An experimental study of end plate shear connections

An experimental study of end plate shear connections

KARL VAN DALEN Department of Civil Engineering, Queen's Universiiy, Kingston, Ont., Canada K7L 3N6

AND

MARK VAN DALEN Building Engineering, Morrison Hershjeld Limited, 1980 Merivale Road, Nepeatl, Ont., Canada K2G 1G4

Received January 29, 1990

Revised manuscript accepted March 4, 1991

The shear behaviour of both conventional end plate connections and clipped end plate connections, in which the upper corner of the end plate was removed, was examined in a laboratory investigation of 29 full-scale test specimens. The study included connections of varying geometry, bolt diameter, and end plate depth. The majority of the connections failed in one of two modes: a failure of the bolt group component or a web-tear failure. For connections failing through the bolt group, the ratio of the shear capacity of the clipped connection to that of the conventional connection is the ratio of the number of bolts in the clipped and conventional connections. Even when failure is not through the bolt group, the shear capacity of an end plate connection is reduced when the upper corner of the end plate is clipped. The experimentally determined shear capacities are compared with the calculated resistances determined using the resistance equations in CAN3-S16.1 in conjunction with the assumptions of load transfer mechanisms and stress distributions embodied in the design aid for end plate connections in the Handbook of Steel Construction.

Key words: beams, connections, bolts, end plates, shear strength.

Le comportement au cisaillement des plaques d'appui de raccordement conventionnelles et des plaques d'appui dont le coin supCrieur est tronquC a CtC examine dans le cadre d'une Ctude en laboratoire portant sur vingt-neuf Cchantillons de taille nor- male. Ces Cchantillons comprenaient des plaques de gComCtrie, dlCpaisseur et de diamktre des orifices des boulons variables. La majorit6 des essais se sont soldCs soit par une rupture au niveau des boulons, soit par une rupture de l 'ime. Dans les premiers cas, le rapport entre la resistance au cisaillement des plaques tronqutes et celle des plaques conventionnelles Ctait Cgal au rapport du nombre de boulons de la plaque tronquCe sur celui de la plaque conventionnelle. M&me lorsque la rupture ne se produit pas au niveau des boulons, la rCsistance au cisaillement de la plaque diminue lorsque le coin supCrieur est tron- quC. Les rtsistances au cisaillement dCterminCes par les essais ont CtC compartes B celles dCterminCes au moyen des equations de resistance de la norme CAN3-S 16.1 et des hypothkses ayant trait aux mtcanismes de transfert de charge et aux distributions des contraintes qu'on trouve dans la section des aides B la conception des plaques de raccordement du Handbook of Steel Construction.

Mots clPs : poutres, raccordements, boulons, plaques d'appui, resistance au cisaillement. [Traduit par la rCdaction]

Can. 1. Civ. Eng. 18, 818-829 (1991)

Introduction Various forms of flexible shear connections have been used

in steel construction over the last few decades. One of the most recent of these is the shear end plate connection. This connection consists of a predrilled steel plate welded to the end of a beam web in a plane normal to its longitudinal axis. The connection is easily made in the field if it involves bolting a single end plate to the flange or web of the supporting member. How- ever, the erection of beams which frame into both sides of a supporting member and are to be connected by end plate connections sharing a common set of bolts involves operations that can be hazardous and expose the erectors to unacceptable risks. An economical and safe solution to this problem involves cutting away the upper right-hand portion of each plate in the shop. This modification permits the first of the two beams to be fastened to the supporting member with a single bolt in the upper left-hand corner of the end plate. The second beam can then be aligned directly opposite the first in spite of the protruding bolt, since that portion of the end plate, which would otherwise have interfered with the previously installed

NOTE: Written discussion of this paper is welcomed and will be received by the Editor until February 29, 1992 (address inside front cover).

bolt, has been removed during fabrication. This type of shear end plate connection is known as a clipped end plate connection and is shown in Fig. 1.

A clipped end plate connection must comply with all the requirements of a conventional shear connection. That is, in addition to transmitting the vertical shear forces imposed on it, a clipped end plate connection must have sufficient flexibility to accommodate the rotations imposed on it when the beam it supports attains its full flexural resistance. The moment- rotation characteristics of clipped end plate connections have been reported in an earlier paper (Van Dalen and MacIntyre 1988). This paper examines the effect of clipping the end plate on the shear capacity of the connection.

Test program Two series of full-scale static shear tests were conducted on

various shear end plate connections. In the initial investigation (Nowack 1987), eight clipped and eight conventional end plate connections were studied. The connections tested included a range of commonly varied connection parameters, namely the number of bolts in a vertical row, the end plate thickness, the gauge of the bolts, and the size of the supported beam. This test series is referred to as the "geometry series" in this paper.

