Axial, Shear and Moment Interaction of Single Plate “Shear Tab” Connections

International Journal of Impact Engineering 31 (2005) 861–876

Finite element analysis of steel beam to column connectionssubjected to blast loads

Tapan Sabuwala, Daniel Linzell*, Theodor Krauthammer

Department of Civil and Environmental Engineering, The Pennsylvania State University, State College, PA 16802, USA

Received 28 August 2003; received in revised form 20 April 2004; accepted 23 April 2004

Available online 26 June 2004

Abstract

The behavior of fully restrained steel connections subjected to blast loads was examined using finiteelement analysis. Two connections that were tested as part of the AISC Northridge Moment ConnectionTest Program (Report for AISC, 1994) were studied using ABAQUS. Models were validated by comparingnumerical results against AISC Program experimental data. Validated models were then subjected tosimulated blast loads and their efficiency against those blast loads was verified based on criteria specified inTM5-1300 (Department of the Army, Structures to resist the effects of accidental explosions, 1990).Adequacy of TM5-1300 criteria was investigated and critical zones in the connection details were identified.Based on the results of the study, recommendations for modifications to TM5-1300 criteria were made andthe effectiveness of the chosen connection details under blast loads was summarized. The results showedthat the TM5-1300 criteria for steel connections subjected to blast loads are inadequate. Also theunreinforced (pre-Northridge) connection detail performed poorly under blast loads with excessivedeflections and above yield stresses in the connection region while the reinforced connection detail showedimproved resistance against blast loads and this connection may be an option when detailing steel framedconnections to resist blast loads.r 2004 Elsevier Ltd. All rights reserved.

Keywords: Blast loads; Finite element modeling; Steel connections

1. Introduction

The study of steel connections subjected to dynamic loads was initiated in the 1960s by Popov[1] wherein tests were conducted to study the cyclic behavior of steel moment-resisting

ARTICLE IN PRESS

*Corresponding author. Fax: +1-814-863-7304.

E-mail address: [email protected] (D. Linzell).

0734-743X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijimpeng.2004.04.013

connections. Since these early studies, investigations have generally focused on their behaviorunder cyclic loads, such as those generated during an earthquake. However, since September 11th,there is rising concern in the United States over the safety of building structures subjected to blastloads. When a structural steel frame is subjected to blast loads, the beam-to-column connections,which are responsible for load transfer between different members within the frame, play a majorrole in structural response. Thus, a better understanding of the behavior of structural steelconnections under blast loads is of prime importance. However, few studies have been conductedwhich analyze the interaction of blast loads and structural components of building structures.Case studies based on past attack on buildings subjected to blast loads have been presented suchas by Caldwell [2] which focus on the pattern and severity of blast damage sustained by thestructure. Most of these studies however take a macro view of the situation and analyze the effectof blast loads on the buildings as a whole instead of identifying the behavior individual structuralcomponents of the building structure under such loads. No experimental studies have beenreported that analyze the behavior of steel connections under blast loads. Only one theoreticalstudy [3] and one numerical study [4] investigating steel connection behavior under blast loads hasbeen reported.While some general publications, as that published by Conrath et al. [5], dealing with the design

of structural systems to resist blast loadings exist, there are only a limited number of codedocuments that exist related to blast design. The principal code currently used for the design ofstructures in the United States to resist blast loads is TM5-1300, Structures to resist the effects ofaccidental explosions [6]. This document provides guidelines for the safe design of structuralelements subjected to short duration dynamic loads (i.e. blast loads) and criteria contained withinTM5-1300 are oriented toward industrial building applications common to ammunitionmanufacturing and storage facilities, (i.e., relatively low, single-story, multi-bay structures). Theapproach presented in TM5-1300 is centered on the response of structures and structural elementsthat are idealized as equivalent lumped-mass single degree of freedom systems. While generalcriteria for proportioning low-rise framed military structures to resist blast loads are provided,this publication does not provide specific design guidelines or performance criteria for steelconnections under blast loads. Hence the performance of a steel connection has to be judgedbased on the performance of the steel frame. However, the adequacy and effectiveness of criteriagiven in TM5-1300 for steel frames is not well understood due to limited research. Hence, giventhe absence of specific criteria governing connection behavior under blast loads, this study aimedat verifying the adequacy of criteria presented in TM5-1300 for steel frames containing selectconnection details. Moreover due to the lack of research and specific design guidelines for blast-loaded steel connections, existing connection details proven to be effective against dynamic loadswere selected in order to obtain an initial understanding of their effectiveness and behavior underblast loads which can then be used as a stepping stone for future studies in this area.

