BOLTED LINKS FOR ECCENTRICALLY BRACED STEEL FRAMES

Transcription

1 BOLTED LINKS FOR ECCENTRICALLY BRACED STEEL FRAMES A. Stratan, the Politehnica University of Timisoara, Romania D. Dubina, the Politehnica University of Timisoara, Romania ABSTRACT Eccentrically braced steel frames represent a suitable solution for multi-storey buildings located in seismic areas. A bolted connection between the link and the beam is suggested to facilitate replacement of damaged dissipative zones (links) after a moderate to strong earthquake, which reduces repair costs. A full scale testing program was carried out in order to demonstrate the feasibility of this concept and to evaluate the performance of bolted links. The paper summarises the results of the testing program. INTRODUCTION Design of multi-storey structures in high-seismicity areas is usually based on dissipative structural response, which accepts significant structural damage under the design earthquake. It is believed however, that design criteria specified in modern seismic codes will prevent structural collapse, ensuring life safe. The earthquakes of Loma Prieta (1989), Northridge (1994) and Hyogoken-Nanbu (1995) showed that generally, modern structures behaved as expected. However, the unexpectedly high economic losses following these earthquakes urged for a limitation of damage to structures in future earthquakes, leading to the development of Performance-Based Design (PBD), Hamburger, 1996 (1). Its objectives include minimizing structural and non-structural damage under low and moderate earthquake intensities, which is equivalent to reduction of the total cost (initial and repair). ed e Figure 1. Bolted link concept. On the other hand, capacity-based design, applied in most of the current seismic design codes allows design of structures that promote plastic deformation in predefined areas only, called dissipative zones. In the case of a bolted connection between dissipative zones and the rest of the structure it is possible to replace the dissipative elements damaged as a result of a moderate to strong earthquake, reducing the repair costs. Application of this philosophy to eccentrically braced frames, where link elements serve as dissipative zones, is presented in figure 1. The connection of the link to the beams is realized by a flush end-plate and highstrength bolts. Bolted connection allows he link element to be fabricated from a lower-yield steel grade, assuring an elastic response of the elements outside removable link element. This system may be applied to both homogeneous structures (eccentrically braced frames alone) and dual ones (eccentrically braced spans combined with moment-resisting spans). The latter system has the advantage of more uniform transient and smaller permanent lateral displacements, which is beneficial for replacing damaged links, as well as for the building function, Stratan and Dubina, 2002 (2). Extended end-plate bolted connections for Connections in Steel Structures V - Amsterdam - June 3-4,

2 eccentrically braced frames with link-column connection configuration were previously suggested and investigated experimentally by Ghobarah and Ramadan, 1994 (3). Their inelastic performance was found to be similar to fully-welded connections. EXPERIMENTAL PROGRAM Specimens and experimental set-up An experimental program was carried out to determine cyclic performance of bolted links and to check the feasibility of the suggested solution. The removable link was fabricated from IPE240 profile of S235 grade steel, while the rest of the structure from S355 grade steel. Four link lengths were considered (e=400, 500, 600 and 700 mm, see figure 1), to study the influence of moment to shear force ratio. All links are classified as short ones according to AISC, 1997 (4). Another parameter considered was the spacing of web stiffeners, provided to prevent web buckling and to improve rotation capacity of the link. Two limit values of stiffener spacing were considered to AISC, 1997 (4): "close" spacing - 30t w -h/5, for a rotation capacity 0.08 rad, and "rare" spacing - 52t w -h/5, for 0.02 rad rotation capacity. support actuator link Figure 2. Experimental set-up for removable bolted links. For combination of link length and stiffener spacing, three specimens were tested: one monotonically and two cyclically. A total of 24 specimens were thus obtained, each being denoted as L[x][n]-[t], where: [x] L for "rare" spacing of stiffeners, H for "close" spacing of stiffeners; [n] 7, 6, 5, 4 for link length; [t] m for monotonic, c1 for cyclic 1 and c2 for cyclic 2 specimens. Thus, LL7-m specimen is one with rare spacing of stiffeners (L) of length 700 mm (7), monotonically loaded (m). The complete ECCS 1985 (5) loading procedure was applied, consisting of one monotonic and two cyclic tests. The monotonic test was used to determine the yield force F y and displacement D y, at the intersection of the initial stiffness and the tangent to the F-D curve having 10% of the initial stiffness. Yield displacement was determined for each monotonic test with rare stiffeners, and used to apply cyclic loading to the specimens of the same length. The cyclic tests consisted of four cycles in the elastic range (±0.25D y, ±0.5D y, ±0.75D y and ±1.0D y ), followed by groups of three cycles at amplitudes multiple of 2D y (3x±2D y, 3x±4D y, 3x±6D y, etc.) The loading was applied quasistatically, in displacement control. Bolts were preloaded to 100% of the full preload value for friction-grip bolts in the case of the monotonically loaded (m) and the first of the cyclically loaded (c1) specimens, and 50% for the second cyclically loaded specimen (c2). Previous experimental research on beam-column joints with end plates, Dubina et al., 2000 (6) showed a series of problems that undermined their cyclic performance: (1) fillet welds are inappropriate in the case of cyclic loading; (2) full-penetration 1/2V weld with the root at the exterior part of the beam cross-section promotes fragile ruptures, due to cracks initiated at weld root; (3) weld-access hole acts as a stress concentrator, causing brittle ruptures of the 224 Connections in Steel Structures V - Amsterdam - June 3-4, 2004

