Welded Steel Moment Connections Research Articles

Cyclic Seismic Performance of Weak-Axis RBS Welded Steel Moment Connections

In this study, cyclic seismic performance of weak-axis reduced beam section (RBS) welded steel moment connections was experimentally investigated. Experimental results of this study again showed that the weak-axis RBS moment connection designed based on the procedure proposed by Gilton and Uang (J Struct Eng 122:1292–1299, 2002) performs well and developed an excellent seismic performance much better than 4% story drift with 80% of beam plastic capacity. However, it was also shown that the sharp corner of C-shaped fillet weld, used for attaching the beam web to the shear plate, can be the origin of crack propagation owing to stress concentration there. Slight trimming of the sharp corner is recommended to reduce notch effect. One specimen in this study eventually failed due to low cycle fatigue fracture as a result of cyclic flange local buckling that occurred at the butt joint between the beam flange and the horizontal continuity plate. The size of the weld access hole beneath the butt joint should be minimized if possible to suppress the occurrence of flange local buckling around the butt joint. The factors affecting the size of C-shaped fillet weld and the beam shear eccentricity for the weld design were also examined through a case study and simple analytical modeling. The sectional factor governing the beam shear eccentricity was shown to be the plastic section modulus ratio of beam section at RBS center to the beam flanges at welded joint. The elastic method neglecting the ductile behavior of the C-shaped fillet weld group was shown to be too conservative and should not be used in design in order to minimize welding heat affection to the thin beam web as well.

Description of asymmetric hysteretic behavior based on the Bouc-Wen model and piecewise linear strength-degradation functions

The Bouc–Wen model is one of the widely employed models in engineering for the description of hysteretic behavior. One of the major challenges in applying this model is how to describe the asymmetric hysteresis behavior. This paper first reviews previous models ranging from the original Bouc-Wen model to its several generalized models that have evolved. From a phenomenological perspective, this study combined the Bouc-Wen models with piecewise linear functions that can capture the hysteretic behavior with asymmetric strength degradation. The proposed model was verified on the basis of the cyclic seismic test results of welded steel moment connections with composite floor slabs.

Seismic retrofit of welded steel moment connections with highly composite floor slabs

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent in welded steel moment frames. The composite action due to the presence of a concrete floor slab was speculated as one of the critical causes of the prevalent bottom flange fracture. Close review of past experimental studies recently conducted by the authors clearly indicated that conventional steel moment connections with highly composite slabs are much more vulnerable to the bottom flange fracture. In this study, three seismic retrofit schemes are presented for welded steel moment connections with highly composite floor slabs typical of existing steel moment frames in Korea. Because top flange modification of existing beams is not feasible due to the presence of a concrete floor slab, beam bottom flange or web modifications by using welded triangular or straight haunch and heavy shear tab were cyclically tested. Test results of this study showed that all the retrofit schemes used are effective in eliminating the detrimental effect caused by high composite action and can ensure excellent connection plastic rotation exceeding 4% rad. Side effects resulting from retrofit as well as design recommendations are also discussed.

바닥슬래브를 고려한 용접철골모멘트접합부의 내진보강

바닥슬래브를 고려한 용접철골모멘트접합부의 내진보강

Resilient welded steel moment connections by enhanced beam buckling resistance

This study develops two (2) simple but effective techniques for enhancing buckling resistance of welded steel moment connections (WSMCs). The ANSI/AISC 358-10 prequalified connections satisfy the 4% interstory drift requirement, however experimental studies have shown that their strength degradation may initiate as early as 3% drift. This strength degradation has been observed to be initiated by buckling of the beam web which is followed by buckling of the beam flange and twisting of the beam. Consequently, buildings with the prequalified connections may sustain significant buckling damages under severe earthquakes and it is questionable as to whether these connections are capable of resisting gravity loads or lateral loads from strong aftershocks following a severe earthquake. To improve upon these shortcomings, two (2) performance enhancing techniques are proposed and investigated through finite element analysis (FEA). The more promising of the two involves reinforcing the beam web in the expected plastic hinge with a web reinforcement plate. Finite element analysis demonstrated that this reinforcement enhances the beam buckling resistance of WSMCs and thereby significantly reduces the beam buckling damages even at 5% interstory drift. The potential of this technique is analytically and experimentally demonstrated for the recently developed heat-treated beam section (HBS) WSMC. Test results confirm that the web reinforcement plate was effective in reducing local buckling damage and associated strength degradation, thereby improving the performance of HBS WSMCs. Areas for application and future development of the proposed techniques are identified.