2. Experimental program

Connections selected for the current study were tested as part of the AISC Northridge MomentConnection Test Program. The study was conducted by Engelhardt et al. [7] immediately after theNorthridge earthquake to provide insight into causes of steel connection failures and to provide

ARTICLE IN PRESS

T. Sabuwala et al. / International Journal of Impact Engineering 31 (2005) 861–876862

preliminary guidelines regarding retrofitting any connections to increase their effectiveness againstinduced seismic loads. The behavior of eight fully restrained connection details was examinedexperimentally at full-scale under cyclic loads (Fig. 1).Based on tests of unreinforced, pre-Northridge shear tab connections, modifications were made

to the connection details to attempt to improve their efficiency against seismic loads. It wasdemonstrated by Engelhardt et al. [7] that the cover plate retrofitted connection detail in Fig. 2bdemonstrated improved performance against cyclic loads by providing higher values of beamplastic rotation, energy dissipation and ductile failure modes when compared to the originalconnection details. Connections selected for the current study included an unreinforcedconnection (Fig. 2a) and the reinforced connection (Fig. 2b). Analytically examining these twoconnections permitted studying the effects of the additional structural elements used for thereinforced connection on blast load behavior.

3. Analytical approach

The general purpose ABAQUS finite element code was selected for the current study.Information from previous studies indicated that, for high rate dynamic loads, ABAQUSperformed better than other commercially available finite element codes as per a study conductedby Krauthammer [8]. Finite element models were validated by comparing results to experimentaldata from the AISC Northridge Test Program.Due to small time durations and high pressure loads required for the current study, the finite

element models were created using 8-noded continuum (brick) elements with reduced integration(C3D8R). Wedge elements (C3D6) were used to model curved regions of the beam, which

ARTICLE IN PRESS

Fig. 1. Experimental setup (Engelhardt et al. [7]).

T. Sabuwala et al. / International Journal of Impact Engineering 31 (2005) 861–876 863

included weld access holes at the top and bottom of the web, and the welds. An isometric view ofthe finite element models is shown in Fig. 3. Numerical models for certain connection componentsare shown in Fig. 4.

ARTICLE IN PRESS

Fig. 2. (a) Unreinforced connection (Engelhardt et al. [7]). (b) Reinforced connection (Engelhardt et al. [7]).


Elasto-plastic material properties with isotropic hardening were selected to simulate materialbehavior of all components in the finite element model except the welds, which were assigned anelasto-plastic material model with perfect plasticity as a brittle material model for steel wasunavailable in ABAQUS Explicit.

ARTICLE IN PRESS

Fig. 3. Numerical study models: (a) Numerical model for validation studies (unreinforced). (b) Numerical model for

blast studies (unreinforced).


Member constituent relationships for model validation were based on test data from the AISCNorthridge Test Program. Since design and analysis procedures presented in TM5-1300 are basedon nominal material properties, blast studies that followed the validation process utilized nominalproperties from the Structural Welding Code [9] for the welds and from other sources [10] for

ARTICLE IN PRESS

Fig. 4. Overview of components of numerical model.


