specific heat, and other load transfer mechanisms are temporarily zeroed out for these deactivated elements. The element's strain is set to zero as soon as it is killed.

In a similar manner, when elements are "born," they are not actually added to the model; they are simply reactivated. All elements, including those to be born in later stages of the analysis, must therefore be present prior to the initial simulation step. When modeling a construction sequence, all elements except those that make up the initial "foundation" are killed prior to the first solve. In each subsequent load step, elements are manually "birthed".

When an element is reactivated, its stiffness, mass, element loads, etc. return to their original values. This might not always be realistic. For example, accurate modeling of concrete curing often needs to be simulated in several independent load steps where the gravity loading is applied immediately but the stiffness is gradually increased to its final 28 day strength. 

Elements that are reactivated have no record of strain history (or heat storage, etc.), even though they may undergo significant deformation before activation due to the motion of the surrounding structure.

Some helpful hints for modeling construction sequences:

  • It is often necessary to fix displacements of elements in their killed state since they are still active in the analysis. Fixing nodal degrees of freedom of killed elements will prevent excessive deformation caused by either the displacements of the surrounding elements or free rigid body motion. If you add these artificial constraints, be sure to remove them in a manner such that artificial stresses are not induced in the model upon activation.
  • Annealing can be simulated by simply killing and then reactivating elements in subsequent load steps.
  • If contact elements are attached to the regions being killed, be sure to also kill the contact pair.
  • Activation or deactivation generally occurs instantaneously. This stepped change nonlinearity (similar to changing contact status) can cause convergence issues when a large number of elements are being killed or reactivated. Activation/Deactivation of small sections at a time will minimize these problems. Figure 3 illustrates the staged construction of tunneling where this procedure is a must.
  • For materials  whose "born" stiffness needs to be gradually ramped on, temperature dependent material properties in conjunction with dummy temperature loads can often be used to simulate this gradual process.
  • When postprocessing, be careful to deactivate the killed elements to avoid unrealistic results.  Inaccurate stress results, in particular, can occur if the killed element stresses (which are set to zero) are averaged with active elements.
Figure 3 - Staged Construction of Tunneling

The ANSYS process described above can be implemented in a similar manner in other FEA General Purpose codes.  Can anyone else share some tips and tricks for simulating construction/assembly sequences?