Home > Downloads > Extreme Loading for Structures

Extreme Loading® for Structures Version 2.3

Frequently Asked Questions

Q1. Is the Applied Element Method (AEM) based on stiffness method?
Q2. What is the difference between AEM and FEM
Q3. For modeling of columns, girders and slabs, do you have frame and shell elements in AEM? These elements exist in FEM.   
Q4. What are the material libraries you have?
Q5. Are these material models nonlinear models?
Q6. Can ELS solve steel structures?
Q7. How can you model plastic hinges in steel structures?
Q8. What kind of loading do you have?
Q9. What is the difference between AEM and the Discrete Element Method (DEM)?
Q10. What type of dynamic loading do you calculate? Implicit or explicit?
Q11. How does the program assume or calculate crack location element cracking? What criteria are you using?
Q12. What material properties determine the crack point?
Q13. Can ELS model fabric structures?
Q14. Do you use an energy dissipation model?
Q15. What blast model do you use?
Q16. Are applied stresses dampened when colliding with structural elements?
Q17. How many modules does ELS use?
Q18. How is the blast loading calculated in ELS Release 4-2005?
Q19. Is the blast loading coupled with the response, uncoupled, or is impulsive loading assumed?
Q20. How long did it take for the AP Murrah building simulation to run?
Q21. What methods can be used to determine the blast pressure?
Q22. Which elements can be used to analyze the structure?
Q23. What kind of connections can be used as input?
Q24. Does the program use geometrically and materially nonlinear dynamic analyses etc.?

 

Q1. Is the Applied Element Method (AEM) based on stiffness method?
A1. Yes, it is based on stiffness method, the same as the finite element method (FEM).

Q2. What is the difference between AEM and FEM
A2. AEM is not FEM. In FEM elements are connected through nodes that cannot be separated, while in AEM elements are connected through a series of calculation points and springs at the element’s surface. These springs between the elements model the material’s properties enabling the points or springs to become separated anywhere on the element. 

Q3. For modeling of columns, girders and slabs, do you have frame and shell elements in AEM? These elements exist in FEM.   
A3. We do not have frame or shell elements. Using AEM, the user can build columns, girders, or slabs for whatever problem he may have associated with them.

Q4. What are the material libraries you have?
A4. In the current version of ELS, we have material models for concrete and steel.

Q5. Are these material models nonlinear models?
A5. Yes. Nonlinear behavior for concrete and steel under cyclic loadings are included. For concrete, the material model includes tension cracking, compression failure, unloading and reloading of concrete. The post-cracking shear transfer models are not yet included. For steel bars or steel sections, the yield of steel, unloading and reloading, and Bauchinger effects are included.

Q6. Can ELS solve steel structures?
A6. Yes, but with some approximations. The current graphical interface in the ELS does not support drawing the complicated steel structures due to very problematical details in the connections. But, if the user approximates the steel section with equivalent ones of a rectangular cross section, then they can be solved with reasonable accuracy.  The GUI for this capability is under continued development.

Q7. How can you model plastic hinges in steel structures?
A7. The user does not have to model plastic hinges in the model. One of the amazing features in ELS is that the failure locations, due to the cracking of concrete or the collapse of the steel, are determined automatically; and, its effects are included without any user intervention.

Q8. What kind of loading do you have?
A8. ELS possesses both static loading and dynamic loading capabilities.

  • For static loading: own weight, load, hydrostatic pressure, and displacement loads, and lumped mass weights.
  • In dynamic loading: load and displacement loads, earthquakes (seismic), immaculate element removal (demolition), pressure history, and bomb blast loads.

Q9. What is the difference between AEM and the Discrete Element Method (DEM)?
A9. Discrete Element Method does not have a stiffness matrix or accurate element connectivity. This makes the DEM much slower, and DEM does not provide satisfactory accuracy for continuum elements. DEM is primarily used for soil analysis and is not widely used for structural analysis.

Q10. What type of dynamic loading do you calculate? Implicit or explicit?
A10. We are using implicit dynamic analysis.

