Stress Engineering Interview Questions Part 3

Stress Engineering Interview Questions Part 3: Technical

Stress Engineering Interview Questions Part 3: Technical

In Part 1 and Part 2 of this series, we discussed:

1. What is a Free Body Diagram
2. What are SFD and BMD for Beams, and
3. A tricky twist to the SFD and BMD of a simply supported center loaded beam
4. What is a statically determinate structure
5. What is a statically indeterminate structure

If you have not yet done so, check out the first two parts and then come back to this part.

Alright, let’s get going with Stress Engineering Interview Questions Part 3…

Q: What is a ‘shear center’?

Answer: A point on the cross sectional plane of a beam or a section coincident with its planes of symmetry, through which any applied transverse shear load (including oblique shear load) will cause bending without any twisting of the beam, such a point is called a ‘shear center’.

Additional Information:

So in simpler terms, if a shear force acting perpendicular to the axis of the beam passes through the shear center, then it will not produce any twisting of the beam. If we want to limit the beam deflection to bending and shear deflections, then we want the shear load to pass through the shear center.

If you want to learn more about the shear center effect, check this post out:

Beam Bar Elements Shear Center Effect

The shear center is generally not at the same point as the centroid of the section, unless the beam cross section has two or more planes of symmetry. The following are some examples of the shear center and centroid at the same point.

The following are some examples of the shear center not coinciding with the centroid of the section.

If you would like to learn more about calculating shear centers of open and closed sections, refer to sections 16.2.1 and 16.3.2 respectively, in Aircraft Structures by Megson.

Q: What is a shear clip?

Answer: A shear clip is a member of a structural joint whose sole purpose is to transfer shear loads.

Additional Information:

There are probably hundreds of these shear clips in a typical aircraft structure that join both primary and secondary structures. Shear clips are not intended to transfer any axial, moment or twisting loads. Shear clips are usually seen in two forms, either thin bent up sheet metal brackets, or T extrusions. Shear clips are generally made from Aluminum alloys such as 7075 or 2024. The following figure illustrates a typical shear clip joint.

We can see in the figure above, the joint consists of a loaded member ‘a’ that is supported by beams ‘b’ and ‘c’. The applied load ‘P’ is beamed out to the support beams ‘b’ and ‘c’ using shear clips that transfer the load in shear along the direction of P. Shear clips must be designed to a “minimum acceptable” condition. That is, no twisting or bending is allowed due to insufficient number of fasteners on either leg of the clip.

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Q: Draw the free body diagram of a simple shear clip with a transverse load on one leg and two fasteners on each leg

Answer:

The shear clip cannot react out of plane tension loads or bending. At least it is not supposed to, it is not used for that purpose, so it cannot react loading by twisting or bending but in only shear. Given this main criterion, the following figures illustrate the process.

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