Blast-resistant structural steel connections T. Krauthammer and GJ. Oh Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA Email: tedk@psu. edit Abstract This paper describes a study to assess the structural response and the current design procedures for steel connections under blast loads. The approach used in this study was based on adopting current blast resistant design procedures and advanced numerical simulationsfor assessing the behavior of modified seismic-resistant steel connections under blast-induced loads from internal explosions. 1 Introduction and Background Many buildings that could be targeted by terrorists are steel frame structures, and the behavior of the structural steel connections under blast loads is of great interest. Current design guidelines^ for structural steel connections have been developed based on experimental and theoretical investigations. However, recent reports^ on damage to steel structures due to seismic loads suggested a surprisingly poor performance of the steel connections, as compared toexpected behavior^ Important design modification were introduced after extensive assessments of the observed damage". Nevertheless, these modified design details may contain similar deficiencies if the connections would be subjected to high rate dynamic loads, such as those associated with blast effects. Therefore, it could be very important to assess the behavior of such structural steel connects under blast effects,and to identify such possible difficulties. Structural behavior under dynamic loads requires attention to the relationship between the dynamic characteristics of the structure and the applied loads. Design specifications should address this relationship to insure that the various structural details are blast resistant. Although most of the recent attention Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Blast-resistant structural steel connections
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Department of Civil and Environmental Engineering, The
Pennsylvania State University, University Park, PA 16802, USA
Email: tedk@psu. edit
Abstract
This paper describes a study to assess the structural response and the current design procedures for steel connections under blast loads. The approach used in this study was based on adopting current blast resistant design procedures and advanced numerical simulations for assessing the behavior of modified seismic-resistant steel connections under blast-induced loads from internal explosions.
1 Introduction and Background
Many buildings that could be targeted by terrorists are steel frame structures, and the behavior of the structural steel connections under blast loads is of great interest. Current design guidelines^ for structural steel connections have been developed based on experimental and theoretical investigations. However, recent reports on damage to steel structures due to seismic loads suggested a surprisingly poor performance of the steel connections, as compared to expected behavior^ Important design modification were introduced after extensive assessments of the observed damage". Nevertheless, these modified design details may contain similar deficiencies if the connections would be subjected to high rate dynamic loads, such as those associated with blast effects. Therefore, it could be very important to assess the behavior of such structural steel connects under blast effects, and to identify such possible difficulties.
Structural behavior under dynamic loads requires attention to the relationship between the dynamic characteristics of the structure and the applied loads. Design specifications should address this relationship to insure that the various structural details are blast resistant. Although most of the recent attention
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
64 Structures Under Shock and Impact
in this area has been devoted to structural concrete connections, the development of design specifications for blast resistant steel connections is as important. TM 5- 1300® contains guidelines for the safe design of blast resistant steel connections. However, the adequacy of these design procedures is not well defined because of insufficient information about the behavior of the steel connections under blast loads.
High loading rates can influence the mechanical properties of structural materials*'*, and the use of dynamic increase factors (DIFs) for describing strain rate enhancement is well known. A DIP is the ratio of the strain rate enhanced strength to the static strength (e.g., the ratio of the dynamic and static yield stresses for a metal). This effect of higher strain rates on the mechanical properties of steel is important for blast-resistant design. DIFs are used for both design and approximate analyses (e.g., single-degree-of-freedom calculations). Nevertheless, DIFs have to be used with care in advanced numerical simulations. It is well known*'* that the steel yield stress is enhanced by strain rates while the ultimate stress is affected much less. The effects of high rate dynamic loadings on the structural responses were investigated in this study by employing the recommended DIF values in TM 5-1300* for both the design and the numerical simulations.
A useful research procedure for studying the behavior of structural concrete connections'^ was adopted to analyze structural steel connections under explosive loads. Typical modified structural steel connections for seismic conditions" were assessed under explosive loads by employing the design procedure in TM 5-1300*, and additional and empirical analysis tools. The maximum safe amount of the contained explosive charge and the blast resistant capacities of the connections were estimated. A reliable finite element code developed for simulating short-duration dynamic events, DYNA3D", was used to investigate the behavior of the structural steel connections under the expected blast loads. DYNA3D" was validated by simulating impact tests before it was used for the explosive simulations, as discussed below.