3 beam flange. Welding details used for the link to end plate connection were chosen so as to prevent the causes of poor performance mentioned above. Thus, link flange was welded to the end plate with a full-penetration 1/2V weld, realised from the exterior part of the crosssection (weld root at the interior); the weld access hole was eliminated completely, and reinforcing fillet weld was applied at the interior part of the flanges and on the web. Table 1. Characteristics of the materials used for fabrication of removable link specimens. component f y (R eh ), N/mm 2 f u, N/mm 2 f u /f y A, % IPE240 flange IPE240 web t= Standard tensile tests were performed on coupons extracted the materials used to fabricate the link specimens. Results presented in table 1 revealed a higher yield strength of the web in comparison with flanges of the link. Design of connections Bolted connection between the link element and the beam is located in a zone of maximum stresses. There are two possible strategies for connection design. The first one is to provide a sufficient overstrength of the connection over the link shear resistance. The second one is to assure a ductile behaviour of the bolted connection itself. The former strategy was followed in this case, as it facilitates replacement of damaged link elements. Capacity design of the connection involves two steps: determination of the yield strength of the dissipative element (link plastic shear resistance), and of the overstrength to allow for strain hardening. Two design provisions available at the date of the experimental program set-up were considered: Eurocode 8, 1994 (7) and AISC, 1997 (4). Though plastic shear resistance is determined using similar formulations in the two codes, the European seismic design provisions, referring to Eurocode 3 (8), consider the contribution of the fillet radius to the shear area, resulting in a capacity 40% higher than the one of the American code, which considers only the web area. The overstrength required for elements outside links also differ substantially. Previous experimental research, Kasai and Popov, 1986 (9), indicated link ultimate shear resistance about 1.5 times the plastic shear resistance. Eurocode 8 requires an overstrength of only 1.2, while AISC 1997 results in overstrength factors between 1.38 and Reduced overstrength factor in European codes is counterbalanced by higher plastic shear resistance, the maximum shear force estimated to the two codes having similar values. A relatively conservative estimation of maximum shear force was adopted in this study (1.75 factor, applied to the web area, corresponding to a 1.25 factor applied to the Eurocode 3 shear area): ( ) Vmax = 1.75 Vy = h 2 tf tw fy / 3 (1) Maximum moment for connection design was determined as: Mmax = Vmax e d /2 (2) Design of connections to the forces determined to equations (1) and (2) was based on Eurocode 3, 1997 and its Annex J (8). M20 gr.10.9 high-strength bolts were used. End plate thickness (25 mm) was chosen so as to provide a mode 3 (bolts in tension) failure mode of the equivalent T-stub, preventing excessive deformation of the end plate. A linear distribution of bolt forces was then assumed, and the bolts checked for tension, shear, combined tension and shear resistance, assuming a partial safety factor γ Mb =1.25. Demand to capacity ratio for combined tension and shear ranged from 0.7 for the LH4 and LL4 specimens to 0.98 for the LH7 and LL7 specimens. Additionally, bolt slip resistance was checked. Connections in Steel Structures V - Amsterdam - June 3-4,

