Bull Moose Tube HSS Connections Manual
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8/99
Foreword................................................................................................................... ..2 Introduction ............................................................................................................... ..3 Framed Connections - Bolted Type Framing ..............................................4 Simple Shear Connections - HSS Column to Wide Flange Connection .................. ..5 Welds in the Center of the HSS.................................................................6 Shear Tab................................................................................................7 Design Procedure - Shear Tab to HSS ....................................................10 The Single Angle....................................................................................13 Slotted through plate ..............................................................................14 Welds Located near Sidewall of the HSS .................................................14 Double Angle Connections......................................................................15 The Simple Tee......................................................................................16 Beam Seats ...........................................................................................17 ATLSS Connector ..................................................................................17 Simple Shear Connections - HSS Beams to HSS Columns..................................... 18 The Double Tee Connection....................................................................18 The Double Angle Connection.................................................................18 Separated Double Angle Connection .......................................................19 Moment Connections - HSS to wide-flange.............................................................. 20 Continuous Beams……………………………………………………………………20 Through Plates………………………………………………………………………..21 Strap Angles ..........................................................................................21 Flange Diaphragms…………………………………………………………………..22 Column Face Reinforcement………………………………………………………..23 Moment Connection by Reinforcing the Beam Flanges .............................25 Moment Connections - HSS to HSS......................................................................... 26 Stepped Connections with b/D < 0.85 ......................................................28 Connections with b/D > 0.8 .....................................................................28 Reinforced Tube Connections .................................................................................. 30 Plate Stiffener…………………………………………………………………………30 Reinforcement with Haunches………………………………………………………30 Blind Fasteners ......................................................................................31 Conclusions .............................................................................................................. 32 References ............................................................................................33
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Foreword A comprehensive discussion of Hollow Structural Section (HSS) connections is beyond the scope of this booklet. However, a considerable body of design criteria does exist but much of it is scattered in pieces of literature and was not readily available to engineers at the time of its publication. After the initial publication of this booklet, Bull Moose Tube Company, in cooperation with the American Institute of Steel Construction. Inc. (AISC), the Steel Tube Institute of North America (STI), and the American Iron and Steel Institute (AISI) collaborated on a design book titled “Hollow Structural Sections - Connections Manual”, which is published by the AISC. Copies are readily available through AISC. This booklet is limited to rectangular and square HSS and does not include extensive research and criteria that is available for circular HSS. Even with rectangular HSS, most of the information concerns various types of truss connections, where axially loaded branch members are directly welded to tubular chords or main members. This guide is further limited to the case where the connection is intended to transmit moment and shear rather than an axial force. The information presented in this publication has been prepared in accordance with recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, designer, or architect. The publication of the material contained herein is not intended as a representation or warranty on the part of Bull Moose Tube Company, that this information is suitable for any general or particular use or of freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use. Caution must be exercised when relying upon other specifications and codes developed by other bodies and incorporated by reference herein since such material may be modified or amended from time to time subsequent to the publishing of this edition. Bull Moose Tube Company bears no responsibility for such material other than to refer to it and incorporate it by reference at the time of the initial publication of this edition.
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Introduction With superior compression capacities and natural aesthetic appeal, Hollow Structural Sections (HSS) are an excellent choice for columns, trusses and building frame systems. Because of these attributes, the use of HSS continues to increase in the U.S. and the rest of the world. However, designers and fabricators who have not worked with HSS still express uncertainties regarding connections to HSS. With this in mind, Bull Moose Tube recognizes that connections to HSS require particular design consideration to achieve construction efficiency and cost minimization. The connection of beams to HSS columns can be grouped into two general categories. One is the direct connection where the beam is welded to one of the column faces. This is often an HSS-to-HSS connection, although wide flange beams or other shapes could be welded to a column face. The other category is where connecting elements are used allowing for easy bolting of framing elements at once at the job site. This is by far the most economical method of connecting HSS as well as open shapes. In fact, the standard bolted connections that work so well for open profiles are often suitable for HSS. For simple connections, those requiring only shear resistance, HSS offer the same types of connecting elements as wide flange members. In fact, the load tables in the LRFD Simple Shear Connections should be used in the same manner for HSS columns as it is for wide flange beams. This is why the cost of simple connections for HSS is generally the same as wide flange members.
