FAILURE MODES OF EXTENDED SHEAR TAB STEEL CONNECTIONS

Adeeb Rahman1, Mustafa Mahamid2, Al Ghorbanpoor3, and Akef Amro4

University of Wisconsin-Milwaukee, Dept. of Civil Engineering and Mechanics College of Engineering and Applied Science, P. O. Box 784. Milwaukee, WI 53201

1Assistant Professor, adeeb@uwm,edu, 2PhD Candidate, 3Professor, 4Structural Engineer.

ABSTRACT

This paper introduces the findings of the experimental investigation and presents 3D nonlinear FE mode capable of predicting the behavior and validating the failure modes of an extended shear tab steel connection. Design recommendation of such connections will be made to the American Institute of Steel Construction (AISC) to be implemented in the design codes.

The extended shear tab connection does not require coping of the beam flanges and part of the web, which makes the fabrication of the beams and the erection of the overall connection much easier and more economical. The use of the new connection eliminates the failure limit states such as beam lateral torsional buckling, beam local web buckling, and beam block shear associated with the coping process. Extensive experimental work was conducted to investigate the behavior and document the failure modes of these connections. The FE model presents a viable procedure to effectively account for contact behavior and bolt tensioning mechanism. The models are analyzed through failure for each configuration. The model is capable to predict the failure modes of deeper connections up to 12 bolts, which is beyond the scope of the completed experimental investigation.

Evidence from experiments and finite element results of the extended shear tab connection presents new failure limit states such as column web mechanism and plate twist failure that are not recognized as failure limit states in the standard shear tab design procedures. Design equations are developed to predict the failure of the connections due to these failure modes. FE results for deflections, stresses, and strains in the plastic region were within 10% accuracy. More importantly, the failure modes observed in the experiments were accurately predicted in the FE model documenting plate twisting, column web failure and bolt shear for the corresponding geometries and loading. Keywords: Finite Element, Shear tab, Steel Connections, Failure Modes.

INTRODUCTION The design and behavior of steel connections is very important in steel design. The current practice uses steel plates and angles to frame beams into columns’ flanges or webs. This practice requires coping of the flanges of beams in the vicinity of the joint to frame the beam close to the web of the column. The extended shear tab concept is designed to transfer the forces to the

supporting member without the need for coping. Many cases of these connections were tested in the laboratory to help find their failure limit states, and to develop design procedures for shear tab plate connections that could lead to significant time and cost savings in the construction.

This research aims to develop an accurate, well-defined finite element model to evaluate and predict failure modes of the extended shear tab connections. The extended shear tab connections are tested and modeled and their failure modes are compared to the limit states provided by the AISC manual. This research introduces new limit states that should be considered in the design of extended shear tab connections. Connections that are considered include W12x87 beam framed into W8x31 column using unstiffened extended shear tab. The shear tab is welded to the web of the column and connected to the beam by three A325-X bolts. The bolts are fully tightened to a plate with slotted holes. Several failure modes were observed in the experiment and in the finite element model. The connection failed primarily in web mechanism mode with bolt shear as secondary failure mode. The web mechanism failure mode is due to the punching of the shear tab into the web of the column. A mechanism of high plastic strains developed in the web. The model was build using ANSYS commercial code.

EXPERIMENTAL WORK & MODELING Experiment 3U (3-bolted unstiffened). The dimensions for this experiment are shown in Figure 1. Several devices were used to monitor the behavior of the structure. LVDT’s were placed on the top of the beam to measure the deflection. Strain gauges labeled 101, 102, 103, 104, 105 and 106 were mounted on the top and bottom flange of the beam in order to calculate the load eccentricity. Strain gauges labeled 107, 108, 109 and 110 were placed on the plate with the addition of a mounted rosette to monitor the load transfer from the plate to the column. Finally, load cells were used for beam loading. Locations of strain gauges and LVDT’s on the beam and the shear tab are shown in Figures 1, 2 and 3. Four elements are used in the modeling of beams, columns (or girders), plates and bolts. These elements are: 3-D solid elements, contact elements and pre-tensioning elements[3]. FEA divides the bolt into two parts, separated by a pre-tension section, which consists of pre-tensioning elements. A pre-tension force must be applied on this section to create the pre-tensioning force in the bolt. Friction plays a main role in transferring forces between the surfaces and only contact can count for friction. In this structure, the beam’s web is in contact with the shear tab, there is contact also between the bolts and both the shear tab’s holes and the beam’s web holes. Surface to surface, flexible-to-flexible contact type is used between steel surfaces

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Figure 1. Structure Setup and strain gauge locations on the beam for Model 3U

Figure 2. Structure Setup and strain gauge locations on the shear tab for Model 3U

Figure 3. LVDT’s Locations

Figure 4. 3-D FE Model for Model 3U

Material Properties The shear tab has a steel grade of 36 Ksi, bolts are A325-X, and the beam and the column have a steel grade of 50 Ksi. Several factors and coefficients should be defined in order to analyze this structure: Coefficient of friction: this factor indicates the roughness of the surfaces. A coefficient of friction equal to 0.30 is used according to the AISC Manual [5]. Stress-Strain Curve: Tests were made to obtain the stress-strain curve of the plate, bolts and the column’s web. For models 3U three materials are defined, with properties shown in Figures 5, 6, and 7.