4 Data processing The instrumentation consisted of the actuator load cell, and a series of displacement transducers used to measure both absolute and relative displacements. The basic forcedisplacement relationship used to characterize the monotonic and cyclic response of bolted links was actuator force (equal to link shear force) and total displacement D T, which includes slip in connection and endplate deformations. Response of link elements is characterised by the shear distortion angle γ - shear force F relation. For classical links, the distortion γ is determined as the difference of end displacements divided to the link length, Engelhardt and Popov, 1992 (10). With the notations from figure 3, γ is expressed as: γ = DT / b (3) Assuming that the edges of the panel bounding the link remain straight after deformation, the same angle γ may be determined from the deformations of the diagonals (DD1 and DD2): γ = ( 2 1) 2 2 a + b DD DD 2 a b (4) γ a b d d+dd2 π +γ2 d+dd1 d π -γ1 a DT b (a) (b) (c) Figure 3. Deformation of a classical link (a), idealisation of the panel zone (b) and its deformation (c). θ γτ γ θ γl γl θ γ DT DALJ DALS (a) (b) Figure 4. Deformation of a bolted link (a) and its idealisation (b). θ Values of angle γ determined according to equations (3) and (4) have close values in the case of classical links. However, in the case of removable bolted links, the behaviour of the link is more complex, and angle γ determined from equations (3) and (4) will be different. Total link deformation is given by the sum of: (1) shear distortion of the link panel - γ, (2) rotation in the two connections θ M =θ S +θ j, and (3) slip in the connections, characterised by the equivalent rotation γ AL =(D ALS +D ALJ )/e d, and can be expressed as: γt = γ + θm + γal (5) 226 Connections in Steel Structures V - Amsterdam - June 3-4, 2004

5 It can be directly obtained from the total displacement D T : γ = D e (6) T T d Instrumentation permitted both direct (6) determination of characteristic deformations, and indirect one (5), using the component deformations. A satisfactory correlation was observed between the two methods. BEHAVIOUR OF SPECIMENS Strength characteristics obtained from nominal and measured geometry and strength are presented in Table 2. Account was taken of the different flange and web yield strength in determining the link plastic moment: My = Wpl, w fy, w + Wpl * fy, f. Measured characteristics of steel showed higher increase of plastic shear force in comparison with plastic moment, which caused a decrease of the 1.6M y /V y limit. Even so, the links are classified as short. At the same time, maximum shear force and moment used for connection design are considerably higher than the initial estimates based on nominal characteristics. Connection strength was checked using estimates of maximum forces determined from measured geometrical and mechanical characteristics, considering a partial safety factor γ Mb =1.0 for the connection. Results indicated that the connection should have responded in the elastic range, though with little reserve for the longer LL7 and LH7 specimens. However, at large displacements, both bolt failures and end-plate deformations were observed during the tests. Two types of bolt failures were observed: (1) by thread stripping, which results in a ductile response (dominant in this experimental program), and (2) by fracture in bolt shank, which results in a brittle response. Table 2. Yield and maximum forces evaluated from nominal and measured characteristics. specimen W pl, W plw, W pl *, cm 3 cm 3 cm 3 V M y, kn y, 1.6M y /V y, V max, M max, knm mm kn knm LH7, LL nominal LH6, LL LH5, LL LH4, LL LH7, LL measured LH6, LL LH5, LL LH4, LL Note: M max determined per equation (2) 400 LL7 c1 400 LH7 c F, kn 0 F, kn γ T, rad γ T, rad Figure 5. Force-total deformation relationship F-γ T for specimens LL7-c1 and LH7-c1. Connections in Steel Structures V - Amsterdam - June 3-4,

