Investigation of Steel Welded Moment Connections Performance under Column Collapse

Progressive collapse of buildings has raised questions on adequacy of the existing regulations
to prevent local and, in turn, global collapses. The present study mostly focuses on the
performance of welded moment connections against progressive collapse. The performance of
moment connections suggested in the FEMA 350, which are proper for seismic forces,
Welded Flange Plate (WFP), Reduced Beam Seaction (RBS), Welded Unreinforced Flange-
Welded Web (WUF-W) and Free Flange (FF), has been studied. The models used include
non-linear behavior of materials and geometrical nonlinear behavior. The behavior of steel
materials used in the structure is the true behavior of steel was stress-strain, which has been
considered in the model completely. The nonlinear stress-strain behavior of steel selected for
modeling the real behavior of beam and column members in the structure. The material
properties of all steel components were modeled using elastic-plastic material model from
ABAQUS. For connection region porous material plasticity was used. The diagram of vertical
force against vertical displacement for each connection was drawn, and the state of each
connection failure was investigated. Making the large scale experimental models to study the
progressive collapse of structures seems too difficult. Using finite element models to study the
behavior of structures are relatively appropriate option with regard to time and cost. In all of
the numerical models, shell (S4) element has been used to simulate the beams, columns and
connections. This is a four-node element, which contains four integration points on the
element. During the calculations, full integration method with more precision was used. For
analysis of the models, dynamic explicit method was used. This method is suitable to analyze
the models with more members having nonlinear characteristics of materials and large
deformations. In this method, the central difference integrating is used to solve the dynamic
equations. In every time step, this method performs simpler than other methods in solving
dynamic equations since there is no need to inverse stiffness matrix in any time stage. The
used numerical method has compared using the laboratorial results, which have tested in 2010
by NIST. The analytical results showed a good agreement with laboratory models. The results
of numerical analyses illustrated that RBS connection has less strength in comparison with
other connections and this connection reaches maximum vertical displacement with less force.
Performance of FF and WUF-W connections is similar to each other. These connections more
resistant in comparison with RBS. WFP connection is more resistant as compared with the
WUF-W, FF and RBS connections against the failure of the column. Failure load in WFPconnection is twice of other connection, and according to the analytical results, this
connection is suitable for HLOP structures. In all connections, rotation capacity
corresponding to collapse prevention against column removal scenario is about twice of the
accepted criteria that FEMA 350 has suggested for seismic loads.