A later study (Van Dalen 1988) consisted of an additional

Prlnccd ~n Canada I Impr~rnf au Canada

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VAN DALEN AND VAN DALEN

FIG. 1 . Clipped end plate shear connection.

series of tests, in which six clipped and seven conventional shear end plate connections were tested to examine the strength of the bolt group component in the connections. This series of tests is hereafter referred to as the "bolt series." To comple- ment the earlier work, the range of the parameters in the series tested by Van Dalen was similar to that in the series tested by Nowack. In both test series, for each clipped end plate connection that was tested, an unclipped (conventional) end plate connection having otherwise the same connection geometry was also tested. This allowed the effect of clipping the end plate to be determined directly for the full range of connections in each series.

The beam component of each specimen was taken from material specified as CSA G40.21M, Grade 300W steel. The end plate component was cut from material specified to be ASTM Grade A36 steel. In the bolt series, the connections were fastened with 12.7 mm diameter bolts. For the geometry series, 19.05 mm diameter bolts were used. In all cases the bolts were A325 high strength bolts, with hardened steel washers under both the head of the bolt and the nut.

Each test specimen was designated by a letter, followed by five numbers and, in some instances, a letter suffix. The first letter is either U or C, representing an unclipped or clipped end plate. The five numbers represent the diameter of the fastening bolts, the number of bolts in a vertical row, the end plate thickness, the bolt gauge, and the beam depth, respec- tively. Thus C-19-2-6-101-310 indicates that the end plate of this specimen was clipped and that 19.05 mm diameter bolts were used with two bolts per vertical row. The end plate thickness was 6 mm, the horizontal distance between vertical rows was 101 mm, and the beam was a W310 x 39.

Three standard wide flange beam sections were used in the tests: W3 10 x 39, W460 x 61, and W610 x 113. Test specimens were flame cut and squared into individual 1.6 m beam lengths. Vertical bearing stiffeners were welded on both sides of the web 110 mm from each end of the beam, so as to be located directly under the load point. An unclipped or similar clipped end plate was welded to each end of the beam to allow two connection tests per beam.

The end plates were flame cut from either 6 or 10 mm thick flat plate. Details of the clip and bolt hole locations are given in Fig. 2.

All plates, whether clipped or unclipped, were specified to be 38 mm below the top of the upper flange of the beams and

64 tr tr I

I I1 RADIUS

m ro

m ro

0 w P-

EXTENT OF END PLATE 0 / / FOR CONNECTION WITH TWO BOLTS PER ROW

DIAMETER OF BOLT + 2

EXTENT OF END PLATE FOR CONNECTION WITH THREE BOLTS PER ROW

FIG. 2. Details of end plates.

welded to the end of the beam web with fillet welds running the full depth of the plate on both sides of the web. E480-14 welding electrodes were specified. The specified fillet weld size was 5 mm for W310 x 39 beams, 6 mm for W460 x 61 beams, and 8 mm for W610 x 113 beams. Clipped plates were positioned so that the clip would be in the top right corner of the plate, as viewed from the exterior of the beam.

The first five tests in the bolt series were conducted on connection specimens fabricated as described above. Three of these specimens did not fail through the bolts, but, instead, failed when a tear formed in the beam web below the end plate. To ensure that the critical connection component in the balance of the bolt series would be the bolt group, the webs of the remaining specimens in this series were reinforced by welding a 6 mm thick, 25 mm wide flat bar to each side of the beam web. The pieces extended from the top of the bottom flange to a point 12 mm above the bottom of the end plate. The pieces effectively thickened the beam web by 12 mm for a distance of 25 mm from the end of the beam. Specimens with

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820 CAN. J . CIV. ENG. VOL. 18, 1991

webs reinforced in this manner have the suffix R in their desig- nation. The beams in the test s~ecimens were much shorter than the beams found in a typicai floor system. To ensure that the connection would fail in shear, the load had to be applied as a concentrated load close to the connection rather than distributed uniformly along the length of the beam. In the laboratory, the concentrated load was applied over the width of the top flange at a location approximately 114 mm from the connection.

Stiffeners would not usually be found in floor beams supporting a uniformly distributed load. However, it was feared that in these tests unstiffened beams would not be capable of resisting the large concentrated loads which would be applied to the test specimens and that the beam section could fail by local web crippling below the load application point at a load substantially lower than the ultimate load for the connection.

As a check on the need for bearing stiffeners and to determine if the stiffeners influenced the nature of the failures. two supplementary tests (A and B) were conducted. Specimen A had no stiffeners welded to the beam web, but was otherwise similar to connection U- 19-3- 10- 10 1-460. Specimen A failed by web crippling under the point of application of the concentrated load. As the load was applied, the web of the beam began to buckle out of plane. Initially, the out-of-plane move- ments of the web were relatively small and the vertical load could still be increased. As loading progressed, the vertical deformation of the top flange and the out-of-plane deformation of the beam web became very severe. ~ v e n t u a l l ~ , a point was reached where the increasingly severe web crippling was accompanied by a decrease in the vertical load sustained by the specimen. This was deemed to be failure; it occurred at a load of 571 kN for specimen A.

