Reference : MOMENT AND LONGITUDINAL RESISTANCE FOR COMPOSITE BEAMS BASED ON STRAIN LIMITED DESIGN...
Dissertations and theses : Doctoral thesis
Engineering, computing & technology : Civil engineering
Physics and Materials Science
http://hdl.handle.net/10993/44554
MOMENT AND LONGITUDINAL RESISTANCE FOR COMPOSITE BEAMS BASED ON STRAIN LIMITED DESIGN METHOD
English
[en] MOMENT AND LONGITUDINAL RESISTANCE FOR COMPOSITE BEAMS BASED ON STRAIN LIMITED DESIGN METHOD
Zhang, Qingjie mailto [University of Luxembourg > Faculty of Science, Technology and Medicine (FSTM) > Department of Engineering (DoE) >]
11-Sep-2020
University of Luxembourg, ​Luxembourg, ​​Luxembourg
DOCTEUR DE L’UNIVERSITÉ DU LUXEMBOURG EN SCIENCES DE L’INGÉNIEUR
245
Schäfer, Markus mailto
Waldmann, Daniele mailto
Bender, Michél mailto
Francis, Olivier mailto
Hicks, Stephen mailto
[en] Composite structure ; Strain-limited design ; Composite beams
[en] The bending and longitudinal shear design of composite beams of steel and concrete
follows often the plastic design method, which is a simplification based on rectangle
stress blocks. The application of the plastic design method requires cross-section to have
enough rotation capacity allowing most parts of the critical cross-section reach plastic
at failure. There are different types of compact composite beams, such as the slim-floor
beams. For them, the neutral axis position often gets deeper at failure, which reduces
the rotation capacity and brings questions to the bending resistance and longitudinal
shear design according to the plastic design resistance.
For a composite beam with deep neutral axis position, advanced numerical methods
such as strain-limited design and FEM simulations can provide more accurate results than
the plastic cross-section resistance. However, they are challenging to perform for general
design engineers. In this work, simplified non-linear strain-limited design approaches,
a strain-limited design software "SL.com" and an Abaqus add-in "CivilLab" have been
developed to simplify the numerical calculations. They have also been applied in other
chapters of this work to check the conventional plastic design results and to provide
simplified design rules through parametric studies.
With full shear connection, a deep neutral axis position in composite beam under
sagging bending may cause an important part of the steel section not to reach plastic at
concrete failure. In this case, plastic bending resistance calculated based on rectangle
stress blocks can result in an overestimation of the resistance and therefore leads to
unsafe design. Thus, according to EN1994-1-1 [22], a reduction factor β on plastic
bending resistance (Mpl,Rd) needs to be applied for cross-sections with steel grade S420
and S460 and the relative compression zone height (zpl/h) is over 0.15. However, with the
developments in industry as well as the second generation of Eurocode, this reduction
factor still needs to be updated to consider new types of composite beams and wider
ranges of steel grades.
While the conventional plastic design method has its limitations and only applicable
when the beam cross-section has enough rotation capacity to allow full plastic development, the more advanced strain-limited numerical calculation and FEM can be used
for a much wider range regardless of the position of the neutral axis. The investigations
in this dissertation through comparing the plastic bending resistance with advanced
numerical calculation results, have confirmed that besides the cross-sections with high
steel grades (S420, S460), also certain cross-sections with lower steel grades can have an
overestimated plastic bending moment resistance. At least this effect is more important
for compact cross-section types such as slim-floor sections or composite beams with
asymmetrical structural steel profiles or with a small concrete slab effective width. Therefore vast amount of parametric studies based on strain-limited method and FEM have
been developed to check the topics, such as limitation of plastic design methods for
different types of composite beams. Furthermore new reduction β functions on Mpl,Rd for
engineering practice considering much wider variates of composite beam cross-sections
have been deviated.
For the design with partial shear connection, the partial shear diagram developed based
on plastic analysis has been widely used. As discussed above, the plastic design may
not be suitable when the position of neutral axis is too deep, similar problems can occur
for the partial shear diagram. This problem is especially significant for slim-floor beams,
for which due to the compact cross-section, the relative compression zone height (zpl/h) is usually much higher than conventional composite beams. Thus the limitation of using
the partial shear diagram for slim-floor beams is provided, and additional simplified
engineering design rules are proposed.
Plastic development inside the cross-section increases the longitudinal shear force in the
plastic zones, furthermore with ductile shear connectors and respecting the minimum
degree of shear connection, the non-linear redistribution of longitudinal shear force
allows equal distance arrangement of shear connectors by the conventional design.
For which, the full plastic development of the cross-section allowing plastic bending
moment resistance and ductile shear connectors allowing non-linear longitudinal shear
force distribution are the two fundamental conditions. The deep neutral axis position
brings questions directly to the first assumption, as full plastic development of crosssection may not be able to reach. Thus the impact of a deep neutral axis position in the
composite beams on longitudinal shear force distribution has been analysed. For which,
the influence of plastic development inside beam cross-sections on longitudinal shear
force with full shear interaction is theoretically explained. The different stages of nonlinear distribution of longitudinal shear force due to shear connectors are investigated
through FEM parametric studies. Based on the theoretical and numerical calculation,
the design suggestions of composite beams with deep neutral axis position are given.
Researchers ; Professionals ; Students ; General public
http://hdl.handle.net/10993/44554

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