Answers to 10 Common FAQs in Structural Analysis

As structural engineers, e2 engineers endeavor to analyze structural engineering and design projects from multiple angles, including those that present potential unique solutions the competition may miss. To that end, we often need to ask and answer numerous engineering questions for better informed design. In fact, we highly value learning, both as it aids in the design and engineering process as well as in other aspects of life.

As a result, we welcome engineering questions from our clients, particularly as they relate to the very same structural analysis questions we must pose to ourselves throughout the engineering process. Here, we’ve compiled a list of the top 10 structural engineering questions, as well as their answers:

1. What is static analysis?

Static analysis is a discipline utilized by engineers that determines the amount of stress a structure or material is under when subjected to outside forces or loads. During the design process, engineers typically utilize software to determine how a structure will respond to varying external forces. Static analysis alone is used for regular buildings under 240 feet tall and irregular structures under 65 feet tall. It also applies to both regular and irregular structures found in seismic zone 1, or structures found in occupancy categories 4 or 5 in seismic zone 2.

2. What is dynamic analysis and how is it different from static analysis?

Unlike static analysis, which studies forces that accelerate or change very slowly, dynamic analysis studies how structures respond to forces that change very rapidly. Dynamic forces can include things like blasts, earthquakes, waves, or even winds. Typically, dynamic analysis is used for buildings above 240 feet tall and irregular structures over 65 feet tall. Additionally, dynamic analysis may be considered for buildings in poor soil or active seismic zones.

3. What is modal analysis?

Modal analysis studies how the properties of a structure respond to external forces that may cause vibration. More specifically, engineers study the mass and stiffness of the structure to determine when it will resonate – or move at a frequency identical to the external force. In the event of an earthquake, for example, a structure resonating with the frequency of the earthquake may experience significant structural damage; thus, it is important to design structures that will not resonate at this frequency.

4. How many modes should be considered?

Code states that modal analysis must analyze at least 90% of the mass of each structure. While there is no inherent number of modes to consider, engineers must ensure the number studied amounts to at least 90% of the structure’s participating mass.

5. What is torsional eccentricity?

In structural engineering, eccentricity is measured as the degree to which two materials within a structure fail to share a center in common. Torsional eccentricity shows a difference between the center of mass in a building and the center of rigidity of a particular floor. Accidental torsion through exterior forces, such as earthquakes, can cause excess torsion or twisting around the center and cause structural damage.

6. How is torsional eccentricity accounted for in design?

Design codes have attempted to enforce certain adjustments during the design process to reduce damaging torsion. Primarily, codes require symmetrical arrangement of a building’s mass to balance stiffness on all sides of the center of mass. While some degree of torsion is always present, these steps can reduce the degree of torsional eccentricity and damaging shifts in mass.

7. What are structural irregularities and how are they addressed?

In structural engineering, there are three major structural irregularities: irregularities in strength and stiffness between floors of a building (also known as weak story and soft story irregularities), irregularities in mass, and irregularities in vertical geometry where lower floors fail to support the upper floors consistently. Each is analyzed in the design process using static, dynamic, or modal analysis. Then, such issues are avoided in design. Alternatively, engineers use solutions like horizontal diaphragms – which help to distribute horizontal stresses like earthquakes and blasts throughout the vertical supports.

8. What is the difference between rigid and semi-rigid diaphragm?

Horizontal diaphragms can resist horizontal seismic forces in two major forms: rigid and semi-rigid. Rigid diaphragms rotate and translate the horizontal forces to all members spreading the lateral load throughout. However, rigid diaphragms do not deform and spread the load based on the center of mass. Semi-rigid diaphragms spread the lateral load to multiple members of the system equivalently.

9. What are elastic response parameters?

A material or structure’s elastic response refers to the information engineers receive when the structure is exposed to force and deforms. Then, when the force is removed, engineers study how it returns to its original form. The elastic response of a building frame to external loading deteriorates as it approaches critical load. In the event of an earthquake or other sudden load, strain increases as critical load is reached and causes structural damage that will not return to its original form.

10. What are the different structural systems?

In engineering, a structural system refers to the various subsystems within a building project that help resist vertical gravity loads as well as horizontal loads caused by earthquakes and other forces. Interior structural systems include hinged frames, rigid frames that resist movement at the joints, braced frames and shear walled frames that do not allow lateral movement, and outrigger frames with projecting structures that stabilize overhangs.

It is our hope that this brief FAQ has helped you understand some of the circumstances your engineers must analyze throughout the design process. If you have more engineering questions or wish to discuss a potential project with our expert team, contact the premiere engineering firm in the Northeast. Reach e2 engineers at 860-437-3259, or complete our online contact form today.