Computer codes must be validated to insure that they can reproduce real- world behavior. Such procedures were discussed in a recent workshop^, and the approaches employed during these numerical studies are in full agreement with the workshop recommendations. DYNA3D " was validated by simulating an impact test on a mild steel beam'*. The tested fixed-fixed steel beam was four inches long, 0.4 inches wide and 0.25 inches deep. It was struck at its center by an 11-pound weight with an impact velocity of 452.6 inches-per-second, and the force was delivered on a 0.2 inches square contact area. The steel material properties were, as follows: Yield stress of 43 ksi. ultimate stress of 63 ksi. and elastic modulus of 10* ksi. Strain rate effects*'* were incorporated into the kinematic/isotropic elastic-plastic material model in DYNA3D". Until 1.5 milliseconds (ms), the test specimen exhibited slightly larger deflections that the simulated data, and it failed at 2.4 ms. Although the difference between test and numerical data increased after about 1.5 ms, the two behaviors compared well. These results show that DYNA3D" can be used for the assessment of structural steel subjected to short-duration dynamic loads, and that the simulations can be expected to correlate well with actual behavior.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 65
A hypothetical one-story frame structure with seismic resistant corner knee connections was employed for this study. A welded beam-to-column connection in that structure was selected for the analyses, as defined in a previous investigation', and it was considered as a Fully-Restrained (FR). W-shape cross section members of the beams and columns sized to resist earthquake conditions were employed in the present model. The internal blast loads were derived from the detonation of a TNT charge at the geometric center of the floor in the structures. According to TM 5-13001 a load pulse from an internal explosion can be represented by a short and intense shock pressure and a longer duration lower intensity gas pressure. The maximum rotational deformations at the connections are typically used to assess structural damage*. These rotations were investigated to define the structural capacities of the connections under opening explosive loads (i.e., an internal explosion will cause the connections to open). Stresses and strains at critical points of welds and panel zones were checked to identify more detailed deformation mechanisms. Finally, based on the observed preliminary data, wider applications of the approaches were discussed. The research approach is described in further detail, next.
2 Study Description
The steel connection, recommended for seismic applications^, was employed to simulate the present cases. As a first step, the blast resistant capacity of the comer knee connection was analyzed according to TM 5-1300®. The preliminary assessment provided an estimated maximum safe explosive charge weight that the structure can be expected to resist without exceeding recommended damage levels**. The rotational deformations were estimated at the connection, based on the expected structural response. Additionally, this analysis was used also to define the corresponding pressure-time histories that would be applied to the structure in the advanced numerical simulation. Finally, numerical simulations were conducted with DYNA3D" for comparison with the findings based on TM 5-1300. and to obtain useful information about the possible responses of the steel connections under internal explosions. To obtain a better understanding of the structural behavior, various types of connection models were developed and simulated. The behavior of the connection and its deformations at points of interest were investigated by checking the nodal displacements, element deformations, stresses and strains. These activities are summarized next.
TM 5-1300* provides methods of design for explosive safety. Also, the manual provides procedure for deriving the blast load parameters and for calculating the dynamic responses of structural members. The adopted details of the structural model were based on a typical seismic resistant steel knee connection at a corner of a frame". W-14x342 (A572 GR50) and W-36xl50 (A36) were employed for the column and beam members, respectively. The weld was made with E70TG- K2 fluxed core electrodes that have a 70 ksi yield stress. The spans of 30 ft and height of 23 ft of the frame model were selected, considering typical dimensions of
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
66 Structures Under Shock and Impact
for such structures. The design procedures outlined in Section 5 of TM 5-1300® were used to evaluate the maximum blast load that can be applied to the beam and column members, as follows.
1.
2.
3.
4.
Plastic moment capacities were determined for beams and columns using dynamic material properties.
The equivalent uniformly distributed static load per unit length of beam was computed.
The blast overpressure on the roof, P,, was computed.
The computer codes Shock'"* and Frang^ were employed for deriving by trial-and-error the weight of an equivalent TNT charge that when detonated at the geometric center of the floor will produce a maximum gas pressure on the roof that will not exceed P\,. One wall was assumed as a frangible panel (i.e., it was assumed to blow out and permit quick venting of the internal pressures). It was found that 24 Ibs of TNT would satisfy these requirements. The corresponding design load function is illustrated in Figure 1.
