S.E. Exam Question Clarification

[Perception]

Hello everyone,

I wanted to get some clarification on problem 903 (bridge question) of the NCEES SE Practice Exam. I'm not a bridge person, so perhaps I'm missing something.

In the problem, the response modification factor is applied to the overturning moment in multiple column bents. However, the response modification factor is not applied to the overturning axial loads produced in the column as a result of the moment. Why is the factor not applied to the axial loads as well?

Thanks

 

[STrctPono]

That's because the Response Modification Factor "R" is based on the idea that you will have a certain level of inelastic response/yielding of the column. We refer to this as plastic hinging. Some systems will be allowed higher R values because they are more likely to have more inelastic behavior and form multiple plastic hinges (multi column bents vs single column pier). You undoubtedly are familiar with the response of steel when subjected to uniaxial tension. Steel is a very ductile material that allows a significant amount of plastic deformation before rupture. Plastic deformation is a really good way to release energy from a system that is experiencing motion due to unpredictable seismic base excitation. This is the Engineers way of artificially releasing energy from the system by reducing the forces based on the assumption that this plastic hinging will occur. Unfortunately, this Force-Based design procedure is really just a bunch of voodoo science and is a rather poor predictor of actual structural behavior in response to a seismic event. This is why the design community has been slowly moving more to a Performance-Based design philosophy which is a more accurate predictor of actual structure behavior but comes at the cost of a more intense analysis with a higher theoretical understanding from the Design Engineer.

Imagine a column that is being pushed sideways... the cover concrete has spalled off and the longitudinal bars have plasticized, but that does not actually mean the column is going to fail so long as the core concrete stays intact. So unlike building design that allows that plastic hinge or fuse to form in the beam (think dog bone style design) bridge engineers allow the plastic hinging to form in the column. Never the beam! We DO NOT apply the Response Modification Factor to axial loads or shear loads! These behaviors have almost no ductile response and are therefore treated as a brittle failure mechanism. It is very very important to maintain the integrity of the core of the concrete and to not allow your spiral (confining) reinforcing to buckle outward. The spiral reinforcing not only provides shear capacity but confinement capacity as well. In fact, AASHTO has very restrictive requirements for confining reinforcing steel ratios in the areas of the plastic hinges. When in doubt, keep your longitudinal (flexural) reinforcing steel to a minimum and load up on your spiral reinforcing.

Welded Seismic Hoops > Continuous Spiral Reinforcing > Closed Circular Hoops with 135d bends > Square Ties. This is why circular columns have historically performed much better than square columns in seismic events.

Look at the aftermath of the Christchurch event in New Zealand in 2011. You will see plenty of examples of failure of the confining reinforcing which lead to total failure of the column.

I am curious, why are you asking about the bridge design portion of the test if you are not a bridge guy? Aren't you taking the building portion of the test? Or are you just curious?

 

[Perception]

Thanks, StrtPono. That answer makes perfect sense, as shear and axial deformations are not known for being ductile.

This semester, I am teaching a structural dynamics class. Since I covered the basics of ASCE 7-16 for buildings, I decided to also cover how to compute seismic forces using AASHTO (thank you bridge people in the class...). It just so happens that the SE practice exam had some pretty decent problems to introduce me to lateral forces using AASHTO.

I do plan to take the SE exam in a few years, and yes I will take the building portion. In some sense, I guess I have started to study bridges earlier than expected with this lecture.