Frequently Asked Questions in Structural Analysis

Static Analysis is considered for regular building less than 240 feet (72 meters) and irregular story 65 feet (20 meters) or less. It can also apply to regular or irregular structures in seismic zone 1 and in occupancy category 4 and 5 and in Seismic Zone 2.

Dynamic Analysis is considered for regular building above 240 feet (72 meters) and irregular over 65 feet tall (5 stories) and of those buildings located in poor soils and of a time period with more than 0.7 seconds period.

Modal analysis is the field of measuring or calculating and analyzing the dynamic response of structures and other systems during excitation. As per the code, UBC-97 section 1631.5.2 and ASCE 7-10 section 12.9.5 the total number of modes considered in the analysis shall include at least 90% of the participating mass of the structure or at least 90% of the participating mass of the structure should be achieved. There is no required number of modes to consider in the analysis but make sure that it is sufficient enough to obtain the 90% target.

Torsional Eccentricity is created when the center of mass and center of rigidity or resistance in the building do not coincide. The distance between these two points is referred to as torsional eccentricity.

Structural Irregularities in buildings are composed of the weak story, soft story, and vertical discontinuity. Refer to the previous article - Understanding Seismic Parameters for further understanding.

A Scale Factor or scaling of the results calculated by dynamic analysis is used to reduce the elastic response parameters. This is usually done to normalize the forces and to distribute it properly while the code level forces are being maintained. Refer to this article for more.

For structure where more than one system is used throughout all or part of the structure, the provision for UBC 97 Dual system shall be met. Alternatively, one system may assume to take all the lateral loads and the other is taken as a building frame system. For such cases, the lateral forces need to scale up to ensure that all the lateral loads are carried by the respective system.

1. SRSS or Square root of the sum of squares is used when we want to capture or consider the forces in orthogonal directions of the response spectrum in a dynamic analysis.
2. By defining a load combination which is 100% from Principal direction + 30% from the other direction or vice versa.

RIGID DIAPHRAGM can rotate and it can translate, but cannot deform. The lateral forces will be distributed & redistributed at the subsequent levels based upon the relative stiffness of all the members resisting lateral loads at each level. When applied in software, the beam in the lateral system had no axial force.

SEMI-RIGID or Flexible DIAPHRAGM is used mainly for defines the wind load and give the building the ability to behave as its actual behavior.

The load effect resulting from the vertical component of the earthquake ground motion and is equal to an addition of 0.5C_aID to the dead load effect, D, for strength design and may be taken as zero for allowable stress design.

Damping measures and describes how a system or a building, for example, reacts after a disturbance or movement in the form of an earthquake. Many systems exhibit oscillatory behavior when they are disturbed from their position of static equilibrium. A mass suspended from a spring, as another example, might, if pulled and released, bounce up and down. On each bounce, the system is "trying" to return to its equilibrium position, but overshoots. Sometimes losses (e.g. frictional) damp the system and can cause the oscillations to gradually decay in amplitude towards zero. The damping ratio is a measure of describing how rapidly the oscillations decay from one bounce to the next.

COLLECTOR ELEMENT is a member or element provided to transfer lateral forces from a portion of the structure to vertical element of the lateral force resisting system (LRFS).

Story Drift is defined as the lateral displacement of one level relative to the level above or below or simply the difference of the deflections at the center of mass at the top and bottom of the story under considerations. The difference between the roof and floor displacement is called inter-story drift.