10
GAS PRESSURE
0 10 20 30 40 50 60 70 80 90 TIME (MS)
Figure 1 The Blast Load
5. The expected structural deformations were computed based on the given load and the procedure outlined in TM 5-1300®.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 67
DYNA3D** was used to investigate the behavior of the steel connections under blast loads. The maximum rotational deformations at the connections were investigated to define the structural capacities of the connections under opening explosive loads. Stresses and strains at the critical points of welds and panel zones were checked to identify the more detailed deformation mechanism. These connections between W-shape cross-sections were studied in both two-dimensional (2D) and three-dimensional (3D) frame structures. The 2D frame models had 5,976 3-D solid elements. They were developed to obtain preliminary general information of the responses of the steel connections under the blast loads, and for initial comparisons with the results of the approximate analyses. Simplified two- dimensional vertical and horizontal pressures, in the X and Z directions, were applied to the inner faces of the members of the 2D models. It was also assumed that all pressure loads applied to the wall and roof panels were transmitted to the beams and columns in the vertical (X) and horizontal (Z) directions. The 3D frame models had 7.652 3-D elements. They were developed to analyze the three- dimensional behavior of the system. This part was similar to the two-dimensional approach, and the pressure loads on wall and roof panels were transmitted to the frame members. To investigate the strain rate effect on the steel properties and structural responses, the numerical simulations were done with and without consideration of Dynamic Increasing Factors ( DIP). Additionally, since this structural model could be part of a multi story building, the effects of gravity dead loads were considered. These equivalent gravity loads were represented by a 30 ksi axial dead load on the columns and 0.825 kips/ft vertical dead load on the beams. The conditions described above were represented by eight cases, as shown in Table 1.
Table 1. The Numerical Models
MODEL
2D
D-2D
DL-2D
DL-D-2D
3D
D-3D
DL-3D
DL-D-3D
2-D
2-D
2-D
2-D
3-D
3-D
3-D
3-D
model
model
model
model
model
model
model
model
DESCRIP"
loads
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
68 Structures Under Shock and Impact
3 Findings and Discussion
The steel knee connection models were investigated with the theoretical/empirical and numerical approaches. The rotational deformations of the models were compared with the rotation limits provided in TM 5-1300® to check the safety of the system. To support the findings, additional data (e.g., element motions, deformations, stresses, and strains at critical locations) were assessed for each case, as shown in Table 2. The maximum global support rotations, as defined in TM 5- 1300, were used for damage assessment. These rotations are shown in Table 3. They are compared with the results of the theoretical/empirical approach, and with the limits specified in TM 5-1300.
Table 2 Computed Peak Response Values
Case
2D
DL-2D
D-2D
DL-D-2D
3D
DL-3D
D-3D
DL-D-3D
* Stress and/or strain data indicated probable failure
The result shown in Tables 2 and 3 can be used to summarize the observed behavior for the different cases. Case 2D, a 2-D model without DIP, failed catastrophically by fracture at the weld. Case D-2D showed significant opening deformations at the connection, and the stresses approached the failure condition. The brittle failure of Case 2D occurred at 38 ms. At 82 ms, case D-2D showed about twice the rotational deformation predicted by TM5-1300. Nevertheless, these two different failure mechanisms showed the notable dynamic rate effects on the structural responses. Cases 3D and D-3D showed large horizontal deformations of the beams, highlighting the possible unsafe application of TM 5-1300 for the design of structural steel connections under blast loads. In Cases 3D and D-3D the strains, stresses and rotations exceeded the failure conditions and the limits set by TM 5-
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 69
Table 3 Computed Behavioral Parameters
Models
Beam
X
2.0
0.43
Failed
Failed
0.85
1.37
7.0**
(#1)
18**
(*U
6.8**
(#1)
10**
(#1)
Y
2.0
0.43
NA
NA
NA
NA
5.0**
(#2)
* For stress and strain data TM 5-1300 allowable limit applicable.
indicate probable failure; ** indicates rotation exceeds ; # indicates the Beam number; and NA indicates Not
1300. The DL cases (i.e., the models with consideration of dead load effects) showed similar structural responses to those of the cases without dead loads. However, because of the dead loads on the columns and on the beams, much larger vertical deformations were observed in all cases. The columns of the DL models, subjected to axial dead loads, showed about two times larger rotational deformation values than for the corresponding cases without dead loads. The DL cases showed a fracture failure at the connections like in 2D, and the internal element deformations also exceeded failure conditions. The results of the DL cases showed that the dead loads should be considered in dynamic design procedures, because they enhance the structural deformations. These observations showed that the TM 5-1300 design procedures might not be adequate for extreme dynamic problems. The differences between results based on the TM 5-1300 procedures and the numerical data suggest that the safety of the TM 5-1300 design procedures for the connection subjected to internal explosion loads might be questioned. TM 5-1300 does not consider the large local deformations of the adjoining members, but only
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
70 Structures Under Shock and Impact
their plastic modulus and ultimate strength. However, the results of these simulations showed that the local large deformations of the member flanges, and the possible weld cracks could govern the behavior and failure.
