Seismic Design of Diaphragm Plate in Steel Beam-to-Box Column Joints
Date Issued
2012
Date
2012
Author(s)
Hsieh, Hsin-Lun
Abstract
Built-up box columns are widely used for steel structures in Taiwan because of its bi-axial strength and stiffness. In the welded beam-to-column moment connections, welded internal diaphragms in the column at the beam flange elevations play an important role in transferring the beam flange forces into the column. For an interior column, two beams are connected to the opposite flanges of the box column, while only one beam for an exterior column. In common practice, the thickness of the diaphragm plate is the same as the beam flange thickness in either interior or exterior column. Most of the tests on welded beam to box column moment connections have been conducted for exterior columns. Under the severe earthquake loads, the diaphragm plates having a thickness identical to the beam flange may go into severe inelastic responses when both two beams develop plastic hinge adjacent to an interior column. This research investigates the deformations and stress distributions, quantifies the force demand, and proposes the seismic design method for diaphragm plates in box columns.
First, this paper presents the difference on stress responses in diaphragms between the exterior and interior connections using finite element (FE) software, ABAQUS. The shell element model was calibrated first using a beam-to-exterior box column connection specimen. Then extensive FE analysis results show that diaphragm peak shear stresses near the column web in the interior column is almost twice of those of in an exterior column. However, diaphragm peak normal stresses near the column flange are similar in both two types of connections. In order to allow designers to efficiently size the diaphragm thickness, this paper proposes two approaches to quantifying the seismic force demand on the diaphragm. One is based on the relative flexibilities of the column flange and diaphragm. The other is based on the simplified free body diagram. The simplified approach is convenient and conservative.
The FE analysis results indicate that the deformation and von Mises stress in the interior column diaphragm are much larger when the diaphragm thickness is the same as the beam flange. The diaphragm plate, thickness determined from the proposed method reduces the peak diaphragm deformation by 75% and allows the diaphragm to remain elastic when plastic hinges form on both two beams. The FE analysis results also indicate that the equivalent cumulative plastic strain and rupture index on beam flange edge caused by diaphragm yielding are much higher in an interior connection than those in the exterior connection. The paper concludes that further experimental tests are desired in order to gain insights into the effects of diaphragm plate responses on rotational capacity of welded beam to interior box column moment connections.
Subjects
welded moment connection
box column
diaphragm
finite element analysis
seismic design
Type
thesis
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