Throughout the course of our endeavor as a Structural Engineer, we always come up for questions that challenge us and trying to figure out the solutions. In Structural Analysis, there are tons of questions out there waiting to be answered. Some answer to these questions is easy to find and the others can only be answered by experience. Knowledge is continuous learning, even the most experienced engineer is continuously learning in his/her daily experiences. Sharing the knowledge that we have learned is a noble thing to do to spread the information for those struggling and needed it the most.

This article aims to at least understand and comprehend the frequently asked questions in Structural Analysis. To start with, here are the most frequently asks questions in Structural Engineering particularly in Structural Analysis with its answers. These questions can also be encountered and might be asked by an interviewer during the technical interview.

### What is the difference between Static and Dynamic 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.

### What is the concept of Modal Analysis and how many modes do we need to consider?

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.

### What is Torsional Eccentricity and how it will be accounted for in the design?

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.

### What are Structural Irregularities? Explain.

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.

### How to scale the Elastic Response parameters calculated by Dynamic Analysis and why?

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.

### What are the different structural systems? Explain about Dual System?

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.

### What are the two methods used in the analysis to combine the SPECX and SPECY effect?

**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.- By defining a load combination which is 100% from Principal direction + 30% from the other direction or vice versa.

### What is the difference between Rigid and Semi-rigid diaphragm?

**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.

### How to include the effect of the Vertical Component of the Earthquake in the analysis?

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

### What is the physical meaning of “Damping” and give examples?

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.

### In structures designed to resist seismic, what is a “Collector Elements”?

**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).

### What is Story Drift?

**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.

There are surely a lot more questions in Structural Analysis that you encountered and might think of. Share us your thoughts. Feel free to leave your questions or even with an answer, we are glad to include it on the above lists of FAQs for everyone to be informed of. Let us interact; everyone is welcome to view their opinion whether it is based on facts or their experiences. Cheers!

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nargesHi, I will be modelling a structure composed of two towers (12 floors each) and will have a common podium of 1 floors. I will be using ETABS 2016 because of the “Multiple-Towers” option. Here are my questions: 1. Is that mode working or it is in BETA status? 2. If it is working, can you give me tips on how to use it? I’ve checked YouTube and there are only few videos about it and none of those showed an example. 3. Another concern is structure period, this should be different for each tower, however, ETABS will present only one period which is a function of the top story assigned. 4. In connection with #3, Static Base Shear will also be affected, and we know that the succeeding structural analysis will depend on this. I’m really new in this “Multiple-tower” set up so if someone can help me, I would really appreciate. Thank you

thestructuralworld(Post author)If I were to design a multi-tower with a common podium, I will separate the two towers with an expansion joint. This is to analyze the 2 towers independently.