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Stress-Strain Curve for Column Web Stress (ksi)

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Figure 5: Stress-Strain Curve for Beams web

Stress-Strain Curve for Shear Tab

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Figure 6: Stress-strain curve for the shear

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Stress Strain Curve for A325 Bolt

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Figure 7: Stress-Strain Curve for Bolts

Boundary Conditions and Load In the experiment, the column is fixed to the wall, and the beam is supported on a roller at the far end-point from the connection. An accurate boundary condition is prescribed in the model to represent the experimental setup. Four nodes on the column at the same location as the experiment are restrained in all degrees of freedom, and the beam edge is restrained in both vertical and lateral directions since beam weight and friction with the roller are acting against any displacement in the lateral direction. Contacts also serve as boundary conditions in the connection vicinity. Welding between the extended shear tab and the column has to be well defined. Since the welding in the experiment was not a critical issue, it is modeled as a rigid connection (welded connection) by merging the shared nodes at the boundary between the column and plate. The loading in the experiments was applied as follows:

1. Torque applied to tighten the bolts, represented by applying 30 kips pre-tensioning force for each bolt. This value is taken from AISC manual [5].

2. The beam’s load is applied through load cells placed on the beam as shown in Figure 1.

RESULTS Comparison between FEA and experimental results shows that the difference is within 15%. The FEA of model 3U produces results very close to the experimental results for all outputs except the twist in the beam. Comparison between FEA and experiment is shown in Figures 8. The maximum difference is 17% for model 3U.

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Load Vs Deflection

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Figure 8: Load-Deflection curve for the beam for model 3U

Principal strain in the shear tab was measured using a rosette, which is a device that measures three strains. These strains are combined together in an equation to find the principal strain. The importance of the rosette lies in its ability to show the transfer of forces from the beam to the connection. Comparison between FEA and experiment 3U is shown in Figures 9. For model 3U, the difference is between 2% and 21%, where the 21% difference is at initial loading of 5.4 kip. The experiment cannot give good results at this load level because the structure is re-configuring itself to accommodate the initial load applications.

Load Vs Principal Strain

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Figure 9: Load-Principle Strain Curve in the shear tab for Model 3U

Analyzing the structure shows a behavior similar to that of the experiment. The primary failure modes for the connection are web mechanism and bolt shear, with twist being a secondary failure mode. The web mechanism failure mode is described by punching of the shear tab into the

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column web and by developing of high stress at the top and bottom of the shear tab at the points of connectivity to the column web. Web mechanism and twist failure modes from both experiment and finite element analysis are shown in Figures 10 and 11.

Figure 10. FE Failure Figure 11. Experiment Failure

CONCLUSION In the elastic and plastic ranges the FE model is well constructed and seems to be very adequate in producing results that are in good agreement with the experimental results. Confidence in the FE model is a result of the excellent level of detailing done to accurately reproduce the actual geometry of all the structural components as prescribed by the experimental setup and the AISC specification. Accurate description of the material properties of steel grades is used. The attention given to the level and location of mesh refinement is adequate and contributed to the good agreement of the results obtained. Special attention is given to the description of the boundary conditions to simulate the experimental setup. The results obtained from the FEA for the load-deflection curves of the beam are within 17%, compared to the experimental results for the 3U model. An even stronger correlation is observed when comparing the results in the computation of the rosette principal strains. The FE value agrees with the eccentricity values calculated by the AISC manual. Considering the fact that many potential sources of errors are present, the comparison between the computational analysis and the experiments is better than expected. Sources of errors can be related to both FEA, and the experimental setup. Clearly this work demonstrates that if a proper FE model is constructed as presented in the research, many advantages can be achieved such as tremendous savings in time and cost. The FE model gives flexibility to model different geometries and setups under a variety of loading

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conditions. The FEA provides a full field of results that enables the investigator to view results at any location with ease.

REFERENCES 1. ANSYS theory manual, version 5.6, 2000. 2. ANSYS training manual, 2001. 3. ANSYS user manual, version 6.1, 2002. 4. Sherman and Al Ghorbanpoor. “Testing and design of extended shear tabs report,” December

2001. 5. American Institute of Steel Construction. Manual of steel Construction LRFD Vol I & II,

Second edition, 1998.AISC, Chicago, IL. 6. Dally, James W. “Experimental Stress Analysis,” Second Edition, 1978. 7. Chiew, Lie and Dai. “Moment Resistance of Steel I-Beam to CFT Column Connections” Journal of Structural Engineering, Vol. 127, No. 10, October 2001 8. Abolitz, A.L., and Warner, M.E. “Bending Under Seated Connections.” AISC Engineering Journal, Vol. 2, No. 1(1-5), 1st Qtr., 1965. AISC, Chicago, IL.

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