Specimen B had stiffeners that were fitted to the underside of the top flange, but only continued down the web a distance equal to the depth of the end plate. In all other respects, the specimen was similar to that for connection U- 19-3- 10-101-460. Specimen B failed at a load of 671 kN. The overall behaviour of this specimen was identical to that of the other test specimens with similar connections which had stiffeners extending the full depth of the web. Supplementary tests A and B therefore confirmed that some form of web stiffening was essential to prevent a premature web buckling failure of the specimen and, further, that the nature of the failure in a specimen with stiffening was not sensitive to the length of the stiffeners. It was therefore decided to fabricate all specimens in both test series with a pair of 12.7 mm thick web stiffeners directly under the load application point, extending the full depth of the section.

All connection tests and supplementary tests were conducted in a specially designed frame. As shown in Fig. 3, the test frame consisted of two independent loading assemblies arranged so that a specific rotation could be applied to the connection prior to its being loaded to failure in shear.

For each of the test specimens, a rotation representing the end rotation of a simply supported beam at the onset of yielding at midspan was imposed on the connection. The main shear load was then applied in approximately 10 equal load increments until the connection failed. After the application of each load increment, deflection and load readings were recorded and, if necessary, the rotation was restored to the value that was originally applied. Failure of the connection was defined as either the fracture of any connection component or the point at which the shear load remained constant (discounting instan-

taneous increases in load as the jack was pumped), or dropped in value while the vertical deflection of the connection continued to increase.

Test results Geometry series

The maximum value of the applied load and the mode of failure for each of the connections tested in the geometry series are presented in Table 1. Four modes of failure were observed in the connections tested in this series: a horizontal tearing failure of the beam web just below the bottom edge of the end plate, bearing failure of the end plate, shear failure of the beam web adjacent to the end plate, and shear failure of the fillet welds.

The most common mode of failure was horizontal tearing of the beam web directly below the end plate. This mode of failure is hereafter termed a "web-tear" failure. The sequence of events which characterized a web-tearing failure was gener- ally as follows. Very early in the loading, the mill scale and whitewash applied to the beam web began to flake, indicating that large strains were developing in the area directly below the end plate. As the load was increased, the highly strained area began to grow, until, at failure, a distinct "block" of the web exhibited significant flaking of mill scale and whitewash. As the load approached its ultimate value, a small tear developed in the beam web at a point just below the end plate. With the application of more load, the tear propagated either horizontally through the web or a small distance vertically up the weld and then horizontally into the beam web. Initially, the vertical load could still be increased in spite of the web tear, but eventually attempts to increase the load simply caused the tear to propagate at an essentially constant load. At that stage the specimen was deemed to have failed. At the same time as the tearing progressed, some local web buckling developed at the top of the beam web. Web buckling was observed to be more predominant for the clipped end plate connections than for the unclipped end plate connections. Typical web-tearing failures are shown in Fig. 4 for an unclipped connection and in Fig. 5 for a clipped end plate connection.

A bearing failure of the end plate is evident in Fig. 6 , which shows the end plate of C-19-3-6-101-460 after testing. At the conclusion of the test, the bolt holes in the end plate of this connection exhibited severe distortion; the top of the plate had undergone major inward in-plane deformations, and the lower portion of the plate had developed a vertical tear.

Although failure of the end plate limited the shear capacity of the connection in only one instance, distortion of the end plate was observed in varying degrees in all the specimens with clipped end plates. As might be expected, specimens with the most severe end plate distortions also exhibited the largest web buckling deformations in the region above the end plate.

Shear failure of the beam web adjacent to the end plate occurred in connection U-19-3- 10-101-3 10. In its initial stages, this failure was similar to the web-tearing mode of failure. Once again, as the load approached its ultimate value, a horizontal crack developed in the beam web at the bottom of the end plate. In contrast to the web-tearing failure mode, the crack propagated only a few millimetres horizontally into the beam web and then travelled vertically up the beam web along- side the fillet weld as shown in Fig. 7. No web buckling was observed with this mode of failure.

Shear failure through the throat of the fillet welds restricted the maximum load carried by connections U- 19-4- 10- 140-6 10

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JACK FOR APPLICATION OF ROTATION

JACK FOR APPLl OF SHEAR LOAD

ROLLER ASSEMBLY TEST CONNECTION

W360 x 64 COLUMN STUB

SCREW JACK

FIG. 3. Connection test frame.

TABLE 1. Geometry series test results supplemented the data on web-tear failures obtained from the geometry series, they provided no insight into the strength of

Failure load of Mode of the bolt group component of the connection. As explained Connection connection (kN) failure earlier, the beam webs in eight of the specimens were there-

fore reinforced so as to force-a bolt failuie. These eight speci- C-19-2-6-101-310 323 Web-tear mens and two of the unreinforced specimens failed when shear U-19-2-6-101-310 359 Web-tear fracture of the bolts occurred.

