November 24, 2015
Dean of Faculty of Structural Engineering
Universtiy of Architecture, Civil engineering and Geodesy
1 Hristo Smirnenski Blvd, Sofia 1046
Bulgaria
Dear Sir,
I’m enclosing herewith a review letter on the doctoral thesis of the candidate Dimo Siderov
Zhelev, “DUCTILITY OF BEAM-TO-COLUMN JOINT WITH ENDPLATE CONNECTION”. I’m
also enclosing one copy of conference paper written by the candidate, for purpose of reference.
I’d be happy if this letter helps your official review process of doctoral thesis.
Very sincerely yours,
Yukihiro Harada, Prof., Dr. Eng.
Department of Architecture, Faculty of Engineering
Chiba University, Chiba, Japan
Tel. +81-43-290-3410, Fax. +81-43-290-3167
E-mail: yharada@faculty.chiba-u.jp
November 24, 2015
Yukihiro Harada, Prof., Dr. Eng.
Department of Architecture, Faculty of Engineering
Chiba University, Chiba, Japan
Tel. +81-43-290-3410, Fax. +81-43-290-3167
E-mail: yharada@faculty.chiba-u.jp
Review letter on the doctoral thesis of the candidate Dimo Siderov Zhelev
“DUCTILITY OF BEAM-TO-COLUMN JOINT WITH ENDPLATE CONNECTION”
The candidate, Mr. Dimo Siderov Zhelev, had stayed at Chiba University as an exchange
research student for about nine months from October 2014, and I, the reviewer, was his advisor there.
The reviewer often discussed his research works, especially by giving information on related studies
and literature works in Japan, and hence, the reviewer knows the contents of his thesis very well. Here
the reviewer gives some comments on the candidate's doctoral thesis.
The candidate's thesis discusses the structural behavior of the steel structural frameworks'
beam-to-column joints with endplate connections. Such beam-to-column joints are widely used owing
to their numerous advantages in terms of feasibility and economy. The main objective of the thesis is
investigating how to obtain a qualitative estimate of the structural performance of such a
beam-to-column joint, which is subjected to monotonic or cyclic loading, through numerical
simulations for finite element analyses (FEA). The significant advantage of numerical simulations is
that they allow rapid “what-if” analysis.
The discussions in the thesis can be divided into two main subjects: one deals with the effect
of the prying force in a T-stub, and the other deals with the cyclic behavior of a framework with the
beam-to-column joints with endplate connections.
With regard to the first subject, previous studies have shown that the prying force in a T-stub
is not always situated on the edge of the flange plate. Based on the results of parametric numerical
simulations on T-stubs, the candidate showed that the position of the prying force can be determined
exactly by considering the bearing capacity and rigidity of the components of the T-stub. Based on
these results, appropriate expressions for the prying force's position and tensile strength of a T-stub
were proposed. These discussions about the T-stub's mechanism are essential for evaluating
beam-to-column joints.
With regard to the second subject, the candidate investigated an appropriate material model
to simulate the behavior of steel material under cyclic loading conditions properly. The modeling
parameters were validated with the test results of the material's cyclic loading tests available in
literature works. By using the cyclic material model, the candidate conducted FEA for the numerical
simulation of beam-to-column joints with endplate connections. The accuracy of the FEA results were
validated with previous experimental studies under monotonic or cyclic loading. Thus, the candidate
showed that the FEA can assess the impact of strength and ductility of the individual components of a
beam-to-column joint on the joint ductility. Next, the candidate discussed structural performance of
different types of endplate connections: connections with stiffened endplates, unstiffened endplates, or
endplates with haunches. The connections were expected to perform differently, even if they were
designed to have the same resistance. The candidate showed that the ductility of the connection types
could be ranked by focusing on the strain conditions, i.e., the magnitude of strain concentration in the
connections subjected to cyclic loading. A comparison of different details of a beam-to-column joint
will allow the evaluation of the advantages of some details over others. The reviewer highly
appreciates his original approach on evaluation of the connections and his practical and fruitful
conclusions.
In addition, the reviewer would like to comment on the candidate's contribution to the
reviewer's research project. During his nine month stay in Japan, the candidate participated in our
ongoing research project on the development of a seismic ductile knee brace, mainly through
numerical simulations of the structural behavior of the knee brace. His contribution was significant
enough to get one conference paper published; the paper is enclosed with this letter.
My final evaluation is that the candidate, Mr. Dimo Siderov Zhelev, deserves the degree of
Doctor of Philosophy in Engineering.
Yukihiro Harada
(Encl: A copy of JSSC (Japanese Society of Steel Construction) annual conference paper written by
Mr. Dimo Siderov Zhelev)