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Framed Connections - Bolted Type Framing Welding tube - to - tube connections is difficult and expensive in the field. Therefore, it is important to have framed connections similar to those used with wide flange columns to facilitate field erection. The calculation methods used for bolted connections between hollow sections are basically no different than those used for any other types of connections in conventional steel construction. The closed profile does, however, in some cases lead to a special approach to the calculation process. For simply supported behavior to be achieved, connections must provide a certain degree of flexibility in order to accommodate beam end rotations as a beam deflects under load. A full moment connection, which prevents beam end rotation relative to the column, implies an increased moment transfer to the column with increasing connection stiffness. However, most of the connections that will be examined in this section are considered hinged or very nearly hinged. There are almost invariably two alternatives for hinged type connections obtained as follows. The hinge is located on the axis "a" and a suitable arrangement of bolts gives a slight fixity of the joint upon the beam with the resulting moment, M 2, acting on the latter. The hinge is located on the "b" axis (a single bolt for instance) and it is the column that reacts the moment, M2. Figure 1 shows the basic arrangement of the conventional type of connection indicating the possible loading conditions and appropriate notation
Q: Vertical reaction at support point H: Horizontal reaction M1: Bending moment transmitted by the beam M2: Bending moment due to the eccentricity of the attachment: M2 = Q.e Figure 1 - Load Conditions for Simple Connections
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Simple Shear Connections - HSS Column to Wide Flange Connectioni A major consideration in the design of this type of framing system is the field connection between the beams and columns. Normally a simple connection is shop welded to the beam or column and field bolted once at the job site. The most economical method is to use a simple framing connection that transmits the beam shear with a minimal moment. A satisfactory simple framing connection of an open profile beam to a hollow section column should consider the following: 1) Adequate shear strength to carry the beam reaction 2) Enough flexibility so that the bending moment transmitted to the column will be minimal 3) The ability to carry any small moment without failure of the connection or connectors. 4) A connection configuration that does not cause excessive deformation of the column wall 5) A reasonably clean cost effective detail for fabricators Common practice for simple field connections is to shop weld connecting elements to the beam or column and complete the connection in the field with bolting. A variety of standard simple framing connections have been developed over the years for wide flange columns but most of them have been adopted for HSS columns as well. These include: Double angles Tees Single angles Angle beam seats Shear tabs or wing plates End plates ATLSS connector (self guiding)
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Except for the beam seat, the connecting element is attached to the web of the beam. However, these connections can be used with tubular beams if a structural tee end cap is welded to the beam so that the stem functions in a similar manner as the web of a wide-flange. These connections must be designed to reduce any secondary loads to the minimum in particular by bringing the center lines of the chord and lattice members to meet at the same point. It is advisable, in the case of main structural components, to arrange the web members symmetrically in relation to the median plane of these components and to attach them in a symmetrical manner. Welds in the Center of the HSS The two types of connections that have welds near the center of the face of the HSS the shear tab and the single angle, which will be discussed in detail in the next several pages. The corresponding weld pattern is shown in Fig. 2
Figure 2 - Shear Tab and Single Angle Welds
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Shear Tab One of the most efficient and economical methods of connecting a beam to a tubular column is the shear tab. Recent research by Dr. Donald Sherman at the University of Milwaukee, Wisconsin quite clearly indicates that the shear tab is a safe, economical means for connecting a beam to a tubular column. In fact, this research indicates that through-plating is often an unnecessary connection that can be avoided if certain criteria are considered. The primary purpose of Sherman's research was to develop design guidelines for shear tabs on HSS columns and to establish the limit states of such connections. The conclusion of this study is that the shear tab did not reduce the column capacity in comparison to the through plate and that bolt tightening had more of an impact than the connection type. This conclusion is shown in Graph 1.
Graph 1 – Shear Tab Column Test
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The study of any connection begins with the identification of the critical failure modes encountered under extreme loading. These have been identified1 as 1) shear failure of bolts 2) yielding of gross plate area 3) fracture of the net plate area and 4) bearing failure of the beam web set. To avoid shear failure of the bolts, a relationship for bolt diameter and tab thickness has been established and is shown below:
t tab ≤ dBolt/2 + 1/16
Also, the tab thickness is limited to 9/16" or less. Taking this into consideration, there are certain combinations of HSS thickness, tab thickness and common bolt diameters that should be used. These are summarized in Table 1, below.
t HSS
t tab
3/16
≤ 5/16
3/4, 7/8, 1
1/4
≤ 7/16
3/4, 7/8, 1
5/16, 1/2, 5/8
≤ 7/16
3/4, 7/8, 1
≤ 1/2
t bolts
7/8, 1
≤ 9/16
1
Table 1 - Combinations of Shear Tab Connection Parameters
The other consideration when analyzing failure of the HSS column connection is the additional limit state introduced by the flexible tube wall. The tube wall in this instance may experience a bending failure caused by yield line development or punching shear failure. Figure 3 illustrates the yield line mechanism. However, because the depth of the shear tab is much larger than its thickness, high strains are likely to develop at the edge of the plate producing a localized failure, such as the plate pulling out or punching into the tube wall. This will occur before a sufficient number of yield lines develop and cause failure.