6 Bolted connections had important contributions to the overall link response and in general did not showed an elastic response. Connection suffered important degradations at the Lx7 specimens, and caused a pronounced pinching effect with a reduced energy dissipation capacity (see figure 5). Element degradation started by bending of the end plate and bolt thread stripping, followed by local buckling of link flanges and web. Closer stiffener spacing had as main effect isolation of local flange and web buckling in outer web panels. Failure was attained by complete degradation of bolt threads. 400 LL4 c1 400 LH4 c1 F, kn web breathing F, kn γ T, rad γ T, rad Figure 6. Force-total deformation relationship F-γ T for specimens LL4-c1 and LH4-c1. (a) (b) (c) Figure 7. Failure by connection degradation at the LH6-c2 specimen (a); plastic web buckling at the LL4-c1 specimen (b), and strengthening of the brace to beam welded connection (c). Smaller length of Lx6 reduced the damage to connections and the pinching behaviour. Failure was attained by complete damage to bolts (see figure 7a), but also by web cracking after repeated plastic web buckling in the case of LL6-c2 specimen, with rare stiffeners. Starting with Lx5 specimens, connections were characterised by a more stable response, plastic web buckling being more important and preceding the one of the flanges. Failure of LL5-c1 and LL5-c2 specimens, with rare stiffeners, was attained by tearing of the web on three edges, at the cracks initiated in the base metal at the web-stiffener and web-end plate welds. Closer stiffener spacing in the case of LH5-c1 and LH5-c2 specimens reduced web 228 Connections in Steel Structures V - Amsterdam - June 3-4, 2004

7 tearing due to severe and repeated buckling (but did not eliminate it completely), failure being attained by damage of the connection. Response of specimens from the Lx4 series was dominated by web shear. Connection had a quasi-elastic response. Flange buckling was observed only after important web buckling. Hysteretic response was characterised by "full" cycles with high energy dissipation capacity (see figure 6). Due to higher web slenderness of the LL4-c1 and LL4-c2 specimens, web buckling was pronounced, and plastic web "breathing" was observed, as web buckling wave was changing direction at reversals of load direction (figure 6). Repeated buckling lead to web tearing along the diagonals (see figure 7b). Close spacing of stiffeners at the LH4-c1 and LH4-c2 specimens prevented this phenomenon, failure initiating through web tearing along the stiffener weld, which extended on three edges of the web. High stresses are present at the beam to brace welded connection, next to the beam to link bolted connection. Higher grade steel of the elements outside removable link did not provide sufficient overstrength in this zone. Due to repeated cyclic loading, the lower beam to brace welded connection fractured during the test of LL7-c1 specimen. Removing the weld and applying a new weld did not help, and the lower beam-brace assembly was completely replaced to due to extensive damage in the zone between the lower connection and the brace. To mitigate this problem after similar failure of the new subassembly during testing of the LL6-c1 specimen, stiffeners were added in the affected zone to increase the shear area and provide a smooth transfer of stresses from the bolted link element to the brace and the beam (see figure 7c). The performance of the subassembly modified in this way was satisfactory for the rest of tests. COMPARATIVE ANALYSIS OF RESULTS Elastic response of links was characterised by the total initial stiffness K γt, determined from V-γ T relationship, as well as shear stiffness of the web K γ, stiffness of connections K θj and K θs, determined from M-θ J, and M-θ S relationships. Initial shear stiffness of the link (K γ ) was in good correlation with the theoretical one (K γ th =G A s ), and not influenced much by the different considered test parameters. There was an important scatter in experimental values of connection rotational stiffness. Full preloading increased the stiffness of connection by approximately 50%. Upper connection resulted more flexible in comparison with the lower connection. Unsymmetrical distribution of moments and lack of fit at the upper connection may be attributed to this behaviour. Reduction of total initial stiffness of the bolted link in comparison with the classical solution is important, as a result of both the semi-rigid endplate, and slip in the connection. Therefore, either explicit modelling of the semi-rigid connection behaviour, or consideration of an equivalent link stiffness is necessary for global analysis of frames with bolted links. Table 3. Yield V y and maximum V max shear forces. parameter specimen LL7 LL6 LL5 LL4 LH7 LH6 LH5 LH4 V th y, kn 266.7* m V y, kn c c V th max, kn 400.1** m V max, kn c c Note: average of positive and negative values presented for specimens c1 and c2 * plastic shear resistance based on measured geometry and yield strength ** V th th max =1.5V y Connections in Steel Structures V - Amsterdam - June 3-4,