Web-tear Web-tear Web-tear Web shear Plate bearing Web-tear Web-tear Web-tear Web-tear Web-tear Web-tear Weld shear Web-tear Weld shear

and U-19-4-10-101-610. As the load approached ultimate, a vertical crack propagated up the weld 25-30 mm from the bottom of the connection. The load then dropped suddenly while the crack rapidly spread up the remaining length of the weld.

Bolt series The ultimate value of connection shear load and the mode

of failure are presented in Table 2 for each of the connections tested in the bolt series. The two modes of failure observed in the bolt series were a web-tearing failure and a bolt shear failure. There were no failures of either the end plate or fillet weld in any of the specimens in this series.

The progression of the web-tear failure in these tests was similar to that described for the web-tear failure in the geometry series. Although these failures provided information that

The bolt shear failure occurred in two distinctly different ways, depending on the number of bolts in a vertical row in the connection. In connections C-12-2-6-101-3 10 and C-12-2- 10-101-310, the connection exhibited some rotation about a longitudinal axis as the load increased. This rotation caused the lone bolt at the top of the end plate to move towards the vertical center line of the beam. The redistribution of forces associated with this rotation apparently caused the lone bolt below the clip to carry a disproportionate share of the total load applied to the connection. Failure of the connection occurred when the lone bolt below the clip sheared, and the load carried by the specimen decreased markedly. In both clipped connections with two bolts per vertical row, the remaining bolts exhibited significant shear deformations. There was relatively little rotation of the connection about the longitudinal axis of the specimen in all other specimens with bolt shear failures.

Initially, the application of vertical load to the connection resulted in an essentially linear load-deformation relation- ship. At some stage, however, each application of an increment of load caused an increasingly larger increment of vertical movement of the connection as a result of shearing deformations in the bolts. Finally, a sudden catastrophic failure occurred when all bolts in the group fractured simul- taneously. In some tests, the bolts were installed and marked in such a way that their orientation could be identified after the test. Observation of the residual deformations in these bolts confirmed that the relative movement between the end plate and the face of the support was predominately vertical and that virtually no connection rotation had occurred.

In all the tests that failed through the bolt group, there was essentially no buckling deformation of the beam web above the end plate. Although significant shear deformation occurred in

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CAN. J. CIV. ENG. VOL. 18. 1991

FIG. 4. Web-tear failure in connection U-19-2-10-101-310.

FIG. 5. Web-tear failure in connection C-19-2-10-101-310.

the beam web between the connection and the stiffener, there was very little deformation perpendicular to the plane of the web. However, for the three cases of failure by web-tear, some web buckling just above the end plate accompanied the web-tear failure.

Analysis of results

Introduction of a clip in the end plate of the connection reduces the total number of bolts in the connection by 1 as compared to the usual case. Therefore, if the effects of eccen-

tricity are ignored and it is assumed that all bolts will simul- taneously reach their ultimate shear strength, the ratio of a clipped-to-unclipped connection capacity would be (n - l)ln, where n is the number of bolts in an unclipped end plate connection. This clipped-to-unclipped connection strength ratio, CIU, is 0.750,0.833, and 0.875 for connections with 2, 3, and 4 bolts in a vertical row. These values of CIU are compared with the values of CIU determined from the test results in Table 3.

The theoretical determination of CIU is, of course, only valid for connections in which both the clipped and unclipped

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FIG. 6. Plate bearing failure in connection C-19-3-6-101-460.

connections failed by bolt group shear. This is the case in four pairs of connections, CIU-12-2-10-101-310, CIU-12-3-6-101- 460, CIU-12-3-10-101-460, and CIU-12-4-10-101-460. The CIU value from the tests for the pair of two-bolts-per-row connections C-12-2-10-101-310 and U-12-2-10-101-310R is 0.790, which is 5 % greater than the theoretical CIU value. It should be noted, however, that in this instance the clipped connection failed when a single bolt sheared. The average of the actual CIU values for the two applicable three-bolts-per-row connections is 0.827, which is within 1 % of the theoretical value of 0.833. For the lone applicable four-bolts-per-row connection, the actual CIUvalue is 0.857, which is within 2 % of the theoretical value of 0.875. Thus, the premise upon which the CIU calculations were based, namely that the bolts carry an equal share of the total load, appears to be reasonable not only for symmetrical unclipped connections but also for clipped connections with three or more bolts per row.

This can be demonstrated more clearly by calculating an apparent individual bolt capacity for the bolts in any of the connections where the failure occurred through the bolt group. This capacity is simply calculated as the ultimate shear load carried by the connection divided by the number of bolts in the connection. The apparent individual bolt capacity has been included in Table 3 wherever applicable.