various components of the connection details. In addition, nominal yield stresses wereincreased using dynamic increase factors as required by TM5-1300 to account for theinfluence of high strain rates from the blast loads on the mechanical properties of steel. Theseincrease factors were 1.12 and 1.29 for Grade 50 and Grade 36 steels, respectively. Materialproperties used for the finite element model for the blast load phase of the study are illustrated inTable 1.Surface-to-surface contact interaction capabilities available in ABAQUS were used to account

for the various forces generated between interacting parts of the model. A tied contactformulation was used for the welds and a small sliding formulation was used for the otherinteracting parts such as bolts–bolt holes, bolt–tabs, tabs–beam, and cover plates–beam.For validation, the loads and boundary conditions applied to the numerical model replicated

the experimental setup. The column for the numerical model was fixed at the ends and at the baseas in the experimental setup (Fig. 3). The beam was cantilevered from the column. Cyclic loadscorresponding to those used during the experimental tests were applied to the free end of thebeam. The primary response quantity used for model calibration was the displacement time-history of the beam tip. Predicted and measured displacement time-histories as obtained from thenumerical model and corresponding experimental values for the selected connection details areillustrated in Figs. 5 and 6, respectively. Maximum differences between the numerical andexperimental results are also shown.Figs. 5 and 6 indicate that good numerical prediction of the behavior existed for both

connections. The peak differences between numerical and experimental tip displacements were9.9% for the unreinforced connection and 6.1% for the reinforced connection.The main aim of this study was to analyze the behavior of the selected connection details under

blast loads and benchmark their performance against criteria from TM5-1300. As per TM5-1300,for blast-resistant design only the peak response, from the first cycle, of the structure is important.This first response cycle is minimally affected by damping in the system and damping effects aresubsequently neglected in the theoretical procedure given in TM5-1300 for evaluating blast loadresponse [6]. Thus, damping was not included in the numerical models. Analyses were carried outover a time duration which would produce one cycle of structural response. Peak displacements

ARTICLE IN PRESS

Table 1

Connection component yield stresses and tensile strengths for FE models under blast loads

Connection

component

Yield stress (w/o

dynamic increase

factor) (MPa)

Yield stress (w/o

dynamic increase

factor) (MPa)

Ultimate tensile

strength (MPa)

Beam 248.2 352.3 399.9

Column 344.7 424.7 482.6

Shear tabs 248.2 352.3 399.9

Bolts 586.0 721.9 722.0

Nuts 586.0 721.9 722.0

Weld 482.6 594.3 594.6

Cover plates 248.2 352.3 399.9


and rotations of structural members were subsequently judged based on their response during thisfirst cycle. Blast pressures were applied as uniformly distributed loads to the inner faces of thebeam and column flanges (Fig. 3) and are summarized in Table 2.

4. Theoretical/empirical load development and response prediction

Simulated blast pressures were generated using procedures outlined in TM5-1300, inconjunction with the SHOCK and FRANG computer codes [11] and [12] respectively andloading functions corresponding to these blast pressures were applied to the numerical models.Theoretical estimates of the behavior of the studied steel connection details under these blast loadswere obtained.The procedure outlined in TM5-1300 to estimate an explosive charge size provides blast

pressure for walls of containment structures or cubicles due to an internal or external explosion.Hence, it was necessary to consider the connection details as part of a hypothetical room withinwhich the explosion occurred. Considering this, a simple one-story steel frame structure as shownin Fig. 7 was employed as the theoretical room model. One side of the room was considered to bea frangible panel having a surface weight of 0.96MPa [6] that would provide realistic venting ofthe explosive gases.

ARTICLE IN PRESS

Fig. 5. Numerical vs. experimental beam tip displacements (unreinforced connection).


Only part of the representative room was considered for the numerical model by takingadvantage of symmetry conditions with planes of symmetry located at mid-height of the columnand mid-length of the beam (Fig. 7).

ARTICLE IN PRESS

Fig. 6. Numerical vs. experimental beam tip displacements (reinforced connection).

Table 2

Numerical blast pressures

Specimen Member Load (MPa) Time (ms)

Unreinf. Beam 3.2 0.0

2.0 3.4

0.0 48.2

Column 8.6 0.0

1.5 1.8

0.0 48.2

Reinf. Beam 3.2 0.0

2.0 3.3

0.0 48.1

Column 8.8 0.0

1.5 1.8

0.0 48.1


Theoretical values for the pressures were determined based on the plastic moment capacity ofthe beam and column for each chosen connection detail. TM5-1300 provides different methodsfor calculating the plastic moment capacity from structural members of a frame based on the typeof anticipated collapse mechanism. The present study considered two typical failure mechanismsas shown in Fig. 8.