8 Connection slip was defined when relative displacement between the end plates of a connection exceeded 0.15 mm, according to C133/82 (11). The only specimen that did not slip was the first tested LL7-m. Slip resistance of the connection was reduced by cyclic loading and partial preload of bolts, rendering ineffective limitation of slip deformations. Yield force determined from V-D T relationship was not influenced by the test parameters and was controlled by shear response of the web. Lower experimental values (see table 3) are partially explained by the procedure used to determine yield force, which underestimates it for high initial stiffness. On the other hand, experimental maximum force presents an increase from the longer to the shorter links (effect of connection strength) and is higher for closer stiffeners (prevention of web plastic buckling). The maximum moment determined from equation (2) was lower than the theoretical one used to design the connections. Poor performance of connections could be explained by the fact that vertical displacement in the experimental set-up was constrained, which generated supplementary tension in the connections at large displacements. Further research is needed to validate this assumption and to check its application to real structures. Following the experimental observations in this study, in order to reduce damage in bolted connections, it is recommended to limit the length of bolted links to ed 0.8 My Vy, which corresponds to links LL4 and LH4. Table 4. Ultimate displacement D Tu and corresponding deformationγ Tu. D Tu, mm specimen LL7 LL6 LL5 LL4 LH7 LH6 LH5 LH4 m c c m γ Tu c c Note: minimum of positive and negative values presented for c1 and c2 specimens Ultimate link displacement D Tu, representing the stable hysteretic response is presented in table 4. Cyclic loading reduced by 40% to 70% rotation capacity, with the maximum reduction for short links. A slight reduction of ultimate displacements was observed for short links. In terms of deformations (γ Tu ), rotation capacity increases slightly for shorter links, with the exception of LL4 and LH4 specimens. With the exception of longer links with rare stiffeners (LL7), specimens showed a stable deformation capacity of at least 0.1 rad. Ductilities larger than 10 were observed, with a number of 16 to 22 cycles in the plastic range. Bolt preloading did not affect rotation capacity, as oppose to closer spacing of stiffeners, which improved link deformation capacity. Behaviour of long specimens was much influenced by the response of the bolted connection, characterised by a gradual reduction of strength due to bolt thread stripping, and a pinching cyclic response. The latter effect reduced the energy dissipated in the group of cycles of constant amplitude. Full bolt preloading reduced partially this effect. Response of short specimens was controlled by the shear of the link web, characterised by important hardening and energy dissipation capacity, but a more rapid degradation of strength after web tearing. Stiffener spacing had maximum importance for short links. Their effect was to limit plastic local buckling of the web, increasing the maximum force and deformation capacity, and providing a more stable cyclic response. However, after the attainment of ultimate deformation, failure of LH4 specimens was more rapid in comparison with LL4 specimens. Distribution of ductility demands between end pate and link web resulted in improved overall deformation capacity in comparison with "pure" failure modes, determined by concentration of plastic deformations in connection or web alone. This effect is characteristic of 230 Connections in Steel Structures V - Amsterdam - June 3-4, 2004

9 intermediate length specimens LL6-LL5 and LH6-LH5. However, it is difficult to achieve this response in practice, due to variability of mechanical characteristics of structural steels. CONCLUSIONS Experimental investigations on removable bolted links demonstrated the technological feasibility of the solution. Performance of short removable links and possibility to be easily replaced makes them attractive for dual eccentrically braced frames. Very short links, that assure an elastic behaviour of the connection are preferred, due to much easier replacement of damaged links. Concentration of damage in the removable link (performing like passive energy dissipation devices) may be accomplished by the capacity design principles, including fabrication of the link from a steel with lower yield strength in comparison with rest of the structure. The beam zone between the link end plate and brace is subject to high stresses, therefore its reinforcement by stiffeners is recommended. Welding details between the link and end plate showed a very good performance, which is attributed to: (1) elimination of weld access hole; (2) full penetration weld in 1/2V between the link flange and end plate, realised from the exterior of the profile; (3) a fillet weld on the interior contour (web and flanges) of the cross-section. Lack of weld access hole has the advantage of reduced fabrication cost in addition to higher connection performance. Longer links and closer stiffener spacing imposed higher demands on the connection. Cyclic response elements for which connection represented the weaker element were characterised by: (1) a reduction of maximum force in comparison with elements dominate by web shear; (2) a pinching behaviour with stiffness and strength degradation in cycles of constant amplitude; (3) failure by gradual strength degradation due to bolt thread stripping. Post elastic connection response was ductile, due to thread stripping. This failure mode is not generally characteristic for bolts. Bolt failure by shank rupture would have caused a more brittle response of long links. Response of short links was governed by web shear, stiffener spacing being important for their performance. In the case of rare spacing of stiffeners, inelastic response of short links was determined by plastic web buckling, which lead to strength degradation by alternative buckling in the direction of the two diagonals. Closer stiffener spacing limited plastic web buckling, leading to: (1) attainment of the maximum possible shear strength; (2) a stable hysteretic response; (3) a larger rotation capacity, but also (4) a more rapid failure by web tearing on the panel edges. With the exception of very short links, connections were partial-strength. On the basis of present experimental program, in order to prevent excessive connection damage, it is recommended to limit link length e d to 0.8 M y /V y. Design strength of short removable links limited to this length may be computed as for classical short links. Full bolt preloading resulted in higher initial stiffness, a more stable hysteretic response and a larger deformation capacity, and therefore is recommended for removable short links. Semi-rigid connections with flush end plate reduce substantially initial stiffness of removable short links in comparison with classical solution. Global analysis of eccentrically braced frames with removable links requires either explicit modelling semi-rigid connections, or consideration of an equivalent shear stiffness of the removable link. ACKNOWLEDGEMENT Support of the Romanian National Education Ministry (MEC-CNCSIS) and World Bank through the C16 Grant Reliability of Buildings Located in Strong Seismic Areas in Romania" and MEC-CNCSIS grant AT10/218 "Seismic response of dual eccentrically braced frames with removable links" is gratefully acknowledged. Connections in Steel Structures V - Amsterdam - June 3-4,