For Case 2D, the element tensile stresses and strains in the joint region reached failure conditions. This resulted in a large interface nodal acceleration of 18E+6 in/ins* at the critical points. The other models, governed by flexure, also showed large deformations at the connections. Element stresses and strains at most critical points of the connections reached failure conditions. However, the connections did not fail since the failed regions did extend over the entire joint, and no separations were observed. Typical deformations of the connection are illustrated in Figure 2.
Figure 2 Deformation Shape of Case D-3D at 80 ms
Significant strain rate effects on the structural responses were observed for the 2D model by comparing Cases 2D and D-2D, as shown in Table 3. Case D-2D underwent larger strains and stresses, and showed lower levels of motions. The 3D model showed comparably lower strain rate effects. Nevertheless, strain rate effect should be considered as an important parameter that can influence the structural response under severe dynamic loads. In addition, the connections' panel zone deformations and local buckling of stiffeners were investigated to check the applicability of the TM 5-1300 design procedures. This investigation showed that the TM 5-1300 approach panel zone deformations and local buckling of stiffeners is applicable for connection design, and that it can prevent local unstable behavior
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 71
at such regions.
4 Conclusions
An investigation using empirical and numerical approaches was conducted to understand the behavior of moment-resisting structural steel corner connections subjected to high explosive loads. This preliminary study produced findings that raised two major concerns: The first about the blast resistance of moment-resisting structural steel connections, and the second, about the safety of using TM 5-1300 for the design of structural steel connections. It was shown that moment-resisting structural steel welded connections subjected to 'safe' explosive loads may fail due to weld fracture. Furthermore, it was shown that the corresponding deformations of the structural elements, for those cases whose connections did not fail, may exceed the limits set in TM 5-1300. These deformations exceeded also the values predicted based on the analysis procedures in TM 5-1300. It was shown further that dead loads have an adverse effect on the behavior. This was due to the added bending and twisting of the beams once they were deformed by the blast effects. Since TM 5-1300 does not address such effects, current design procedures should be modified to reflect the structural damage caused by weak axis deformations.
All the numerical simulations were based on combining DIP values and validation against precision test data. Although steel is not expected to be very sensitive to strain rate effects, as compared to concrete, serious strain rate effects were note in the structural steel connections. Nevertheless, it was observed that the current DIFs. as defined in TM 5-1300. need to be modified to address more detailed pressure levels and the differences between 2-D and 3-D behavior. It is recommended that more detailed DIP applications, associated with pressure levels, might be necessary to avoid possible overeslimalion of strain rate effects. This could be very important when 2-D analysis are used to design 3-D structures.
Improved design approaches for both structural concrete and structural steel welded connections arc needed. Such design approaches should be derived based on additional studies that must be supported by combined theoretical, numerical and experimental efforts.
References
[1] AISC. Load and Resistance Factored Design Specifications for Structural Steel Buildings. Am. Inst. of Steel Constr.. 1986.
[2] Engelhardt, M.D., Sabol, T.A., Aboutaha. R.S., and Frank, K.H., Testing Connections, An Overview of the AISC Northridge A foment Connection Test Program. Modern Steel Construction, pp. 36-44, May 1995.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
72 Structures Under Shock and Impact
[3] Kaufmann. E.J., Xue, M.. Lu L.W., and Fisher, J.F., Achieving Ductile Behavior of Moment Connections, Modern Steel Construction, pp. 38-42, January 1996.
[4] Tsai. K.C., Wu, S., and Popov, E.P., Experimental Performance of Seismic Steel Beam Connection Moment Joints, Journal of Structural engineering, ASCE. Vol. 121. No. 6. pp. 925-931. June 1995.
[5] Engelhardt. M.D., and Husain, A.S., Cyclic-Load Performance of Welded Flange Bolted Web Connections. Journal of Structural Engineering, Vol. 119, No. 12. pp. 3537-3550. Dec. 1993.