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Figure 3 - Yield Line Failure
Figure 4 - Punching Shear Failure
Figure 4 illustrates the punching shear failure. Failure in this instance is defined as the point at which applied load exceeds the shear resistance of the tube wall around the perimeter of the tab. To prevent this failure, thickness of the tube wall must exceed some fraction of the thickness of the tab so that the shear tab yields before the tube wall fractures in shear. The equation for the tube wall thickness is defined below and is used in conjunction with the equation for bolt shear failure to produce Table 1. Fy(tab) t (tab) ≤ 1.2 Fu (HSS) t (HSS) For those connections, which failed during Sherman's study, all experienced a shear yielding of the gross area of the tab. However, all of the connections exhibited the possibility of multiple failure modes. To remove the possibility of weld failure it is recommended that welding to HSS be done in accordance to AWS section D1.1 that gives procedures to assure good welding practice. Local moments are an important consideration in the design of connecting elements. The moment developed in the connection depends on the reaction eccentricity, which is defined as the distance from the face of the HSS column to the location of zero bending moment. This eccentricity was shown to be less than three inches, the dimension between the weldline and the bolt line, except when the flexible beam was used with the stiffest HSS column face (b/t)= 8. It was demonstrated that the minimum thickness of the column face should be related to the shear tab thickness in order to force yielding of the tab rather than allowing possible punching shear failure of the column face. It is known that reaction eccentricity increases with the number of bolts, the size of the bolts and the thickness of the plate. An empirical equation was then developed for calculating the eccentricity:
t 1.3 tw (L d )1.35 d pl (inches ) e = 0.08 wt
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Based on the results of extensive shear tab research, Sherman has refined the design procedure for shear tabs and has indicated the basic rules as follows:
General Requirements (Shear Tabs) - If the shear tab is at the end of a long unbraced length, a brace point should be established near the shear tab. - The area of the tube wall to receive the weld should be free from mill scale and some preheat should be applied before welding. - The welds are fillet welds along the entire length of both side of the plate and should be terminated just short of the top of the shear tab. - It is left to the designer as to whether to use tubes with high b/t ratios. However, it is recommended that
b t < 253
F
y
- The design is applicable for both fully torqued and snug tight bolts.
Design Procedure - Shear Tab to HSSii 1) Calculate the number of bolts required to resist the shear force. Assume the force acts at the bolt line:
n=
R r v
2) Calculate the maximum thickness of the shear tab allowed to insure yielding of the shear tab before punching shear failure of the tube wall:
t
pl
≤t
1.2 F u tw F
y
tw
pl
3) Calculate the length or thickness of the shear tab needed to resist the shear force:
d
pl
≥
R 0.4 F
y
t pl
pl
10
4) Calculate the reaction eccentricity: a.) Determine "e":
t 1.35 1.3 tw L d e = 0.08 pl (w t ) d b.) If e< 3, the point of eccentricity is between the bolt line and the weld line and:
e = e ' = 2.25 b w c.) If e >3, the point of eccentricity is outside of the bolt line and:
e = (n − 1) − 3 b e
w
=n
5.) Recalculate the capacity of the bolt group, using Table XI of the AISC-ASD Manual:
( )
R =C r b v
where L = e
b
6) Recalculate the length or thickness of the shear tab using the reduction factor, which takes into account the reduction in shear capacity due to the distortion in the tube wall:
d
pl
≥
R ξ 0.4 F t y pl pl
where ξ = 0.80 if e ≤ 3 and b t ≥ 15
= 1.00 for all other cases 7) Check for fracture along the net section:
R
(
)
= d − n d + (1 16) t 0.3F pl ns pl b u pl
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8) Calculate the weld size, in sixteenths, to develop the shear capacity for the tab, using Table XIX from the AISC-ASD Manual:
D=
R
pl
C d pl
where aL = ew. The weld size need not exceed 0.75 t pl 9) Check the bearing capacity of the bolt group:
P = 1.2 F t d n b u pl b pl 10) Check for a block shear failure if the beam is coped
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The Single Angle The single angle shown in Fig. 5 is yet another option for bolting HSS columns to beams. Single angles are simple to shop weld placing the bolts in single shear at the connection. The angle does create an eccentric moment at the connection, which can affect the design of the fastening element. The amount of moment developed in an angle can be approximated as, m =R*e, where "R" is the reaction shear and "e" is the reaction eccentricity. The reaction eccentricity depends on a number of factors, such as the number of bolts, the dimensions and material of the angle. To provide necessary flexibility, there should be no weld at the top or heel of the angle: this applies to angles welded to both HSS and wide flange columns. To prevent overturning of the beam, the distance between the center of the top and bottom connecting bolts should equal or exceed, one half the T-distance of the supported member.iii The simple angel connection is therefore considered more complicated than the tab because of the complications introduced by the eccentricity. However, the wall distortion of the HSS should be less than for the shear tab since some of the force is transmitted to the weld near the side wall. Therefore, a yield line mechanism that may cause failure for a shear tab as shown in Fig. 3 will not develop, and the HSS column strength should not be affected. The basic design procedures for selecting a single angle connection are included in LRFD simple connections Table XII and apply for HSS columns as well.