Investigation of Composite Slab Effect on Seismic Performance of Steel Moment Connections

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent. Increased strain demand in the beam bottom flange due to the presence of concrete slab and resulting composite action was speculated to be one of the critical causes of the prevalent bottom flange fracture. Past experimental studies available in the literature after the 1994 Northridge earthquake were reviewed first in this paper. The review showed that different observations and recommendations were made for the slab effect on seismic performance among researchers depending upon the details of beam-to-column connections and concrete slab. In order to augment the test database and emphasize possible negative effect of a composite slab on seismic performance, full-scale cyclic seismic testing was also conducted for three welded steel moment connections. The pre-Northridge (PN) type connection specimen designed with a low degree of composite action (15% fully composite) exhibited a significant upward shift of the neutral axis under a positive (sagging) moment, thus experiencing larger strains in the bottom flange, and showed a poor seismic performance because of the bottom flange fracture at 3% story drift. The specimen designed by using reduced beam section (RBS) connection and with the details of minimized composite action exhibited a superior seismic performance without experiencing any fracture or concrete crushing, close to that of bare steel counterpart. Based on the review of previous experimental studies and test results of this study, it is clearly shown that welded seismic steel moment connections and concrete floor should be designed and constructed to minimize the composite action, if possible, especially when shallow beams and conventional connections are involved.

Effects of Composite Floor Slab on Seismic Performance of Welded Steel Moment Connections

Traditionally, domestic steel design and construction practice has provided extra shear studs to moment frame beams even when they are designed as non-composite beams. In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange side was prevalent. The upward moving of the neutral axis due to the composite action between steel beam and floor deck was speculated to be one of the critical causes. In this study, full-scale seismic testing was conducted to investigate the side effects of the composite action in steel seismic moment frames. The specimen PN700-C, designed following the domestic connection and floor deck details, exhibited significant upward shift of the neutral axis under sagging (or positive) moment, thus producing high strain demand on the bottom flange, and showed a poor seismic performance because of brittle fracture of the beam bottom flange at 3% story drift. The specimen DB700-C, designed by using RBS connection and with the details of minimized floor composite action, exhibited superior seismic performance, without experiencing any fracture or concrete crushing, almost identical to the bare steel counterpart (specimen DB700-NC). The results of this study clearly indicate that the beams and connections in seismic steel moment frames should be constructed to minimize the composite action of a floor deck if possible.

Seismic Performance of Pre-Northridge Welded Steel Moment Connections to Built-Up Box Columns

Results of experimental and analytical investigations of the seismic response of fully restrained steel moment connections between deep W-shape beams and deep and square built-up box columns are presented. Prototype beam-column connections were selected from the steel moment-resisting frames in a building built before the 1994 Northridge earthquake. Two full-size, single sided, pre- Northridge box column connection assemblage specimens were fabricated using the prototype connection data. Pretest analyses showed that the responses of box column connections can be substantially different from W-shape column connections. Both specimens were tested using a standard connection prequalification test protocol and each failed by brittle fracture in the complete joint penetration CJP welds of the beam flanges at connection drift angles less than 0.8% rad. The pre-Northridge beam flange CJP welds are likely to fail in a brittle manner regardless of the type of column cross section. ABAQUS finite-element models of the specimens were validated using the test results. The locations of the cracks in the beam flange CJP welds coincide with the region of high axial stresses, which varies depending on the column type, presence of continuity plates, and thickness of box-column plates.

기존 용접형 철골 박스기둥 접합부의 내진성능

본 연구에서는 1994년 미국 노스리지 지진 이전에 건설된 기존 철골 건물의 용접형 철골 박스기둥과 보 접합부의 내진성능을 반복재하 실험에 의해 평가하였다. 실험결과에 의하면 노스리지 지진 이전 접합상세를 갖는 박스기둥과 보 접합부에서도 H-형강 기둥과 보 접합부 실험결과와 유사하게 취성파단이 발생하였다. 하지만, 박스기둥과 보 접합부에서의 플랜지 응력전달 경로가 H-형강 기둥과 보 접합부에서의 것과 상당한 차이가 있고 또한 박스기둥의 형상에 따라 균열 전파의 형태가 다를 수 있다는 것이 밝혀졌다. 따라서 용접형 철골 박스기둥과 보 접합부의 내진성능을 향상시키기 위해서는 H-형강 기둥과 보 접합부에 대한 기존 연구결과를 그대로 적용하기보다는 박스기둥에 적합한 접합부의 보강상세 개발이 요구된다. In this study the seismic performance of welded steel moment connections to built-up box columns in the existing building built before 1994 Northridge earthquake was evaluated by cyclic tests. According to the test results, the pre-Northridge steel moment connections to the box columns also suffered from brittle fracture similar to that in the H-shaped column connections. However, the flange force transfer mechanism of the box column connections was substantially different from that of the H-shaped column connections, and the patterns of crack propagation may be changed due to the shape of the box column. Therefore, it is required to develop proper details for the box column connections instead of using the research results for H-shaped column connections in order to enhance the seismic performance the connections.