[6] Richard. R.M.. Partridge, R.M.. and Radau. S., Finite Element Analysis and Tests ofBeam-to-Column Connections, Modern Steel Construction, October 1995.
[7] AISC. Specifications of Seismic Provisions for Structural Steel Buildings, Am. Inst. of Steel Constr. 1992.
[8] Department of the Army. Structures to Resist the Effects of Accidental Explosions. TM 5-1300. November 1990.
[9] Soroushian. P. and Choi. K.B., Steel Mechanical properties at Different Strain Rates. Journal of Structural Engineering, ASCE. Vol. 113, No.…
Pennsylvania State University, University Park, PA 16802, USA
Email: tedk@psu. edit
Abstract
This paper describes a study to assess the structural response and the current design procedures for steel connections under blast loads. The approach used in this study was based on adopting current blast resistant design procedures and advanced numerical simulations for assessing the behavior of modified seismic-resistant steel connections under blast-induced loads from internal explosions.
1 Introduction and Background
Many buildings that could be targeted by terrorists are steel frame structures, and the behavior of the structural steel connections under blast loads is of great interest. Current design guidelines^ for structural steel connections have been developed based on experimental and theoretical investigations. However, recent reports on damage to steel structures due to seismic loads suggested a surprisingly poor performance of the steel connections, as compared to expected behavior^ Important design modification were introduced after extensive assessments of the observed damage". Nevertheless, these modified design details may contain similar deficiencies if the connections would be subjected to high rate dynamic loads, such as those associated with blast effects. Therefore, it could be very important to assess the behavior of such structural steel connects under blast effects, and to identify such possible difficulties.
Structural behavior under dynamic loads requires attention to the relationship between the dynamic characteristics of the structure and the applied loads. Design specifications should address this relationship to insure that the various structural details are blast resistant. Although most of the recent attention
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
64 Structures Under Shock and Impact
in this area has been devoted to structural concrete connections, the development of design specifications for blast resistant steel connections is as important. TM 5- 1300® contains guidelines for the safe design of blast resistant steel connections. However, the adequacy of these design procedures is not well defined because of insufficient information about the behavior of the steel connections under blast loads.
High loading rates can influence the mechanical properties of structural materials*'*, and the use of dynamic increase factors (DIFs) for describing strain rate enhancement is well known. A DIP is the ratio of the strain rate enhanced strength to the static strength (e.g., the ratio of the dynamic and static yield stresses for a metal). This effect of higher strain rates on the mechanical properties of steel is important for blast-resistant design. DIFs are used for both design and approximate analyses (e.g., single-degree-of-freedom calculations). Nevertheless, DIFs have to be used with care in advanced numerical simulations. It is well known*'* that the steel yield stress is enhanced by strain rates while the ultimate stress is affected much less. The effects of high rate dynamic loadings on the structural responses were investigated in this study by employing the recommended DIF values in TM 5-1300* for both the design and the numerical simulations.
A useful research procedure for studying the behavior of structural concrete connections'^ was adopted to analyze structural steel connections under explosive loads. Typical modified structural steel connections for seismic conditions" were assessed under explosive loads by employing the design procedure in TM 5-1300*, and additional and empirical analysis tools. The maximum safe amount of the contained explosive charge and the blast resistant capacities of the connections were estimated. A reliable finite element code developed for simulating short-duration dynamic events, DYNA3D", was used to investigate the behavior of the structural steel connections under the expected blast loads. DYNA3D" was validated by simulating impact tests before it was used for the explosive simulations, as discussed below.