Figure 5 - Single angle connection
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Slotted through plate The slotted through plate is shown in Fig. 6. The through plate is the stiffest simple connection because of the reduced flexibility of the plate vs. the column wall yet it is also the most expensive to fabricate. On the other hand, through plates minimize column deformation at the connection, and the connection behavior is virtually independent of the cross-sectional dimensions of the column.
Figure 6 - Slotted Through-plate Welds Located near Sidewall of the HSS The double angle, tee and seat connections are attached to the column with vertical welds at the two edges of the connecting elements. This is shown in Fig. 7. If the connecting elements are not as wide as the HSS, the welds are on the flat wall of the section near the corner. On the other hand, if the elements are as wide as the HSS, flare bevel groove welds would be required. For double angles, tees, and beam seats, the width of the element should be slightly less than the column width.
Figure 7 - Welds near Sidewall
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When using connections with side welds the local distortion of the face of the HSS is not a strength consideration and since any end moment in the connection is small, the side wall compression is not a critical factor. The primary additional strength consideration in using these connections is that the welds are in the vicinity of the corners that have been extremely cold worked by the direct-form manufacturing process. This often raises the tensile properties of the steel by more than 50%, offsetting any tensile strength lost by the weld region. The consideration in using these connections with HSS columns is the rotational flexibility. In practice, these connections are considered as simple hinges in the analysis of a structure. When used with wide flange columns, the welds are toward the flexible edges of the flanges that are not directly supported by the web, while in the HSS the sidewalls provide stiffness to the tube face at the welds. Since most of the connection distortion is assumed to take place in the connecting elements, the flexibility should still exist in these elements with HSS columns. In fact, it is not anticipated that either the strength or the flexibility of the connection will be a critical factor or compromised with HSS columns.
Double Angle Connections Quite likely the most suitable connection for general purpose is the traditional double angle arrangement shown in Fig. 8. It provides the strength of bolts in double shear combined with excellent flexibility and, being symmetrical, the connection avoids any lateral torsion. Fabricators can select standard detail angles from stock rather than prepare special components such as tees. Engineers should consult the LRFD Simple Shear Connections manual to make the selection of the appropriate angle based on load conditions and bolt sizes. The connection is completed by shop-welding the two vertical angles to the column with a short return along the top portion of the angle that is 2x the weld width and completed by field bolting to the web of the beam.
Figure 8 - Double Angle
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The Simple Tee Figure 9 shows a connection consisting of a tee section welded along both flange edges of the HSS column. The tee section is offset from the center line, allowing the beam web to be centered onto the column face. iv The beam is then easily field bolted to the tee. This type of connection permits reasonable rotation through distortion of the tee and does not induce high stresses or distortions on the HSS column wall as compared to the shear tab. The basic behavior of this type of connection is somewhat similar to conventional framing angles although it is stiffer because of the continuity of the tee flange across the entire HSS face. A minimum width-to thickness ratio of ten is suggestedv by White and Fang. Again, the primary difference is the location of the weld and the connection eccentricity. Table XI in LRFD manual gives tabulated strengths for tee sections.
Figure 9 - Simple Tee
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Beam Seats Some very common connections for heavier shear loads are the use of beam seats with the option of standard tees and shear plates. The beam seat usually consists of an angle welded to the HSS face transmitting the shear load to the face of the HSS as shown in Fig. 10a. Alternatively, traditional tees and shear plates can be welded to the HSS face with a smaller angle welded across the HSS as shown in Fig. 10b.
Figure 10a - Beam Seat
Figure 10b - Beam Seat for Large Loads
ATLSS Connector A series of new beam- to- column connections, known as ATLSS connections, are currently under development. The emphasis of these new designs is on a self-guiding feature that will minimize human assistance during construction and result in a quicker, safer, less expensive erection procedure. The concept is based on using a trapezoid solid tenon piece on the beam, which slips into a three-dimensional mortice guide. This concept is shown in the photograph below. The ATLSS Connector may prove ideal for HSS since it can be welded flush to the side of a tube. In fact, research is under way to to assess the behavioral characteristics of the ATLSS when it is welded to tubes of varying thickness.