ARTICLE IN PRESS

Fig. 7. Theoretical room used for blast study.

Fig. 8. Theoretical model failure mechanisms.


A representative pressure time-history for the connection details is graphically depicted inFig. 9. For the numerical model, an effective pressure time-history as depicted by solid line inFig. 9 is used. This time-history shows shock and gas pressures acting on a side wall of thehypothetical room. These pressures are then transferred to the column considering the tributaryarea of the side wall. A similar procedure is used for the roof and the pressures are thentransferred to the beam using the effective tributary area. These transferred loads on the beam andthe column are distributed over their flanges and these resultant pressures as applied to thenumerical model are summarized in Table 2.

5. Performance evaluation

In addition to estimating blast load response using ABAQUS, additional estimates wereobtained using TM5-1300 procedures for comparative purposes. Response was characterized interms of the maximum deflection at mid-span, Xm, and the corresponding rotational deformationat the member end, y. Design charts from TM5-1300 that related dynamic properties of thestructural elements, such as the natural period of vibration (TN), resistance (R), and deflection(X), to those of the blast overpressures that consisted of the load (P) and time duration (T) wereused. A ductility ratio, m, associated with the ratios T/TN and P/Ru, can be obtained from thesedesign charts. Table 3 summarizes values of maximum deflection, Xm, ductility ratio, m androtational deformation, y, for structural elements of the studied connection details along with thesafety limit for rotation as specified in TM5-1300. These theoretical estimations indicated thatthe representative room could withstand the loads from the explosive charge according to theTM5-1300 limiting criteria.

ARTICLE IN PRESS

Fig. 9. Unreinforced connection side wall load.


6. Results and discussion

6.1. Unreinforced connection

Beam response was evaluated based on end rotation, end displacement, and Von Mises stresses.Numerical model displacements and rotations for both the reinforced and unreinforcedconnections under the prescribed blast pressures are summarized and compared to theoreticalpredictions from TM5-1300 in Table 4.As shown in the table, numerical model beam rotation for the unreinforced connection was well

within deformation criteria specified by TM5-1300 and hence it is considered as safe under theapplied blast loads based upon these criteria. However, TM5-1300 predicted values are 38%higher than those predicted by the numerical model and hence the beam was over-designed for theapplied blast loads according to TM5-1300.

ARTICLE IN PRESS

Table 4

Predicted displacements and rotations

Spec. Member Response

quantity

Peak

numerical

value

Peak theoretical

value (TM5-

1300)

%

Difference

Limiting

criteria (deg)

Unreinf. Beam Rotation (y) 0.2� 0.3� 38.3 2>0.3

Displacement

(X)

9.8mm 15.7mm 38.3 —

Column Rotation (y) 0.1� 1.3� 94 2>1.3

Displacement

(X)

4.2mm 72.1mm 94 —

Reinf. Beam Rotation (y) 0.1� 0.3� 55.5 2>0.3

Displacement

(X)

7.7mm 17.2mm 55.5 —

Column Rotation (y) 0.1� 1.3� 92.8 2>0.3

Displacement

(X)

5.0mm 69.8mm 92.8 —

Table 3

Blast load response from TM5-1300

Connection

detail

Member Max. deflection

Xm (mm)

Ductility

ratio (m)Rotation y(deg)

Rotational limit

ylimit (deg)

Unreinf. Beam 15.7 2.0 0.3 2>0.3

Column 72.1 1.7 1.3 2>1.3

Reinf. Beam 17.3 2.2 0.3 2>0.33

Column 69.9 1.8 1.3 2>1.3


Stress contour plots of the numerical model indicated that the highest stresses were generatednear the weld access hole at the bottom of the beam web due to the reduced area of the web at thatlocation. The observed peak stress in this region was 364MPa, which was marginally higher thanthe dynamic yield stress for the beam (352.2MPa), indicating initiation of local yielding at thislocation. Column response under simulated blast loads was elastic with low stresses anddeformations.Table 5, which summarizes stresses for the reinforced and unreinforced connections under the

prescribed blast pressures, indicates that under the simulated blast loads, stresses in the bottomgroove welds between the beam and column flange reached a maximum value of 587.4MPa,which was marginally smaller than the weld dynamic yield stress of 594.6MPa. These resultsshowed that localized weld failure may occur at these locations.