Computer codes must be validated to insure that they can reproduce real- world behavior. Such procedures were discussed in a recent workshop^, and the approaches employed during these numerical studies are in full agreement with the workshop recommendations. DYNA3D " was validated by simulating an impact test on a mild steel beam'*. The tested fixed-fixed steel beam was four inches long, 0.4 inches wide and 0.25 inches deep. It was struck at its center by an 11-pound weight with an impact velocity of 452.6 inches-per-second, and the force was delivered on a 0.2 inches square contact area. The steel material properties were, as follows: Yield stress of 43 ksi. ultimate stress of 63 ksi. and elastic modulus of 10* ksi. Strain rate effects*'* were incorporated into the kinematic/isotropic elastic-plastic material model in DYNA3D". Until 1.5 milliseconds (ms), the test specimen exhibited slightly larger deflections that the simulated data, and it failed at 2.4 ms. Although the difference between test and numerical data increased after about 1.5 ms, the two behaviors compared well. These results show that DYNA3D" can be used for the assessment of structural steel subjected to short-duration dynamic loads, and that the simulations can be expected to correlate well with actual behavior.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 65
A hypothetical one-story frame structure with seismic resistant corner knee connections was employed for this study. A welded beam-to-column connection in that structure was selected for the analyses, as defined in a previous investigation', and it was considered as a Fully-Restrained (FR). W-shape cross section members of the beams and columns sized to resist earthquake conditions were employed in the present model. The internal blast loads were derived from the detonation of a TNT charge at the geometric center of the floor in the structures. According to TM 5-13001 a load pulse from an internal explosion can be represented by a short and intense shock pressure and a longer duration lower intensity gas pressure. The maximum rotational deformations at the connections are typically used to assess structural damage*. These rotations were investigated to define the structural capacities of the connections under opening explosive loads (i.e., an internal explosion will cause the connections to open). Stresses and strains at critical points of welds and panel zones were checked to identify more detailed deformation mechanisms. Finally, based on the observed preliminary data, wider applications of the approaches were discussed. The research approach is described in further detail, next.
2 Study Description
The steel connection, recommended for seismic applications^, was employed to simulate the present cases. As a first step, the blast resistant capacity of the comer knee connection was analyzed according to TM 5-1300®. The preliminary assessment provided an estimated maximum safe explosive charge weight that the structure can be expected to resist without exceeding recommended damage levels**. The rotational deformations were estimated at the connection, based on the expected structural response. Additionally, this analysis was used also to define the corresponding pressure-time histories that would be applied to the structure in the advanced numerical simulation. Finally, numerical simulations were conducted with DYNA3D" for comparison with the findings based on TM 5-1300. and to obtain useful information about the possible responses of the steel connections under internal explosions. To obtain a better understanding of the structural behavior, various types of connection models were developed and simulated. The behavior of the connection and its deformations at points of interest were investigated by checking the nodal displacements, element deformations, stresses and strains. These activities are summarized next.
TM 5-1300* provides methods of design for explosive safety. Also, the manual provides procedure for deriving the blast load parameters and for calculating the dynamic responses of structural members. The adopted details of the structural model were based on a typical seismic resistant steel knee connection at a corner of a frame". W-14x342 (A572 GR50) and W-36xl50 (A36) were employed for the column and beam members, respectively. The weld was made with E70TG- K2 fluxed core electrodes that have a 70 ksi yield stress. The spans of 30 ft and height of 23 ft of the frame model were selected, considering typical dimensions of
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
66 Structures Under Shock and Impact
for such structures. The design procedures outlined in Section 5 of TM 5-1300® were used to evaluate the maximum blast load that can be applied to the beam and column members, as follows.
1.
2.
3.
4.
Plastic moment capacities were determined for beams and columns using dynamic material properties.
The equivalent uniformly distributed static load per unit length of beam was computed.
The blast overpressure on the roof, P,, was computed.
The computer codes Shock'"* and Frang^ were employed for deriving by trial-and-error the weight of an equivalent TNT charge that when detonated at the geometric center of the floor will produce a maximum gas pressure on the roof that will not exceed P\,. One wall was assumed as a frangible panel (i.e., it was assumed to blow out and permit quick venting of the internal pressures). It was found that 24 Ibs of TNT would satisfy these requirements. The corresponding design load function is illustrated in Figure 1.
10
GAS PRESSURE
0 10 20 30 40 50 60 70 80 90 TIME (MS)
Figure 1 The Blast Load
5. The expected structural deformations were computed based on the given load and the procedure outlined in TM 5-1300®.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 67
DYNA3D** was used to investigate the behavior of the steel connections under blast loads. The maximum rotational deformations at the connections were investigated to define the structural capacities of the connections under opening explosive loads. Stresses and strains at the critical points of welds and panel zones were checked to identify the more detailed deformation mechanism. These connections between W-shape cross-sections were studied in both two-dimensional (2D) and three-dimensional (3D) frame structures. The 2D frame models had 5,976 3-D solid elements. They were developed to obtain preliminary general information of the responses of the steel connections under the blast loads, and for initial comparisons with the results of the approximate analyses. Simplified two- dimensional vertical and horizontal pressures, in the X and Z directions, were applied to the inner faces of the members of the 2D models. It was also assumed that all pressure loads applied to the wall and roof panels were transmitted to the beams and columns in the vertical (X) and horizontal (Z) directions. The 3D frame models had 7.652 3-D elements. They were developed to analyze the three- dimensional behavior of the system. This part was similar to the two-dimensional approach, and the pressure loads on wall and roof panels were transmitted to the frame members. To investigate the strain rate effect on the steel properties and structural responses, the numerical simulations were done with and without consideration of Dynamic Increasing Factors ( DIP). Additionally, since this structural model could be part of a multi story building, the effects of gravity dead loads were considered. These equivalent gravity loads were represented by a 30 ksi axial dead load on the columns and 0.825 kips/ft vertical dead load on the beams. The conditions described above were represented by eight cases, as shown in Table 1.