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Simple Shear Connections - HSS Beams to HSS Columns Bolted HSS beam to HSS column connections generally entail the use of detail material such as tees, angles or plates in a manner similar to those shown in Figs. 3 - 9. Figures 11a,b shows a simple connection typical of HSS beam to column connections. Fig 11a illustrates a connection consisting of a tee shop welded horizontally to the column with a short return around the top corners while Fig. 11b is a similar detail utilizing two angles to form a slot. These connections are completed by field bolting the beam to the two vertical angles. This connection, as with any connection involving field bolting, is less costly than a field welded connection.
The Double Tee Connection As shown in Fig. 11a is a means of field bolting this system. The tees are either built up from plates or rolled sections. To maintain proper alignment of the beam and column, the beam tee section can be centered on the column wall. The tee section on the column is shop welded along both flange edges to the column. The tee should not be welded on all four sides to maintain flexibility. A minimum width to thickness ratio of 10 is suggested for the column tee flange. vi
Figure 11a/b The Double Angle Connection This connection shown in Fig. 11b allows the bolts to work in double shear. A minimum b/t ratio of 10 for the tee section and 20 for the column section is suggested by Astenuh (1987).
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Separated Double Angle Connection Figure 12a, below shows a double angle with the beam coped to facilitate bolting. The beam end should be sealed to prevent exposure to a corrosive atmosphere. The column b/t ratio should be greater than 32 and the angles welded close to the edge of the column. Also, plates instead of angles can be used as shown in Fig. 12b allowing for wider beams. The plates are shop welded close to the edges of the HSS column as shown, forming a relatively rigid connection. Another option is to use shear plates welded close to the column side walls as shown in Fig12b. Again, the best method for selecting the correct bolt sizes and angles is to consult the LRFD manual for simple shear connections Table XI.
Figure 12a - Separated Double Angle
Figure 12b - Shear Plates
Plates on the column face as shown in Fig. 13 portray another method of connecting tubes. End plates are welded to face of the HSS and the end of the tube sealing the beam while providing easy installation.
Figure 13 - Plates on the Column Face 19
Moment Connections The basic design criteria for rigid or moment connections are: 1. Sufficient strength 2. Sufficient rotation capacity 3. Adequate stiffness 4. Ease of erection and economical fabrication
Moment Connections - HSS to wide-flange Moment connections are generally more expensive to fabricate than simple connections because of the complicated welds and labor involved. The designer should consider simple framing and use moment connections only when necessary. A number of concepts are used to transmit moments from wide flange beams to rectangular or square HSS columns. They range from in use from continuous beams with column interrupted, to the provision of continuity from beam to beam across the column, to the reinforcing of the column face to accept a beam moment connection. Continuous Beams A continuous beam approach shown in Fig. 14 avoids the task of transmitting moments into the HSS column by running a continuous beam through an interrupted column. Column continuity is provided by reinforcing the wide flange with a split HSS stiffener. This detail presupposes beam flanges that are as wide as the column section., and would not be intended to transmit major moments into the column. The column interruption makes for easy bolting of this connection as end plates are welded to the end of each column section.
Figure 14 - Split HSS Stiffener
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Through Plates A simple arrangement which provides direct moment transfer from beam to beam across a column ( or to a column) is shown in Figs. 15 a,b where the column is interrupted to pass flange plates through it from one beam to another. In this instance, shear tabs can be used for beam webs since beam end rotation ( which limits their use for simple connections) is not a factor. For heavier moments double angle web shear connections may be needed. The corresponding detail for a column top can be seen in Fig. 15b where another column tier can be bolted directly on top of desired. This arrangement can be modified for beams framing from three or four directions, but they all need to be within shimming range of the same depth. Also, the top column can be reduced in size to accommodate lighter loads as building floors increase reducing the need for heavier sections
Figure 15a
Figure 15b
Strap Angles One alternative where there are beams in only one plane at the same depth is to use strap angles which connect the beams to the column faces parallel to the beam is shown in Fig. 16. No weld is used between the beam flange and the face of the column but welds are used to attach the beam web to the strap and the strap to the side wall of the column. The following procedure have been establishedvii to produce good joint behavior: 1) Top angles should be coped with a radius equal to the angle leg minus its thickness 2) Top angles should be longer than bottom angles 3) A short length of the horizontal legs of the top angles should be welded to the beam 4) A Clearance of approximately .5" between column wall and the end of the beam is desirable.