6.2. Reinforced connection

Table 4 indicates that predicted values for beam response in terms of the tip rotation anddisplacement were overestimated for the reinforced connection. Moreover, differences betweenpredicted and theoretical response quantities for the beam were 55% higher than correspondingdifferences for the unreinforced connection. Increased discrepancies between TM5-1300predictions and those from the ABAQUS model were caused by additional stiffness providedto the reinforced connection from the cover plates that were not addressed by TM5-1300 criteria.Thus, as expected, the reinforced connection was more efficient than the unreinforced connectionfor limiting deflections of the beam and the column under blast loads.

ARTICLE IN PRESS

Table 5

Observed stress summary

Specimen Component Stress concentration

region

Maximum

stress (MPa)

Dynamic yield

stress (MPa)

Comment

Unreinf. Beam Lower weld access hole 365.0 352.3 Local yielding

Column Upper and lower

connection point with

shear tabs

124.1 424.7 —

Shear tabs Around bolt holes 248.2 352.3 —

Bolts Bolt shank 262.0 722.0 —

Welds Bottom groove weld 587.4 594.3 Near failure

Reinf. Beam Beam flange at end of

top cover plate

379.2 352.3 Local yielding

Column Lower connection

point with shear tabs

151.7 424.7 —

Shear tabs Around bolt holes 248.2 352.3 —

Bolts Bolt shank 262.0 721.9 —

Welds Bottom groove weld 358.5 594.3 —

Cover plates Tip of top cover plate 358.5 352.3 Local yielding


Table 5 indicates that stresses generated in the beam for this connection were lower whencompared to those in the unreinforced connection. They were concentrated in the beam flange atthe end of the top cover plate with the maximum stress being 379.2MPa, which indicated theformation of a yielding zone at that point. This behavior corresponds with the design philosophyof the reinforced connection for seismic loads whereby cover plates were utilized to positionplastic hinges in the beam away from the connection zone. Beam stresses in the connection regionranged between 103.4 and 124.1MPa, which was approximately 65% lower than the peak stressobserved for the unreinforced connection. As in the unreinforced connection, the columnremained elastic.One of the key problems identified with the unreinforced pre-Northridge connections was the

concentration of stresses at the groove welds. The cover plates added to the reinforced connectionaimed at reducing this stress concentration for induced seismic loads. Response of the reinforcedconnection under simulated blast loads indicated that adding cover plates to reduce weld stressconcentration can be effectively used for blast loads. Reinforced connection groove weld stressconcentrations were significantly reduced, with peak stresses (358.5MPa) being 33% less thanpeak stresses observed in the unreinforced connection (Table 5). Moreover, stresses on the otherweld components were also reduced to a maximum of 255.1MPa.The cover plates were subjected to high stresses at their tips under blast loads with maximum

358.5MPa at the top cover plate tip, which indicated local yielding. Lower stresses occurred forthe bottom cover plate. The reason for the difference in stress was caused by the blast loadapplication procedure, which initially deformed the beam upwards with high pressures. The topcover plate attempted to resist this motion leading to stress concentrations at its tip.

7. Conclusions

Based on the results obtained from the numerical models the following conclusions were maderegarding connection performance and adequacy of the TM5-1300 criteria.

1. According to the TM5-1300 criteria, structural members were over-designed for the blast loadsas peak values of displacements and rotations from the numerical models were lower than thelimiting criteria given in TM5-1300.

2. The reinforced connection performed better than the unreinforced connection under blastloads, exhibiting lower displacements, rotations and stresses.