An arithmetic mean of the apparent bolt capacities can be calculated for the eight connections that failed through the entire bolt group. This value should represent an average ultimate shear capacity for 12.7 mm bolts, based on a sample of eight tests. This calculation yields a mean bolt shear capacity of 88.6 kN, with a standard deviation of 3.5 kN. A compari- son can now be made between this mean value and the apparent bolt capacities calculated for the two connections where failure occurred through the lone bolt.

FIG. 7. Web shear failure in connection U-19-3-10-101-310.

TABLE 2. Bolt series test results

Failure load of Connection connection (kN) Mode of failure

Lone bolt shear Web-tear

Lone bolt shear Web-tear Bolt group shear

Web-tear Bolt group shear

Bolt group shear Bolt group shear



The eccentricity in a clipped end plate must cause some rotation of the end plate, which results in a redistribution of bolt forces. Test results provided visual confirmation that the end plate does rotate in the case of a connection with two bolts in a vertical row, and the fracture of the lone bolt below the clip is evidence that this bolt is more heavily loaded than the remaining bolts. If the distribution of loads amongst the bolts is severely nonuniform, the apparent capacity per bolt, calculated as the ultimate load carried by the connection divided by the total number of bolts, must of necessity be less than the actual capacity per bolt. The extent to which the apparent bolt capacity in a connection with a single bolt failure is less than

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TABLE 4. Measured dimensions and material properties

Beam Bolt End plate

Web Weld Diameter F,* Thickness Fyi FUt thickness Fyt size Xu:

(mm) (MPa) (mm) (MPa) (MPa) Shape (mm) (MPa) (mm) (MPa)

(a) Geometry test series 19.05 1110 6 320 430 W310 X 39 5.7 360 6.54 550

10 330 420 W460 x 61 8.2 330 6.65 530 W610 X 113 11.9 360 8.70 5.65

(6) Bolt test series 12.70 1080 6 375 590 W310 X 39 6.0 300 6.23 460

10 280 450 W460 X 61 8.0 360 7.97 485

"Determined as the ultimate load measured in tensile test on bolt, divided by the cross-sectional area defined by the minor diameter of the thread.

?Determined from tensile coupon tests. *Determined from tests on weld specimens similar to those described in Butler and Kulak (1971).

of failure. For both clipped and unclipped connections, the governing failure mode will be the one resulting in the lowest load capacity for the connection.

Some of the failure modes observed in these tests involve the fundamental components making up the connection, the bolts, the welds, and the web of the connected beam. The resistance of each of these components can be calculated using the resistance equations in CAN3-S 16.1 "Steel structures for buildings - limit states design" (CSA 1984), provided some assumptions are made regarding the distribution of stresses in the connection. If these equations, together with some existing behavioural model (such as the model implicit in the design aid for end plate connections in the Handbook of Steel Construc- tion (CISC 1989)), result in calculated resistances that are com- parable to the load capacities of the connection corresponding to each mode of failure, there would be no need to develop new analytical models specifically to represent the failure modes in end plate connections. Thus, the analysis in this paper is restricted to an examination of whether the resistance equations available in S16.1 and the assumptions implicit in the design aid for end plate connections in the Handbook of Steel Construction can be used to obtain an estimate of the resistance associated with each of the connection components and to explain the changes in the mode of failure and the reduction in the shear capacity of a connection, which are apparently caused by clipping the end plate.

The resistance equations for each of the components of the test connections are given in Sect. 13 of CAN3-S16.1. Clauses 13.10(c) and 13.11.2 provide the limiting conditions for the bearing resistance of the end plate and the shear resistance of the bolt group component. Clause 13.13.1 provides two limiting conditions for shear on the fillet weld component in the connection; the shear resistance of a beam web is given in clause 13.4.1.

Design aids to assist in the design and detailing of end plate beam connections in accordance with these clauses are available in the Handbook of Steel Construction (CISC 1989). Cer- tain assumptions are embodied in the quantities tabulated in these design aids. For example, the "minimum required thickness of supported beam" listed in the design aid appears to be based on the assumption that a uniform shear stress of 0.66Fy is distributed over an area equal to the product of the web

thickness and the depth of beam web welded to the end plate. The remainder of the beam web above and below the end plate is assumed to carry no shear stress.

To be able to compare directly the values calculated using the equations in CAN3-S 16.1 and the test values, the resistance factors must be set equal to 1. Thus, the equations governing the shear resistance of the components of an end plate connection become (i) for plate bearing,

(ii) for bolt shear,

(iii) for weld strength,

[3a] v = 0.67FyA, (base metal)

[3bl v = 0.67XuA, (weld metal)

(iv) for web shear,

The measured weld sizes and web thicknesses, and the measured material properties of the bolts, the end plates, and the beam, were used with equations [ I ] - [4] to calculate the resistances associated with bearing failure of the end plate, bolt shear failure, weld shear failure, and web shear failure. These measured quantities are given in Table 4. The calculated resistances using these quantities and equations [I] - [4], in conjunction with the assumptions which the authors understand to be embodied in the design aid for end plate connections in the Handbook of Steel Construction, are given in Table 5 for the geometry series and Table 6 for the bolt series. No values are given for the fifth and most common mode of failure in these tests, because there is currently no method available for calculating the resistance associated with a web-tear failure.

