# 1. Advanced Solver

When performing an analysis with STAAD, there are several processes that are undertaken, including solving the stiffness matrix which can be the most significant time consuming part of the analysis process. The advanced solver is two solvers in one. This is because it can perform both in-core and out-of-core techniques. The out-of-core method creates and uses temporary data files, while the in-core method holds all the data stored within memory during the matrix formulation, which will be faster than out-of core if the temporary data files need to be written to disk.

- The advanced in-core solver is used for models with under 20,000 joints.
- The advanced out-of-core solver is used for models over 20,000 joints.
- The advanced in-core solver can be 500 to 2,000 times faster than the STAAD solver.
- The advanced in-core solver is between 100% and 200% as fast as the out-of-core solver method.
- The advanced analysis solver is particularly efficient for:
- Large models
- Models with large numbers of primary load cases
- Dynamic analysis
- Master/slave models
- Models requiring iterative solutions

# 2. Geometric Non Linear Analysis

Structures that are subject to large forces which result in significant displacement will introduce secondary effects which are not captured during a typical first order elastic analysis. These secondary effects can be accounted for using a P-Delta analysis or more accurately with a Geometric Non Linear (GNL) analysis. Here the loading is applied incrementally and at each load increment, equilibrium between the applied loading and the internal force distribution is obtained before moving onto the next load step. The displacements at each load step are saved and can be reviewed to establish the response of the structure during the application of the load.

The GNL analysis process can be continued until the structure reaches, but does not exceed, the non-linear buckling stage.

# 3. Additional Meshing Routines

In order to create models with floor and wall surfaces, these panels need to be decomposed into a series of finite element meshes. In STAAD.Pro a tool is provided which allows the boundary to be defined and a suitable method chosen. With the advanced analysis module an additional triangular and quadrilateral method are included. This provides the opportunity to select a mesh that may be better suited to the specific conditions of the structure.

In STAAD(X) this is taken further with a new option added for physical surfaces. These can be decomposed into meshes using higher order finite elements. This is done with mid-edge nodes which means that triangular elements are defined with six nodes and quadrilateral elements with eight nodes. Additionally when dealing with thin surfaces, the analysis can be set to ignore the out-of-plane forces for a plane stress solution.

# 4. Pushover Analysis

Engineers can perform a pushover analysis as per FEMA 356 : 2000 and ATC 40. Pushover analysis is a static, non-linear procedure using a simplified non-linear technique to estimate seismic structural deformations. It is an incremental static analysis used to determine the force displacement relationship, or the capacity curve, for a structure or structural element.

The analysis involves applying horizontal loads, in a prescribed pattern, to the structure incrementally; for example, pushing the structure and plotting the total applied shear force and associated lateral displacement at each increment until the structure is in a collapse condition.

In the current implementation of the pushover analysis, the user can provide hinge properties as per table 5-6 and 5-7 of the FEMA 356 manual (Generalized Force-Deformation Relationship) and also enter the expected yield stress of steel.

At present, the STAAD.Pro pushover analysis is only applicable to steel structures.

# 5. Steady State Analysis

A structure subjected only to harmonic loading, all at a given forcing frequency and with non-zero damping, will reach a steady state of vibration that will repeat every forcing cycle. This steady state response can be computed without calculating the transient time history response prior to the steady state condition.

Ground motion or a joint force distribution may be specified. Each global direction may be at a different phase angle.

Output frequency points are selected automatically for modal frequencies and for a set number of frequencies between modal frequencies. There is an option to change the number of points between frequencies and an option to add frequencies to the list of output frequencies.

# 6. Buckling Load Analysis

STAAD.Pro can now identify the factor by which the loads in the selected load case should be increased (or decreased if less than one) such that Euler buckling would occur. This buckling method is automatically activated if an advanced analysis license is available. When using the advanced solver, the corresponding "buckling modes" are included in the output file.

The buckling modes, shapes, and table are available to be viewed in the post processing mode in a new buckling page.

# 7. Base Isolators

One of the great new features in STAAD(X) is the ability to include base isolators at the base of columns to counteract the effects of vibration under the dynamic effects typically due to a seismic event such as an earthquake. STAAD supports low damping, high damping and lead rubber isolators.

The analysis produces two sets of results, first with the isolator fixed (as though it were locked) and a second with the isolator free to deform as in its definition.

One of the primary goals of a base isolator is to shift the fundamental period of the structure to a higher value and thus away from the peak accelerations which occur in lower frequencies. Once a base isolator which meets the required axial load and displacement demands has been determined, trial and error may be used to vary the fundamental period of the structure. Further, increased damping provided by base isolators can reduce the total seismic acceleration.

Refer to Chapter 17 "Seismic Design Requirements for Seismically Isolated Structures" of ASCE 7-05 Minimum Design Loads for Buildings and Other Structures for additional information on the design of base isolators.