Equivalent Lateral Forces - ASCE 7-10

 

16.1

SCIA Engineer 16.1 extends the possibility for seismic calculation - Equivalent Lateral Forces (ELF) in compliance with the US code: ASCE 7-10.

The concept employed in the ELF procedure is to place static loads on a structure with such magnitudes and directions that closely approximate the effects of dynamic loading due to seismic events (earthquakes). The end goal of this procedure is the calculation of a base shear, as well as the vertical distribution of seismic forces to various levels of the structure. These loads can then be applied to the structure and used for seismic load analysis, instead of performing a dynamic analysis.

Calculation using the equivalent lateral force method

Setting of ELF is very simple - it is just an extension of a seismic load case. There is a new group related to this method.

The user is able to select from several methods for the calculation of equivalent lateral forces. These methods are in compliance with EN 1998 and ASCE 7-10 standards. Equivalent lateral forces are calculated at the background after a model analysis. They are applied as one concentrated force at the mass centre of each storey of the building and then a linear analysis is performed.

Enhancements of ELF based on ASCE 7-10

SCIA Engineer 16.1 performs the seismic calculation fully in accordance with Section 12.8 of ASCE 7-10, where all necessary requirements for the use of the Equivalent Lateral Force Procedure are outlined. Attention has been paid to code requirements and parameters in order to provide a full solution that meets user requirements.

Highlights
Seismic spectrum according with ASCE 7 in the "load library". The user can input the spectrum using code parameters.
Calculation method for ELF forces based on "Approximate fundamental period".
Automatic generation of seismic combinations based on ASCE 7-10 12.4.

Seismic spectrum: code dependent parameters

A seismic spectrum fully based on code parameters for ASCE 7 has been integrated into SCIA Engineer 16.1. For the ASCE 7 spectrum the following variables are required to be specified in the program:

  • site class
  • response modification factor (R)
  • seismic risk
  • category of the structure (I to IV)
  • short period acceleration parameter (Ss)
  • second period acceleration parameter (S1)
  • long period (TL)

Method of calculation

SCIA Engineer offers multiple methods for the determination of equivalent lateral forces required by ASCE 7. When polynomial distribution (ASCE 7-10 12.8-7) is picked, seismic the force can be determined based on:

  • Approximate fundamental period. All variables as defined in the code are available in the software.
  • If the period used in the calculation is defined by a method other than “approximate fundamental period", an option to Limit fundamental period is available as per Table 12.8-1 of ASCE 7-10.

Seismic combinations

SCIA Engineer now supports IBC (ASD) - seismic combination and IBC (LRFD) - seismic combination. With the combination type set, other options for the seismic load combinations as defined in ASCE 7-10 12.4 can be taken into account:

  • include redundancy factor of 1.3 (r) according to ASCE 7-10 12.3.4
  • add vertical component to the combination which enables the user to include the vertical seismic load effect according to ASCE 7-10 12.4.2.2
  • overstrength factor according to ASCE 7-10 12.4.2.2