Q11. How does the program assume or calculate crack location element cracking? What criteria are you using?
A11. Crack location and element separation are arbitrary in the analysis. The user does not have to guess where the cracks will be. The cracking criterion is based on the principal stress calculation compared to the cracking strength.

Q12. What material properties determine the crack point?
A12. If the stresses reach the concrete tensile strength, cracking occurs.

Q13. Can ELS model fabric structures?
A13. Yes, it can. If the fabric compression and tension behavior is close to the concrete or steel behavior, then existing models can be used with modifications of strength and yield values. If the behavior is different from concrete or steel, then a new material model must be added by us to the program. The user should submit the new model to ASI to include in the AEM solver.

Q14. Do you use an energy dissipation model?
A14. Yes, we have many damping models. We have external damping models, which are a function of element velocity, together with damping due to cracking, crushing of concrete, unloading and reloading of concrete and steel, collision between elements, and friction between elements.

Q15. What blast model do you use?
A15. Currently we are using the free-field wave equation, and are currently upgrading to a more accurate blast model. You can also import blast pressure history and apply it to the model manually.

Q16. Are applied stresses dampened when colliding with structural elements?
A16. Yes.

Q17. How many modules does ELS use?
A17.
For analysis modules: Static loads, dynamic loads, seismic loads, blast loads, immaculate removal and impact loads
For general modules: Modeling module, preprocessor, solver, post-processor

Q18. How is the blast loading calculated in ELS Release 4-2005?
We calculate pressure time histories for all elements facing the bomb. We neglect hidden elements that do not face the bomb. We calculate the pressure as a function of the bomb weight, distance to the element, and time. We use free-field wave equations.

Q19. Is the blast loading coupled with the response, uncoupled, or is impulsive loading assumed?
Regarding coupling between pressure history and element motion, we do not have this factored yet due to use of free field equations that assume that pressure is not affected by the existence of structures. In the future however, we have plans to enhance that to include the presence of structures.

Q20. How long did it take for the AP Murrah building simulation to run?
For the Oklahoma City building, it took a longer time for analysis, simply due to the large bomb weight.  The pressure positive phase (pushing time) is less than 0.01 seconds (hence we use time step of 0.0001 seconds).  To model the structure collapse of 4 seconds, it took approximately three days due to the small time step (on a desk-top PC).  Use of a Super Computer would have reduced that calculation time significantly.

Q21. What methods can be used to determine the blast pressure?
We use free-field equations to calculate blast pressure at any location as a function of TNT weight, location, and distance between the element and the explosion source.  Analysis is performed using a small time step to accurately track the wave propagation and structural response.  We are continuing to develop an even more advanced fluid dynamic method to simulate blast effects on surrounding areas of the structure.

Q22. Which elements can be used to analyze the structure?
We use a method called the Applied Element Method. It has been proven to match FEM in certain aspects and can follow a broader range of application than FEM. It has been proven through tens of publications that the method can accurately follow the mechanical behavior in the elastic range and the, nonlinear range; in crack initiation and propagation; in buckling and post-buckling; and in nonlinear dynamic behavior.
  
Q23. What kind of connections can be used as input?
Unlike FEM where elements are connected at nodes, in AEM elements are connected through a series of normal and shear springs that transfer the stresses among the elements. These springs can simulate the full nonlinear behavior of concrete or steel or any material that needs to be analyzed (including glass and foam).

Q24. Does the program use geometrically and materially nonlinear dynamic analyses etc.?
Yes.  Besides geometric and material nonlinear behavior, ELS also provides accurate results for the cracking and crushing of concrete. Additionally, with this program the user can track the crack initiation, propagation and crack closure, shear transfer models, yield, and nonlinear cyclic behavior of steel bars.   Also ELS can determine the separation between elements (which can not be modeled using FEM) and contact between elements during collapse. The modeling of separation and contact are done automatically  without any user intervention to mesh the model in a way that forces the cracking to propagate. Furthermore, contact between elements is arbitrary without any intervention from the user to determine contact time and location.  The outcome of the simulation is visually rendered without user intervention once the simulation is initiated.