Prediction of Ductile Fracture in Steel Connections Using SMCS Criterion

Conventional fracture mechanics approaches have limited capabilities to accurately predict fracture under conditions of large scale yielding or in complex geometries where there are no significant pre-existing flaws. Such conditions are found, for example, in welded steel moment connections that meet stringent detailing and quality assurance requirements for seismic design that have been imposed in response to damage caused by the Northridge earthquake. The stress modified critical strain (SMCS) criterion provides an alternative approach for modeling ductile crack initiation by relating the fundamental process of void initiation, growth, and coalescence to macroscopic stresses and strains, obtained using detailed finite element models. The material-specific parameters of the SMCS model are calibrated for mild A572 Grade 50 steel using a series of notched tensile tests and fractographic data. Accuracy of the SMCS criterion is demonstrated for specimens with a range of triaxial constraint conditions, including both sharp-crack fracture specimens and blunt-notch specimens, the latter being examples of cases where conventional fracture mechanics approaches would not apply. The SMCS model is then applied in a practical demonstration to relate data from idealized pull-plate tests to the fracture resistance of beam–column moment connections.

Review of force transfer mechanism of welded steel moment connections

Employing the classical beam theory for the design of welded steel moment connections has been brought into question since the 1994 Northridge earthquake. In this study, the force transfer mechanism in various welded steel moment connections is comprehensively reviewed based mainly on the author’s recent studies. Available analytical and experimental results showed that the beam theory does not give accurate predictions of the stress path in almost all the welded steel moment connections because of the effects of the boundary conditions. Vertical plates near the connection, such as the beam web, the web of the straight haunch, and the rib, act as a strut to some extent. The force transfer mechanism in the reduced beam section (RBS) connection is essentially the same as that in the pre-Northridge type connections. Some simplified analytical models that can be used as the basis of a practical design procedure are also briefly presented.

용접 철골 모멘트접합부의 응력전달 메커니즘 재평가

용접 철골 모멘트접합부는 일반적으로 평면유지의 가정을 전제한 초등휨이론에 의해 설계되어 왔다. 그러나 1994년 노스리지 지진 이후 보-기둥 접합부의 설계에 초등휨이론을 적용하는 것은 타당치 않음이 몇몇 연구자에 의해 제기된 바가 있다. 본 연구에서는 필자의 최근 해석 및 실험연구를 주 근거로 하여 다양한 형식의 접합부의 응력전달 메커니즘을 재평가하고, 거의 모든 용접 모멘트접합부의 설계에 초등휨이론을 적용하는 것이 부적절함을 보이고자 하였다. 보의 웨브, 수평헌치의 웨브, 리브 등과 같은 수직 플레이트 접합요소는 모두 스트럿 작용에 의해 응력을 전달하는 유사성이 있음을 해석적, 실험적으로 확인하였다. 또한 최근 가장 큰 주목을 받고 있는 고연성 RBS 접합부의 전단력 응력전달 메커니즘은 PN형식 접합부의 그것과 크게 다르지 않음을 확인하였다. 아울러 접합부 설계에 유용하게 활용될 수 있는 단순화된 해석적 응력전달 모형을 소개하였다. Employing the classical beam theory for the design of welded steel moment connections has been brought into question by several researchers since the 1994 Northridge earthquake. In this study, the load transfer mechanism in various welded steel moment connections is comprehensively reviewed mainly based on recent studies conducted by the writer. Available analytical and experimental results showed that the load path in almost all the welded steel moment connections is completely different from that as predicted by the classical beam theory. Vertical plates near the connection such as the beam web, the web of the straight haunch, and the rib act as a strut rather than following the classical beam theory. The shear force transfer in the RBS connection is essentially the same as that in PN type connection. Some simplified analytical models that can be used as the basis of a practical design procedure are also presented.