Table 1. The Numerical Models
MODEL
2D
D-2D
DL-2D
DL-D-2D
3D
D-3D
DL-3D
DL-D-3D
2-D
2-D
2-D
2-D
3-D
3-D
3-D
3-D
model
model
model
model
model
model
model
model
DESCRIP"
loads
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
68 Structures Under Shock and Impact
3 Findings and Discussion
The steel knee connection models were investigated with the theoretical/empirical and numerical approaches. The rotational deformations of the models were compared with the rotation limits provided in TM 5-1300® to check the safety of the system. To support the findings, additional data (e.g., element motions, deformations, stresses, and strains at critical locations) were assessed for each case, as shown in Table 2. The maximum global support rotations, as defined in TM 5- 1300, were used for damage assessment. These rotations are shown in Table 3. They are compared with the results of the theoretical/empirical approach, and with the limits specified in TM 5-1300.
Table 2 Computed Peak Response Values
Case
2D
DL-2D
D-2D
DL-D-2D
3D
DL-3D
D-3D
DL-D-3D
* Stress and/or strain data indicated probable failure
The result shown in Tables 2 and 3 can be used to summarize the observed behavior for the different cases. Case 2D, a 2-D model without DIP, failed catastrophically by fracture at the weld. Case D-2D showed significant opening deformations at the connection, and the stresses approached the failure condition. The brittle failure of Case 2D occurred at 38 ms. At 82 ms, case D-2D showed about twice the rotational deformation predicted by TM5-1300. Nevertheless, these two different failure mechanisms showed the notable dynamic rate effects on the structural responses. Cases 3D and D-3D showed large horizontal deformations of the beams, highlighting the possible unsafe application of TM 5-1300 for the design of structural steel connections under blast loads. In Cases 3D and D-3D the strains, stresses and rotations exceeded the failure conditions and the limits set by TM 5-
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 69
Table 3 Computed Behavioral Parameters
Models
Beam
X
2.0
0.43
Failed
Failed
0.85
1.37
7.0**
(#1)
18**
(*U
6.8**
(#1)
10**
(#1)
Y
2.0
0.43
NA
NA
NA
NA
5.0**
(#2)
* For stress and strain data TM 5-1300 allowable limit applicable.
indicate probable failure; ** indicates rotation exceeds ; # indicates the Beam number; and NA indicates Not
1300. The DL cases (i.e., the models with consideration of dead load effects) showed similar structural responses to those of the cases without dead loads. However, because of the dead loads on the columns and on the beams, much larger vertical deformations were observed in all cases. The columns of the DL models, subjected to axial dead loads, showed about two times larger rotational deformation values than for the corresponding cases without dead loads. The DL cases showed a fracture failure at the connections like in 2D, and the internal element deformations also exceeded failure conditions. The results of the DL cases showed that the dead loads should be considered in dynamic design procedures, because they enhance the structural deformations. These observations showed that the TM 5-1300 design procedures might not be adequate for extreme dynamic problems. The differences between results based on the TM 5-1300 procedures and the numerical data suggest that the safety of the TM 5-1300 design procedures for the connection subjected to internal explosion loads might be questioned. TM 5-1300 does not consider the large local deformations of the adjoining members, but only
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
70 Structures Under Shock and Impact
their plastic modulus and ultimate strength. However, the results of these simulations showed that the local large deformations of the member flanges, and the possible weld cracks could govern the behavior and failure.