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Naturally, beam flange widths should be equal to or less than column widths. For connections where the beam flange is substantially narrower than column width, it should not be assumed that shear force is taken by the strap angles. A symmetrical web connection must be designed to take the total shear force. Equations for the working strength of strap angles have been developed
viii
and should be followed:
Flange Diaphragms Another method of connecting beam flanges to an HSS column is shown in Fig. 17. A plate diaphragm is fitted around the column for each of the beam flanges, and vertical web plates are located between them. The beams are connected by simple shear connections at the extremities of this assembly where beam contra flexure points are expected.
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This arrangement can be adapted to locations with two, three, or four beams framing at a column. Research conducted by Kato et. al. (1981) and Tabuchi (1988) to determine the design criteria for this arrangement has been established and is included on the illustration. Column Face Reinforcementix The most direct approach for moment- connecting a beam to an HSS column is to reinforce the column face to accept the flange forces from the beam. This approach is shown in Fig. 19. Essentially, a plate diaphragm is fitted around the column for each of the beam flanges and vertical web plates are located between them. The beams are connected by simple shear connections at the extremities of this assembly where beam contra flexure points are expected. This arrangement can be adopted for locations with two, three, or four beams framing at a column. According to research conducted by Dawe and Grondinx to analyze the critical design criteria, four basic modes of failure should be considered (Illustrated in Fig. 20). a.) Beam tension flange to column doubler plate rupture b.) Punching shear of the doubler plate at beam tension flange c.) Web crippling of the column side walls at the beam compression flange d.) Punching shear of the column face along the edge of the doubler plate, either near the beam tension flange or near the beam compression flange.
Figure 19 - Column Face Reinforcement
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Figure 20 - Critical Failure Modes
The conclusion of this research is that the governing failure mode is by failure Mode (a) in which the connection moment resistance can be estimated by:
M* = h F t b r1 b yb b e
Where
10 be is given by = b t 0 0
F y 0 t 0 F t yi i
bi but ≤ bi
t b is the beam flange thickness hb is the height of the beam
Alternatively, the connection moment resistance governed by failure Mode (b) can be estimated by:
M* = h r1 b Where
F
yp
t b 3 p ep
bep is given by [9.10] T p is the reinforcing plate thickness
Failure Mode(c), involving web crippling of the column side walls can be related to plate reinforcement methods and can be estimated by:
M * = 0.5F t h + 5 t + t 2 r1 k 0 b p 0 Where Fk is the buckling stress of the column side walls, and can be taken as .8 Fy. 24
The column side walls are stabilized against buckling when there are beams framing into them with connecting material mounted on the vulnerable area of the walls. Mode (d) failure involves punching shear of the column face at the edges of the doubler plate, either outwards at the beam tension flange or inwards at the compression flange. It is assumed that the connecting moment resistance for this failure mode, assuming uniform punching shear stress all around the reinforcing plate, is :
M * = 0.5 r1
F
t L 2 + 2 L B p p 3 0 p
y0
However, a further potential failure mode could be yield line failure mechanism in the reinforcing plate. This research indicates that the moment capacity of beams could be developed with doubler plates if the failure modes discussed above are kept in mind. Further, it has been demonstrated that 1) stiffness of the connection is enhanced when the doubler is nearly as wide as the flat portion of the column face and , in fact, the maximum connection stiffness would be achieved if the plate width = column width and 2) strength is more a function of the doubler than of the width to thickness ratio of the column, and 3) connections with seat angles distribute the bottom flange loads into the column more effectively than do those with only a flange plate. Moment Connection by Reinforcing the Beam Flanges An interesting alternative to reinforcing the face of an HSS is to broaden the beam flanges with stiffeners to deliver flange forces directly into the column side walls and often completely around the HSS face. One such research study by Tingxi et. al. (1991) concluded that the use of tee stiffeners effectively increased both strength and stiffness of the connection. A variety of arrangements have been proposed and are shown in Figures 21a. and 21b.
Figure 21a
Figure 21b
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Moment Connections - HSS to HSSxii The terminology and notation used to describe tubular joints is important, but not always familiar to those who have not worked with this type of member. Branches are members that frame into continuous main members at a joint. In matched connections branches and main members have the same width while in stepped connections, the branch is narrower. Moment connections for connecting HSS columns to HSS beams involve direct welding and are analyzed by examining the behavior, strength, and flexibility of a Vierendeel type truss where the connection transmits shear and moment with an inverted type "T " position as shown in Fig. 22 (with the appropriate notation.)
Figure 22 - Notation for Tube-to-tube Connections For all practical design purposes the moment capacity of a connection can be determined in a manner similar to axially loaded HSS T connections, whereby the strength is characterized by an ultimate bearing capacity or by a deformation/rotation limit. Again, failure analysis is used to determine connection criteria as shown in Fig 23. (Wardenheir 1982). These assume that neither the welds nor the members themselves are critical.