It is evident that the failure load of the connection should be less than, or for the governing failure mode equal to, the calculated resistance for each failure mode for each connection listed in Tables 5 and 6. Ratios of the failure load of the connection to the calculated resistance for each mode of failure

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TABLE 5. Failure loads and calculated resistances - geometry series

Calculated resistance of connection component

Plate Bolt Base Weld Web Failure load of bearing shear metal metal shear

Connection connection (kN) (kN) (kN) (kN) (kN) (kN)

TABLE 6. Failure loads and calculated resistances - bolt series

Calculated resistance of connection component

Plate Bolt Base Weld Web Failure load of bearing shear metal metal shear

Connection connection (kN) (kN) (kN) (kN) (kN) (kN)

C-12-4-10-101-460R 594 1186 576 912 1113 571* U-12-4-10-101-460R 693 1356 658 912 1113 571*

*Web shear resistances are calculated ignoring the contribution of reinforcing plates.

have been tabulated in Tables 7 and 8. Ratios with values less values, the amount by which the ratio exceeds 1.0 indicates the than 1 have not been entered, as these simply indicate that the extent by which the load capacity of a component of the con- connection did not achieve a load as large as the calculated nection exceeds the calculated resistance of that component. In resistance for that failure mode because of the prior occur- those cases where the failure mode under consideration is not rence of some other form of failure, and thus shed no light on the one that defined the failure of the connection, the amount the validity of the calculated resistances. by which the ratio exceeds 1.0 provides a lower bound esti-

The ratios of connection failure load to the calculated resis- mate of the same indicator. tance for a specific failure mode are underlined in Tables 7 and Considering first the plate bearing failure mode, one can see 8 whenever that failure mode was identified as the cause of the from Table 7 that in the one instance where the capacity of the connection failure in a particular test. For these underlined connections was governed by this failure mode, the maximum

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TABLE 7. Failure load of connection/calculated resistance for specific failure mode - geometry series

Failure load/calculated resistance for specific failure mode

Plate Bolt Weld Connection bearing shear shear

Web shear

Observed mode of failure

Web-tear Web shear

Plate bearing Web-tear

Web-tear Web-tear

Web-tear Weld shear

Web-tear Weld shear

TABLE 8. Failure load of connection/calculated resistance for specific failure mode - bolt series

Failure load/calculated resistance for specific failure mode

Plate Bolt Weld Web Observed mode Connection bearing shear shear shear of failure

C-12-2-6-101-3 10 - 1.07 - 1.45 Lone bolt shear U-12-2-6-101-3 10 - - - 1.74 Web-tear

C-12-2-10-101-310 - 1.07 - 1.45 Lone bolt shear U-12-2-10-101-310 - - 1.67 Web-tear U-12-2-10-101-310R - 1.01 - 1.83* Bolt group shear

C-12-3-10-101-310 - 1.04 - 1.57 Web-tear U-12-3-10-101-3 10R - - 1.13 1.04 2.03" Bolt group shear

C-12-3-6-101-460R - - 1.12 - 1.07' Bolt group shear U-12-3-6- 10 1 -460R - 1.09 - 1.26" Bolt group shear

C-12-3-10-101-460R - 1.06 - 1.02* Bolt group shear U-12-3-10-101-460R - 1.11 - 1.28" Bolt group shear

C-12-4-10-101-460R - 1.03 - 1.04* Bolt group shear U-12-4-10-101-460R 1.05 - 1.21 * Bolt group shear

*True ratio is less than the value shown, as effects of reinforcing plates are ignored in the calculation of web shear resistance.

load carried by the connection exceeded the calculated resistance by 18 %. The other entries for the plate bearing mode of failure show values for the ratio of connection failure load to calculated plate bearing resistance which varied from 1.05 to 1.13. Thus it can be concluded that the assumption that each fastener carries an equal share of the load and the application of equation [ l ] leads to calculated plate bearing resistances that underestimate the plate bearing capacity by at least 5 % - 13 %, with a probable maximum of 18 % .

Examination of the ratios for the bolt shear failure mode indicates that in the ten cases in Table 8 where failure of the bolts governed the capacity of the connection, the assumption that each fastener carries an equal share of the load and the application of equation [2] leads to calculated resistances of the bolt component of the connection that underestimate the bolt shear capacity by amounts varying from 1 % to 13 % , with an average and a standard deviation of 7% and 4% respec- tively. Thus, it can be concluded that this procedure for cal-

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828 CAN. I . CIV. ENG . VOL. 18, 1991

culating the resistance of the bolt component of an end plate connection underestimates the bolt capacity by approximately 7 % .