For Case 2D, the element tensile stresses and strains in the joint region reached failure conditions. This resulted in a large interface nodal acceleration of 18E+6 in/ins* at the critical points. The other models, governed by flexure, also showed large deformations at the connections. Element stresses and strains at most critical points of the connections reached failure conditions. However, the connections did not fail since the failed regions did extend over the entire joint, and no separations were observed. Typical deformations of the connection are illustrated in Figure 2.
Figure 2 Deformation Shape of Case D-3D at 80 ms
Significant strain rate effects on the structural responses were observed for the 2D model by comparing Cases 2D and D-2D, as shown in Table 3. Case D-2D underwent larger strains and stresses, and showed lower levels of motions. The 3D model showed comparably lower strain rate effects. Nevertheless, strain rate effect should be considered as an important parameter that can influence the structural response under severe dynamic loads. In addition, the connections' panel zone deformations and local buckling of stiffeners were investigated to check the applicability of the TM 5-1300 design procedures. This investigation showed that the TM 5-1300 approach panel zone deformations and local buckling of stiffeners is applicable for connection design, and that it can prevent local unstable behavior
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
Structures Under Shock and Impact 71
at such regions.
4 Conclusions
An investigation using empirical and numerical approaches was conducted to understand the behavior of moment-resisting structural steel corner connections subjected to high explosive loads. This preliminary study produced findings that raised two major concerns: The first about the blast resistance of moment-resisting structural steel connections, and the second, about the safety of using TM 5-1300 for the design of structural steel connections. It was shown that moment-resisting structural steel welded connections subjected to 'safe' explosive loads may fail due to weld fracture. Furthermore, it was shown that the corresponding deformations of the structural elements, for those cases whose connections did not fail, may exceed the limits set in TM 5-1300. These deformations exceeded also the values predicted based on the analysis procedures in TM 5-1300. It was shown further that dead loads have an adverse effect on the behavior. This was due to the added bending and twisting of the beams once they were deformed by the blast effects. Since TM 5-1300 does not address such effects, current design procedures should be modified to reflect the structural damage caused by weak axis deformations.
All the numerical simulations were based on combining DIP values and validation against precision test data. Although steel is not expected to be very sensitive to strain rate effects, as compared to concrete, serious strain rate effects were note in the structural steel connections. Nevertheless, it was observed that the current DIFs. as defined in TM 5-1300. need to be modified to address more detailed pressure levels and the differences between 2-D and 3-D behavior. It is recommended that more detailed DIP applications, associated with pressure levels, might be necessary to avoid possible overeslimalion of strain rate effects. This could be very important when 2-D analysis are used to design 3-D structures.
Improved design approaches for both structural concrete and structural steel welded connections arc needed. Such design approaches should be derived based on additional studies that must be supported by combined theoretical, numerical and experimental efforts.
References
[1] AISC. Load and Resistance Factored Design Specifications for Structural Steel Buildings. Am. Inst. of Steel Constr.. 1986.
[2] Engelhardt, M.D., Sabol, T.A., Aboutaha. R.S., and Frank, K.H., Testing Connections, An Overview of the AISC Northridge A foment Connection Test Program. Modern Steel Construction, pp. 36-44, May 1995.
Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509
72 Structures Under Shock and Impact
[3] Kaufmann. E.J., Xue, M.. Lu L.W., and Fisher, J.F., Achieving Ductile Behavior of Moment Connections, Modern Steel Construction, pp. 38-42, January 1996.
[4] Tsai. K.C., Wu, S., and Popov, E.P., Experimental Performance of Seismic Steel Beam Connection Moment Joints, Journal of Structural engineering, ASCE. Vol. 121. No. 6. pp. 925-931. June 1995.
[5] Engelhardt. M.D., and Husain, A.S., Cyclic-Load Performance of Welded Flange Bolted Web Connections. Journal of Structural Engineering, Vol. 119, No. 12. pp. 3537-3550. Dec. 1993.
[6] Richard. R.M.. Partridge, R.M.. and Radau. S., Finite Element Analysis and Tests ofBeam-to-Column Connections, Modern Steel Construction, October 1995.
[7] AISC. Specifications of Seismic Provisions for Structural Steel Buildings, Am. Inst. of Steel Constr. 1992.
[8] Department of the Army. Structures to Resist the Effects of Accidental Explosions. TM 5-1300. November 1990.
[9] Soroushian. P. and Choi. K.B., Steel Mechanical properties at Different Strain Rates. Journal of Structural Engineering, ASCE. Vol. 113, No.…
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