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# Frequently Asked Questions

##### What are the features included in Extreme Loading for Structures version 3.1?

A comprehensive list of features for ELS 3.1 can be downloaded here: ELS 3.1 Features PDF.

##### Is the Applied Element Method (AEM) based on the stiffness method?

Yes, it is based on the stiffness method, just like FEM.

##### Is there a significant difference between AEM and FEM?

Most definitely, in FEM elements are connected through nodes that cannot be separated. In AEM elements are connected through a series of calculation points and springs at the elements’ edge. Springs between the elements model the material properties. These points or springs can be separated any where on the element.

##### For modeling of columns, girders and slabs, do you have frame and shell elements in AEM? (These elements exist in FEM)

ELS does not have frame or shell elements. Using AEM, the user can build columns, girders, and slabs for whatever problem may be associated with them.

##### What are the material libraries you have?

##### Are these material models non-linear models?

Yes. Non-linear behavior for concrete and steel under cyclic loading is 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 Bauschinger effect is included.

##### Is Poisson's ratio considered during analysis?

Poisson’s ratio is considered as zero for all cases. It has been disregarded because in cases of concrete Poisson’s ratio has effects only before cracking occurs. Once cracking occurs, the effect of Poisson’s ratio is negligible.

##### How can plastic hinges be modeled in steel structures?

The user does not have to specifically model plastic hinges. One of amazing features in ELS is that the failure locations, due to the cracking of concrete or the steel yield, are determined automatically; The effects are included without any user intervention.

##### What kind of loading options exist in Extreme Loading® for Structures?

Static loading: own weight, load and displacement loads, lumped mass weights, uniform pressure, hydrostatic pressure, moving loads and line load. In Dynamics, ELS has load and displacement loads, uniform pressure, moving loads, earthquake, and blast loads based on the TM5-1300.

##### What is the difference between AEM and the Discrete Element Method (DEM)?

The Discrete Element Method does not have a stiffness matrix or accurate element connectivity, making DEM much slower. Furthermore, DEM does not generate satisfactory accuracy for continuum elements. DEM is primarily used for soil analysis and is not widely used for structural analysis.

##### What type of dynamic loading is calculated; implicit or explicit?

AEM incorporates the global stiffness matrix; hence it is implicit dynamic analysis.

##### How does the program assume or calculate crack (location) and element cracking; what criteria is being used?

Crack location and element separation are arbitrary in the analysis. The user does not have to guess where cracks will be. Cracking criteria are based on principal stress calculations compared to cracking strengths.

##### What material properties determine the crack point?

If the stresses reach the concrete tensile strength, cracking occurs.

##### Can ELS model fabric structures?

Yes, it can. If the fabric compression and tension behavior is close to concrete or steel behavior, then existing models can be used with modifications of strength and yield values. If the behavior is different from that of concrete or steel, then a new material model would have to be added to ELS by ASI. Contact ASI for pricing or more information.

##### Does ELS use an energy dissipation model?

Yes, there are many damping models. ELS has external damping models which are a function of element velocity together with internal damping due to cracking, crushing of concrete, unloading and reloading of concrete and steel, collision between elements, and friction between elements.

##### What blast model is used in ELS?

Currently ELS uses the free-field wave equation based on the TM5-1300, other blast loads currently under development.

##### Are applied stresses dampened when colliding with structural elements?

Yes, energy dissipation is controlled by the unloading stiffness associated with each material.

##### How many modules does ELS use?

ELS employs several modules:

- Analysis modules: Static loads, dynamic loads, seismic loads, blast loads, immaculate removal and impact loads.
- General modules: Modeling, preprocessor, solver, post-processor.

##### How is the blast loading calculated in ELS release v3.0?

Pressure time histories are calculated for all elements facing the bomb; hidden elements that do not face the bomb are neglected. Pressure is calculated as a function of the bomb weight, distance to element, incident angle and time. Free-field wave equations are based on the TM5-1300. The user can also apply blast loads as external pressure-time history to any elements he wishes.

##### Is blast loading coupled with the response, uncoupled, or is impulsive loading assumed?

Regarding coupling between pressure history and element motion, this is not presently factored due to the use of free field equations which assume that pressure is not affected by the existence of structures.

##### How long did it take to run the simulation of the AP Murrah building?

For the Oklahoma City building, the large bomb weight was a necessary consideration. The pressure positive phase (pushing time) is less than 0.01 seconds (hence we use time step of 0.0001 seconds); so, to model the structure collapse of 4 seconds, it took approximately 1-2 days due to the small time step (on a desktop PC with ELS v2.0).

##### What methods can be used to determine the blast pressure?

As previously noted, Free-field equations are used to calculate blast pressure at any location as a function of TNT weight, location, incident angle and distance between the element and the explosion source. The user can calculate the blast pressure using external tools, import them into ELS as uniform pressure, and apply it to the desired elements.

##### Which methods can be used to analyze structures?

ELS uses the Applied Element Method (AEM). It has been proven to match FEM before collapse occurs and can be applied to a broader range of applications where elements begin to separate and structural collapse is expected. AEM has been proven and verified through several publications to accurately follow the mechanical behavior in elastic range, non-linear range, crack initiation and propagation, buckling and post-buckling, and non-linear dynamic behavior until complete collapse of the structure.

##### 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 full non-linear behavior of concrete or steel or any material that needs to be analyzed.

##### Does the program use geometrically and materially nonlinear dynamic analyses etc.?

Yes. ELS incorporates both material and geometric non-linear behavior. Since ELS is based on theApplied Element Method (AEM) , a user can track crack initiation, propagation and crack closure, shear transfer models, yield, and nonlinear cyclic behavior of steel bars. Additionally, 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 kind of user intervention to mesh the model in a way that forces the cracking to propagate. Also, contact between all elements is arbitrary, without any intervention from the user in determining time and location. Once the simulation is initiated, the outcome is visually rendered without user intervention.

##### Can ELS be utilized for 'push-over' analysis?

Yes, ELS can be calculated “push-over” analysis taking into account all non-linear material behavior and effects of lateral deformations at the columns internal forces.

##### How does ELS produce internal force diagrams?

Unlike 3D FEM software where you only get stress and strain, ELS allows the user to integrate stresses for any element to create internal force diagrams (Normal, shear, moment, torsion, etc) base on fully non-linear analysis.