Seismic & Wind Lateral Load Generation for “Other Structures”
The consideration of accurate seismic and wind load models is an essential step in the process of modeling, analysis and design of the main force resisting system (MFRS) of a structure. Seismic and wind loads may be difficult to calculate because they are unpredictable, subject to interpretation and at times based on a company’s experience (in-house standards). But there are standard calculations that can provide a good prediction of a structure’s performance based on the code-prescribed recommendations.
Current building codes provide wind and seismic design criteria written mostly to address floor-based (commercial and institutional) structures. Frame-based (industrial) structures, although similar, possess different flow characteristics (geometries, framing systems, mass and stiffness) that can vary significantly from floor-based structures.
In many cases, structural engineers attempt to use codes and standards which may have inadequate provisions for industrial structures, which ultimately could significantly over/under-estimate their loads (see Figure 1 for different load applications).
GT STRUDL has implemented wind- and seismic-independent load generators – based on ASCE Standards 7-05, 7-10, Wind Loads for Petrochemical and Other Industrial Facilities (ASCE Petrochemical Energy Committee) and IBC 2012 – to assist with seismic and wind lateral loading analysis. In addition to MFRS, this application also extends to generic structures (pipe racks, open/partially clad frames and vessels) commonly found in the process industry.
There are three basic steps for the wind load calculation to be member-applied to structural types designated as “Other Structures,” including industrial structures, per the documents indicated earlier:
1. Wind Velocity Pressure qz = 0.00256 Kz Kzt Kd V2 (I) 2. Calculation of Design Wind Force Fm = qz G Cf Af 3. Member Load Distribution as uniformly distributed load Fm/Lm
Note: qz minimum is taken as 10 psf under ASCE 7-05 and 16 psf under ASCE 7-10; I is applicable only to ASCE 7-05; Cf is based on Cdg/e. Seismic Loads
The current seismic load implementation is applied as a set of joint loads that represent the prescribed lateral, vertical and accidental torsion components that are computed by the following steps:
1. Mapped MCE spectral response accelerations SS + S1 and the design spectral response accelerations SDS + SD1 are specified or computed.
2. Seismic base shear is computed and distributed as a lateral seismic force Eh to each specified story level.
3. Seismic lateral force Eh at each story level is distributed as a seismic story joint load to each joint this story level. If selected:
a. The vertical component of the seismic load, Ev, is computed and distributed as a joint load to each joint on each specified story level.
b. The accidental torsional moment load is applied to each specified story as a set of self-equilibrating joint loads that produce the specified accident torsional moment.
Many building codes provide wind and seismic design criteria written to address floor-based (commercial and institutional) structures. Frame-based (industrial) structures possess different characteristics that can vary significantly, and therefore require different design considerations.
This brief review was written to highlight and provide some guidance on the new GT STRUDL wind and seismic load generators. Information cited is based on industry standards and documentation, company standards and documentation, and other published articles. For more information on the actual GT STRUDL implementation, please contact the author at firstname.lastname@example.org.
Mitch Sklar, P.E., is a subject matter expert at Intergraph CADWorx
& Analysis Solutions. He has an extensive project background and
situational knowledge of design, analysis and construction with more
than 28 years of evaluative expertise in structural, pipe stress and
finite element analysis.