10 NOTATION e, e d clear length of the link between braces, and length of the bolted link t f, t w, h flange thickness, web thickness, and cross-section height D y, F y yield displacement, yield force f y (R eh ), f u, A (upper) yield stress, tensile strength, elongation at rupture V y, M y plastic shear resistance, plastic moment V max, M max maximum shear force, maximum moment γ Mb partial safety factor for bolt resistance D T total link displacement a, b link panel dimensions γ link shear distortion angle γ AL equivalent link rotation angle due to connection slip θ M, θ S, θ j average, bottom, and top connection rotation γ T total link distortion angle DD1, DD2 measurements of link diagonal displacement transducers D ALJ, D ALS measurements of link slip displacement transducers f y,w, f y,f web and flange yield stress W pl,w, W* pl plastic modulus of the web and flanges (W* pl = W pl - W pl,w, W pl ) K γt, K γ, K θj and K θs total initial, web shear, and connection stiffness D Tu, γ Tu ultimate displacement, ultimate deformation REFERENCES (1) Hamburger, R.O. (1996). "Implementing performance-based seismic design in structural engineering practice". In: Proceedings of 11th World Conference on Earthquake Engineering, Acapulco, Mexico. Paper no Oxford: Pergamon. (2) Stratan, A., and Dubina, D. (2002). "Control of performance of dual frames with eccentric bracing", Proc. Stability and Ductility of Steel Structures SDSS 2002, Budapest, Hungary, september (3) Ghobarah, A. and Ramadan, T. (1994). "Bolted link-column joints in eccentrically braced frames". Engineering Structures, Vol.16 No.1: (4) AISC-97, (1997). "Seismic Provisions for Structural Steel Buildings". American Institute of Steel Construction, Inc. Chicago, Illinois, USA. (5) ECCS (1985). "Recommended Testing Procedures for Assessing the Behaviour of Structural Elements under Cyclic Loads", European Convention for Constructional Steelwork, Technical Committee 1, TWG 1.3 Seismic Design, No.45 (6) Dubina, D., Ciutina, A., Stratan, A., (2000). "Cyclic Tests on Bolted Steel Double Sided Beam-to-Column Joints". The International Workshop Connections in Steel Structures IV: Steel Connections in the New Millenium. October 22-25, 2000, Roanoke, Virginia, USA. (7) Eurocode 8 (1994). "Design provisions for earthquake resistance of structures". CEN European Committee for Standardisation. (8) Eurocode 3 (1997). "Design of steel structures. Part 1-1: General Rules and Rules for Buildings ", CEN. European Committee for Standardisation. (9) Kasai, K., and Popov, E.P., (1986). "General Behaviour of WF Steel Shear Link Beams", ASCE, Journal of Structural Engineering, Vol.112, No.2: (10) Engelhardt, M.D. and Popov, E.P. (1992). "Experimental performance of long links in eccentrically braced frames". Journal of Structural Engineering, Vol.188, No.11: (11) C133/82 (1982). "Technical guide for connections id steel structures with high strength friction grip bolts ". ICB, INCERC (in Romanian). 232 Connections in Steel Structures V - Amsterdam - June 3-4, 2004