Figure 23. - HSS to HSS Moment Connection Analysis by Failure
26
Failure modes affected by geometry are distortion or punching shear in the connection face of the main member or crippling of the side walls. For joints with in-plane bending of the branch member, the primary variables affecting the strength are: 1.) The width ratio, b/D 2.) D/T of the main member 3.) The thickness ratio, t/T 4.) The ratio of branch depth to main member width, h/D With this many variables, there is little wonder that design criteria become complicated. However, there has been extensive testing and evaluation of tube-to-tube connections by CIDECT, an international organization concerned with tubular construction. Researchers Konig and Wardenheir (1985) concur that both the strength and flexural rigidity of an unstiffened connection decreases as the chord slenderness ratio D/T increases and the branch to chord width ratio b/D decreases. Complex design criteria for tube joints involving procedures using both equations and monographs have been established by Packer and Henderson and included in their book, Design Guide for Hollow Structural Sections. A summary of their findings are described below: Most HSS Vierendeel connections exhibit some lack of rigidity. Thus, when computing moments and deflections for this type of connection, a knowledge of the level of rigidity in the connection is required. An empirically derived expression that approximates the stiffness of tube- to- tube connections is:
K = 560 × 10 6
(b D )2 (h b ) (D T )3
,
(in − kips
rad )
However, for all practical design purposes, the moment capacity of a connection can be determined in a manner similar to that used for axially loaded HSS T - connections. In fact, section 10 of the AWS Structural Welding Code contains criteria for tube-to- tube connections. These are general criteria for rectangular tubes in a variety of truss configurations. The criteria are based on punching shear stress through wall thickness of the main member. Adjustments have been made in constants to account for the yield line distortion failure mode and provisions for web crippling are included. The criteria for the tube- to - tube connection with in-plane bending can, however, be extracted as follows.
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Stepped Connections with b/D < 0.85 The limiting bending stress in the branch, f b, is given by:
Q Q F t q f y f ≤ ≤ 0.4 F y T b .3(D T ) where
Q = 1 for b D ≤ 0.5 q
and
0.25 Q = for b D > 0.5 q b D (1 − b D ) Q Q
f f
= 1 for f
0.6 F ≤ 0.44 y
a
= 1.2 − .5 f
a
0.6 F
y
for f
a
0.6 F > 0.44 y
with f a being the stress in the main member. Qq is a constant, which provides for increase in the wall strength for wide branches and Qf a constant that decreases the wall strength if the column carries high axial stresses. Basically the criteria limits the bending stress in the branch to prevent a distortion failure of the wall of the main member, which is a function of D/t. An upper limit on the failure is that the shear through the wall cannot exceed the shear yield. Qq provides for an increase in the wall strength for wide branches and Qf provides decreases the wall strength if the column carries high axial stresses.
Connections with b/D > 0.8 For matched connections or stepped connections with b/D > 0.8, AWS provides critical design criteria. Design is governed by the more critical failure mode 1) the reduced branch member capacity (effective width ), and 2) the chord side wall bearing or buckling capacity. The side wall forces are based on the full shear yield of the main member for stepped connections and on web yielding of the main member for matched connections. The criteria is written to apply to an axial force in the branch and it is conservative to use the equation for b/D 0.8 if the value of Qq is limited to 1.56, corresponding to b/D = 0.8. The equations can be rearranged to give limiting ratios of D/T as a function of t/T to develop a given stress level in the branch member. D/T [ Qf Q q
but not to exceed 8.33 Qf Q q for yield
These limiting ratios are plotted in Figure 24 for an allowable stress in the branch equal to 0.6 Fy and Q f for the stress level in the main member equal to one. The lower line represents Q q equal to one and applies to b/D < 0.5. The upper line is the limit when b/D is equal to 0.8 and can be used larger values of the ratio. Higher stress levels in the branch or in the main member would shift the curves to the left, but would not effect the plateaus.
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Korol conducted tests in Canada that showed full moment transfer in matched connections should not be expected unless D/T were less than 16. This appears to follow the trend indicated in Fig. 24. If the limits in Fig. 24 are not met, either the stress level in the branch must be reduced, or the connection must be reinforced.
Figure 24 - Limits of D/t and t/T AWS criteria also permits a yield line analysis in stepped connections with b/D < 0.8. Stockwell derived the ultimate moment based on a yield line analysis of the web of a W column with a beam welded to it. Since the yield line pattern is similar to that which would occur in the face of a tubular column, this equation can also be used for the tube- to -tube connection.