3. The bottom groove weld was an area of concern for the unreinforced connection as it wassubjected to high stresses exceeding its dynamic yield stress. This behavior matchesobservations made during testing of this connection detail under cyclic loads for the AISCNorthridge Test Program [7] where the connection detail failed due to localized fracture of thebottom groove welds.

4. Flange cover plates added to the reinforced connection were efficient in reducing the stressconcentrations on the groove welds.

5. The formation of plastic hinges in the reinforced connection would occur away from theconnection zone at the end of the cover plate as indicated by initiation of yielding of the beamflange at this point.

ARTICLE IN PRESS


6. Failure of the unreinforced connection would appear to occur in the connection zone asindicated by local yielding of the region near the bottom weld access hole and failure of thebottom groove welds.

These conclusions appear to indicate that criteria presented in TM5-1300 used to judge theadequacy of a steel frame based purely on rotations of the structural members is not adequate andshould be revised. The studied connection details satisfied these criteria but were shown to besubjected to high localized stresses that were indicative of failure. Thus, it would be advantageousto incorporate a strength based criterion into the TM5-1300 document that would augmentexisting serviceability criteria to evaluate steel frame performance under blast loads. In addition,any strength criteria added to TM5-1300 should account for contributions to the connectionstrength and stiffness, and the subsequent increase in blast resistance, which results from theaddition of cover plates and column stiffeners to the connection region.It is also recommended that unreinforced connection details similar to those included in this

study should not be used for blast-resistant structures as they show undesirable failures within theconnection zone near the column face. The studied reinforced connection details, which includedcover plates for the beam flanges and stiffeners for the column web, showed improvedperformance under blast loads with reduced stresses in the connection region and indication ofplastic hinge formation in the beam away from the connection.

Acknowledgements

This work was supported by The US Army Corps of Engineers through a project with theProtective Technology Center (PTC) at Penn State University entitled ‘‘Protective TechnologyResearch, Development and Implementation in Support of DoD Force Protection Needs.’’ Theassistance of PTC personnel and personnel at the US Army Corps of Engineers ERDC isgratefully acknowledged. In addition, data and background information provided by Dr. MichaelD. Engelhardt at the University of Texas was invaluable to this investigation.

References

[1] Popov EP. Seismic moment connections for moment-resisting steel frames. Report No. UCB/EERC-83/02,

Earthquake Engineering Research Center, Berkeley, CA; 1983.

[2] Caldwell T. Bomb blast damage to a concrete-framed office building Ceylinco House - Columbo, Sri Lanka.

Proceedings of Structures Congress, New Orleans, LA; 1999.

[3] Krauthammer T. Structural concrete and steel connections for blast resistant design. Int J Impact Eng 1999;

22(9–10):887–910.

[4] Krauthammer T, Oh GJ. Blast resistant structural concrete and steel connections. Int J Impact Eng 1999;22:

887–910.

[5] Conrath EJ, Krauthammer T, Marchand KA, Mlakar PF. Structural design for physical security. State of the

practice report, American Society of Civil Engineers. 1999.

[6] Department of the Army. Structures to resist the effects of accidental explosions, TM5-1300. 1990.

[7] Engelhardt MD, Sabol TA, Aboutaha RS, Frank KH. AISC Northridge moment connection test program. Report

for AISC, 1994.

ARTICLE IN PRESS


[8] Krauthammer T, Lim J, Oh GJ. Findings from three computer code validations with precision impact test data.

Proceedings of the 29th Department of Defense Explosive Safety Seminar, New Orleans, LA; 2000. p. 18–20.

[9] American Welding Society. Structural Welding Code for Steel/ANSI/AWS Dl.1–94. 1994.

[10] Salmon GC, Johnson EJ. Steel structures: design and behavior, emphasizing load and resistance factor design.

New York: HarperCollins College Publishers; 1994.

[11] Wager P, Connett J. SHOCK User’s Manual, Naval Engineering Lab, Port Hueneme, CA; 1989.

[12] Wager P, Connett J. FRANG User’s Manual, Naval Engineering Lab, Port Hueneme, CA; 1989.

ARTICLE IN PRESS


Documents

Axial, Shear and Moment Interaction of Single Plate “Shear Tab” Connections