In the two cases in Table 7 where the capacity of the connection was governed by a failure through the weld, the maximum load carried by the connection exceeded the weld resistance calculated by the application of equation [3] by 8 % and 15 % . The other entries for the weld shear failure mode show values for the ratio of connection failure load to calculated weld resistance which varied from 1.01 to 1.05. Thus, it can be concluded that the procedures for calculating the resistance of the welds in an end plate connection underestimate the capacity of the welds by at least 1 % -5%, with a probable maximum of 15%.

It will be recalled that seven specimens in the bolt series were reinforced specifically to prevent the occurrence of a web-tear failure. Thus, there were 22 tests in which a web-tear failure was possible in the two series of tests. Inspection of Tables 7 and 8 shows that the ultimate strength of the connection was governed by a web-tear failure in 15 cases and by a shear failure of the web in only one case. Indeed, it can be questioned whether the failure of connection U- 19-3- 10- 10 1 - 3 10, which was designated a web shear failure, is in fact a distinctly different phenomenon from the web-tear failure. The onset of failure in each case was marked by the formation of a horizontal tear in the web of the beam just below the end plate. In every case except connection U- 19-3- 10- 10 1-3 10, this crack extended horizontally into the web as the load was increased. For connection U- 19-3- 10- 10 1-3 10, the application of more load caused the tear to turn through 90" and to travel more or less vertically up the web. It was in recognition of this difference in the path of the web tearing that the failure mode was recorded as a web shear failure. However, as both the web-tear failure and the web shear failure began with the formation of a horizontal tear in the web, the web shear failure has been grouped with the web-tear failures in the following analysis.

There exists, at present, no analytical procedure in S16.1 for calculating the resistance of a beam web other than the procedures associated with the application of equation [4]. The ratio of the failure load to the calculated web shear resistance using these procedures is recorded for each connection in Tables 7 and 8. For the 16 connections that exhibited some form of web failure, the ratios varied from 1.37 to 1.90, with an average of 1.63. That is, in these 16 tests, the capacity of the web exceeded the calculated resistance by an average of 63 % , with a standard deviation of 13%. It should also be noted that for four of the five pairs of connections in which a web failure occurred in both the clipped and the unclipped connection, the ratio of the failure load to the calculated resistance was smaller for the clipped connection than for the unclipped connection.

The large spread in the ratios of web capacity to calculated web resistance, the undesirably large amounts by which the capacity exceeded the calculated shear resistance of the web, and the inability of the analytical procedures for calculating this shear resistance to differentiate between webs connected to clipped and unclipped end plates suggest that web-tearing should be treated as a distinct failure mode. It is not surpris- ing, therefore, that the equations in S16.1 for web shear resistance in conjunction with the assumptions adopted from those embodied in the design aid for end plate connections in the Handbook of Steel Construction do not give a reliable estimate of the shear resistance of an end plateconnection when that

resistance is limited by tearing of the web of the supported beam.

Clearly, there is a need for a failure model that would explain this common type of failure and serve as the basis for resistance equations for the web component of these connections. Preliminary analyses of stress distributions in finite element models of some of these test connections suggest that the state of stress in the web just prior to the formation of the web tear could be represented by a block shear model based on the concept originally proposed by Birkemoe and Gilmor (1978). Details of the model will be reported when the additional analysis to permit this block shear model to be extended to the full range of connections for which experimental data are available is completed.

Conclusions Two series of full-scale static tests were conducted on 29

shear end plate connections. The objective of the tests was to study the effect of clipping the end plate on the shear capacity of the connection and to evaluate the applicability of existing resistance equations and behavioural models to this type of connection. The following conclusions were drawn:

1. The shear capacity of a conventional end plate connection is reduced when the upper corner of the end plate is clipped.

2. The reduction in shear capacity is directly proportional to the ratio of bolts in the connections when the failure occurs through the bolt group. However, the reduction in capacity is neither directly related to nor as severe as would be indicated by the ratio of bolts in the connections when the failure does not occur through the bolt group.

3. The eccentricity inherent in a clipped end plate causes noticeable secondary forces, due to rotation of the end plate, only in the bolts of connections with shallow end plates. How- ever, even for these connections, the additional rotational forces are insignificant relative to the vertical shear forces. Therefore, in a clipped end plate connection, it is acceptable to design the bolt group component as if the total vertical shear load were equally shared by each bolt, and the effect of the eccentricity in the end plate may be ignored.

4. It is appropriate to use the resistance equation for bolts in S16.1 to determine the shear capacity of the bolt group component of an end plate connection, whether clipped or unclipped. In the tests conducted in this investigation, these equations (with resistance factors set equal to 1) resulted in shear resistances for the bolt groups that were approximately 7 % smaller than the experimentally determined shear capacities.