Seismic damage evaluation of a steel building using stress triaxiality

This paper presents a procedure for the evaluation of the seismic damage in steel moment frame buildings. The rotational capacities, of typical connections of a building in the Los Angeles area, which suffered extensive damage during the Northridge earthquake, were determined using stress triaxiality as an indicator for fracture. The rotational demands of the same connections were obtained from a three-dimensional finite element analysis of the building model. Comparisons of the rotational demands at several locations in the building to the rotational capacities determined by the stress triaxiality analysis showed that fracture of steel moment connections of the building are governed by the triaxiality of stresses. Based on a number of moment connections analyzed, the paper suggests a moment capacity equation for the pre-Northridge welded steel moment connections taking into account the fracture incidence due to stress triaxiality effect. The moment capacity of such connections as dictated by stress triaxiality compared favorably to FEMA-350 requirements.

Cyclic Testing of Moment Connections Upgraded with Weld Overlays

Experimental studies were conducted to investigate the application of fracture tough overlay welds for the upgrading of welded steel moment connections. The experimental program consisted of cyclic tests on three exterior connection specimens that were initially fabricated using pre-Northridge welding procedures. Two specimens were fabricated from small size sections and one was fabricated from an intermediate size section. Overlay welds were placed using self-shielded flux core arc weld (FCAW-ss) with a fracture tough electrode (E70T-8). The application of weld overlays to the small size specimens resulted in a significant improvement in their cyclic performance. The plastic rotation capacity of both specimens was either close to or exceeded 3%. The application of weld overlays to the moment connection of the intermediate size specimen also improved the rotation capacity. However, the upgraded specimen failed in the parent material of the beam flange, away from the weld region at a plastic rotation of 2.7%.

Design of Welded Steel Moment Connections Using Truss Analogy

A new design procedure for fully-restrained welded beamto- column moment connections is presented in this paper. This procedure is based on truss analogy approach developed by the authors (Goel, Stojadinovic, and Lee 1996 and 1997). The truss analogy approach accounts for boundary effects occurring in the connection because the configuration of the connection restrains the deformation of the connected elements (Lee, Goel, and Stojadinovic, 1997). The resulting stress distribution, or "load path" can be modeled using a truss. The diagonals of the truss model represent the resultants of normal and shear stresses in the web of the beam that go through the regions near the beam flange welds. A new connection design is also presented in this paper. This connection configuration is designed by following the load path model using truss analogy. The idea behind the new connection design is to indirectly connect the beam to the column. The connection elements are to be placed near the top and bottom flanges of the beam to handle the forces in the truss chords and diagonals. Each flange connection comprises a horizontal flange plate and a set of vertical ribs designed to transfer the force in the truss members. The middle portion of the beam web need not be connected to the column except for field erection purposes because it is virtually stress-free. The combination of the new design procedure and new connection design is intended to ensure adequate ductility of the moment-resisting frame. To achieve this, it is desirable that plastic hinges form in the beams at a safe distance away from the beam-to-column connections.

Truss Analogy for Steel Moment Connections

Unprecedented widespread failure of welded moment connections in steel frames caused by the 1994 Northridge and the 1995 Kobe earthquakes have alarmed the engineering communities throughout the world. Beam-to-column connections in steel frames have been traditionally designed by using classical Euler-Bernoulli beam theory which leads to assumptions that the flanges transfer moment while the web connection primarily resists the shear force. The results of a recent finite element study at The University of Michigan have shown that stress distribution in the vicinity of moment connections drastically differs from the pattern assumed following the classical beam theory. This is in agreement with the boundary effect postulate expressed by the famous Saint Venants Principle. The finite element study has shown that the magnitude and direction of the principal stresses in the connection region are better approximated by using truss analogy rather than the classical beam theory. Accordingly, both the bending moment and the shear force are transferred across the connection near the beam flanges through diagonal strut action. Thus, the beam flange region of the traditionally designed connection is overloaded. This conclusion explains, to a large extent, the recently observed steel moment connection failures. This study uses the finite element analysis results and the truss analogy in order to establish a more realistic load path in the welded steel moment connections, a practical and more rational analysis and design procedure has been developed. The new connection design uses a combination of flange cover plates and vertical rib plates to carry the normal and shear stresses created by the truss action. The design procedure was successfully validated through cyclic load testing of a nearly full size specimen.