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Civil Engineering/Preferable structural connectivity

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Question
Hey, I'm having a project where I have two 130m-towers linked by five (7m*7m) bridges. I've been ordered to make several scenarios concerning the connectivity of bridge extremities. I've excluded the cases of Rigid-Roller & Hinge-Roller after making models and analyzing them, but I'm having a difficulty in determining the best structural connectivity between Rigid-Rigid, Rigid-Hinge, & Hinge-Hinge.

The latter types of connectivity have relatively comparable effect on the buildings' drift and displacement, so choosing the best connectivity will be dependent on its effect on the bridge itself.

The analysis results are as follows -considering the bridge @ Elev. 127.8 m- :

*Displacement of bridge due to EQ (allowable displacement= 0.7m):

Rigid-Rigid: 0.6504 m/ Rigid-Hinge: 0.6587/ Hinge-Hinge:0.6614

*Displacement of bridge due to Wind (allowable displacement= 0.236m):

Rigid-Rigid: 0.1963 m/ Rigid-Hinge: 0.2308/ Hinge-Hinge:0.2319

*Moment @ bridge extremities due to ultimate case (KN.m):

Rigid-Rigid: -283.3/ Rigid-Hinge: -263.711.

At the first look I though Hinge-Hinge was the best option, but at second thoughts allowing rotation on both sides of bridge doesn't seem to be a good idea. Also, Rigidity at both ends is resulting in high moment, which leaves me with the Rigid-Hinge case.

Now I can't go to my supervisor and tell him: "I FEEL that the latter case is the best structurally".

My question is: Is Rigid-Hinge case the best, or do you have a different point of view? Any explanation related to the preferable structural connectivity is highly appreciated.

Thanks!!!

Answer
Dear Mohammad

Feelings of an engineer are very important facts governing a design, because you cant calculate all effects and investigate them with a calculator.
But I have a magical tip which is helpful for me most of the times. I consider the structure being loaded extremely to be able assess the effect of yielding of essential members on the stability of the structure. 2 consequences may occur:

1- redundancy of your rigid connection make your system turning to a collapse mechanism more later than a lower redundant structure, then you will have a more efficient design.

2- If the  members prone to yielding  force the perpendicular members to yield sooner than the horizontals then you will have a not very good design.

Then I suggest you to investigate your system in its limits.

Regards
BEH

Civil Engineering

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Babak Esmailzadeh Hakimi

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I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!

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