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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: Hi Mohammad,

I think hinge-hinge is the best option, as that will avoid fatigue problems with the ridid joint due to constant stress reversals from lateral deflections of towers due to wind loading. Also, by having hinge-hinge on both sides it means you will have the same details for all bridges for both sides.

Don't forget to cater for axial stresses caused by out-of-phase seismic drifts of the two towers and also due to temperature expansion/contraction. These stresses can be avoided by having a roller at one end, but then you will have problems with seismic pounding! So stick with hinge-hinge.

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QUESTION: Thanks for your helpful reply.

I do know steel bolts at connections undergo fatigue due to cyclic loading, but I haven't been giving much attention to fatigue in concrete rigid connections due to cyclic loading previously.

Can you provide me any link, .pdf file,etc... concerning this phenomenon?

Answer
Hi Mohammad,

If you think about it, a rigid connection in RC relies on the rebar for all tensile (& possibly compressive) resistance when bending, so it will be the rebar that will be subjected to cyclic loading.

As for the concrete, I do not have any papers about fatigue but if you Google, you will finds lots of info, such as:

http://www.ceat.uiuc.edu/PUBLICATIONS/publications/Final%20FAA%20COE%20Reports/C

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Arshad Khan

Expertise

I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

Experience

Employment history: 40 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

Publications
•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

Education/Credentials
BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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