5. It is appropriate to use the equation for bearing resistance in S16.1 to determine the resistance in bearing of the end plate in an end plate connection. In the one test in which a plate bearing failure occurred, the resistance equation (with resistance factors set equal to 1) resulted in a calculated resistance for the plate that was approximately 18% smaller than that determined from the test.

6. It is appropriate to use the formulae in S16.1 for the resistance of fillet welds to determine the resistance of this component of an end plate connection. In the two tests in which a weld failure occurred, the resistance equations (with resistance factors set equal to 1) resulted in calculated resistances for the welds that were 8 % and 15 % smaller than those determined from the tests.

7. The predominant failure mode for end plate connections that did not fail through the bolts was tearing of the web at the bottom of the end plate, combined with yielding of the beam

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VAN DALEN AN ID VAN DALEN 829

web adjacent to the end plate. The results of this study indicate that the web-tear failure mechanism may be similar to the block shear concept originally proposed by Birkemoe and Gilmor (1978). The equation in S 16.1 for the shear resistance of a beam web, in conjunction with the assumption that the shear area is equal to the product of the web thickness and the depth of beam web welded to the end plate, results in calculated shear resistances that are much less than the loads at which web-tear failures occurred. In the 16 tests in this study that exhibited web failure, the ratio of the capacity of the web to its calculated shear resistance varied from 1.37 to 1.90, with an average of 1.63.

The calculation of web shear resistances is independent of whether the end plate is clipped or unclipped. The test results show, however, that a web-tearing failure occurs in a clipped connection at approximately 94% of the load at which a web- tearing failure occurs in the corresponding unclipped connection.

Guidelines for the calculation of the resistance of the web to web-tearing in clipped and unclipped end plate connections must be developed, to enable designers to obtain a reliable estimate of the resistance of this component of an end plate connection.

The resistance equations in S16.1 in conjunction with the assumptions embodied in the design aid for end plate connections in the Handbook of Steel Construction are satisfactory for evaluating the bearing resistance of the end plate, the shear resistance of the bolt group, and the resistance of the fillet welds for both clipped and unclipped end plate connections. However, the resistance equations for web shear in S16.1, in conjunction with the assumptions embodied in the design aid for end plate connections, result in calculated resistances for both clipped and conventional end plate connections that are significantly smaller than the capacity of the connection when its capacity is limited by a web-tear failure. A failure model that explains the occurrence of web-tearing in clipped and unclipped end plate connections must be developed to serve as the basis for a resistance statement for the resistance of the web of an end plate connection when its capacity is limited by this mode of failure. Finite element analyses of some of the connections described in this paper have been conducted with the object of developing a simple model to explain the web- tearing phenomenon. Preliminary indications are that the state of stress in the web immediately before the onset of a web- tearing failure may be represented by some variation of the block shear concept developed for bolted double angle connections. The work is continuing with nonlinear, nonelastic finite element analyses being performed on the full range of connection geometries for which experimental data are available.

Acknowledgments

The work described in this paper was supported through a Steel Structures Education Foundation grant, and by the Natural Sciences and Engineering Research Council of Canada. This financial support and the helpful advice from those at the Canadian Institute of Steel Construction are greatly appre- ciated.

BIRKEMOE, P. C., and GILMOR, M. L. 1978. Behaviour of bearing critical double-angle beam connections. Engineering Journal of the American Institute of Steel Construction, 15(4): 109-1 15.

BUTLER, L. J., and KULAK, G. L. 197 1. Strength of fillet welds as a function of direction of load. American Welding Society Journal Supplement, 50(5): 23 1s -234s.

CISC. 1989. Handbook of steel construction. Canadian Institute of Steel Construction. Willowdale, Ont.

CSA. 1984. Steel structures for buildings - limit states design. National Standard of Canada CAN3-S16.1-M84, Canadian Stan- dards Association, Rexdale, Ont.

NOWACK, R. P. 1987. The shear behaviour of clipped end plate connections. M.Sc. thesis, Department of Civil Engineering, Queen's University, Kingston, Ont.

VAN DALEN, K., and MACINTYRE, J. R. 1988. The rotational behaviour of clipped end plate connections. Canadian Journal of Civil Engineering, 15(1): 117- 126.

VAN DALEN, M. 1988. The shear capacity of end plate connections. M.Sc. thesis, Department of Civil Engineering, Queen's Univer- sity, Kingston, Ont.

List of symbols

cross-sectional area of one bolt based on nominal diameter area of fusion face between base metal and the leg of a fillet weld shear area of beam web; effective throat area of weld factored bearing resistance of end plate ratio of shear strength of a clipped connection to shear strength of a similar unclipped connection diameter of bolt; overall depth of a beam section end distance from the centre of the end bolt to the nearest end of the plate specified minimum tensile strength; ultimate tensile strength specified minimum yield strength; tensile yield strength number of shear planes in a bolted joint number of bolts in a connection thickness of end plate factored shear resistance of a member or component ultimate strength as rated by the Electrode Classifica- tion number

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An experimental study of end plate shear connections

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