Where
( c + b)hT ( cD + h 2 )T 2 6hT 3 + + M =F u y 12 2c c c=
D−b 2
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Reinforced Tube Connections Plate Stiffener The chord flange stiffener is least obtrusive and most easily method of reinforcing a tube- to- tube connection as shown in Fig. 25. Korolxiiiet. al. (1982) developed a yield line analysis which led to reinforcing plate parameters that result in sufficient strength to resist the bending or axial capacity of the framing to the column. Recommendations by Korol to obtain a full strength connection are: 1. Plate width should be at least equal to the flat width of the HSS face that is taken to be ≥ bo - 4t o. 2. Plate length should be twice the HSS column width, i. e. 2bo. 3. Plate thickness depends on whether axial or bending loads dominate. For full axial compression capacity of the branch, t p ≥ 4 t1 - t o. For full moment capacity of the branch, t p ≥ 0.63(b1t 1)0.5 - t o.
Figure 25 - Plate Stiffener Reinforcement Reinforcement with Haunches Another efficient and aesthetic form of connection reinforcement is to use 45° haunches as shown below. In fact, cuttings from the branch member provide a convenient haunch for either end. Both of the reinforcing methods provide adequate methods of resisting in plane moments. This is shown below in Fig. 26.
Figure 26 - Reinforcement with Haunches
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Blind Fasteners Recently, a new type of blind fastener was developed by Huck International Inc. for use in situations where the rear side of the connection is inaccessible as in the case in connecting to HSS columns. In fact, high strength blind fasteners seem to have a good potential in moment connections as reported in research by Mourad, 1993. The connection between w-shape beam and HSS column using blind fasteners is shown in Fig 27. In general, twist-off blind bolts could be used in all applications where access to one side of the structure is difficult or restricted. Productivity improvements in the order of 2 to 3 times are expected from using the Twistoff blind bolt (TBB) system, and improvements are specially more noticeable for the beam- to tubular column connections where elimination of welding, diaphragms and cumbersome assembly procedures will no longer be needed. The joining process offered by TBB allows the user to either partially assemble on the job site or do the assembly on the job site. Huck expects the TBB fastener to be in full production by the end of 1st quarter 1994.
Figure 27 - Huck International Fastener In addition to the Huck fastener, flow-drilling is another method of producing a blind bolted connection. This process requires a hole being forced through a plate by a carbide conical tool rotating at sufficient speed to produce heating of the metal and softening the material in a local area. The material being displaced forms a truncated hollow cone or bushing on the inner surface and a small upset on the outer surface. A cold - formed tap is then used to roll a thread into the hole without any chips or removal of material. This system has recently been tested and shows potential for use in blind bolting to HSS columns. However, this assumes the availability of high speed drilling equipment by the fabricator.
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Conclusions Hollow structural sections are an excellent choice for engineers and architects because of their natural aesthetic appeal, structural efficiency, and due to the significant cost savings they offer the user. Most important of the savings are in terms of material cost, transportation costs, and when used as exposed structural, the cost of hiding the structural member. In the past, connections to hollow structural sections were thought to be cumbersome and costly. However, they are not as difficult as presumed by many engineers. In fact, there is a large quantity of supporting research that has been done to refine design criteria for HSS. This booklet has drawn upon these sources and is intended to give engineers basic ideas about making simple and moment type connections using HSS.
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References
i
Sherman, D.R. , Framed Connections to HSS Columns," Tubular Structures V, Proceedings of the Fifth International Symposium , Coutie & Davies ed., E&FN Spon, United Kingdom, August 1993. ii Sherman, Donald R. , Ales, Joseph M. , " The Design of Shear Tabs with Tubular Columns," Master of Engineering Project, University of Wisconsin at Milwaukee, 1990 iii Load and Resistance Factor Design of Simple Shear Connections, American Institute of Steel Construction, 1990 iv Hollow Structural Sections, Design Manual for Connections, Stelco, 2nd edition, 1981 v White, R. N. and Fang, P.J. ," Framing Connections for Square Structural Tubing ," Journal of the Structural Division , American Society of Civil Engineers, 92( ST2): 175 - 194 vi White and Fang vii Giroux, Y.M. , and Picard, A., " Rigid Framing Connections for Tubular Columns, " Canadian Structural Engineering Conference, 1976 viii Stelco, Design Manual for Connections, Hollow Structural Sections ix Packer, J.A. ,and Henderson J. E. , Design Guide for Hollow Structural Sections Connections , Canadian Institute of Steel Construction, 1992, p. 246- 250 x Dawe, J.L. and Grondin, G. Y. , "W- Shape Beams to RHS Column Connections," Canadian of Civil Engineering 4(2): p. 134- 144 xi Ting, L.C. Shanmugan, N. E. , and Lee, S.L. , "Box Column to I-Beam Connections with External Stiffeners," Journal of Construction Steel Research, 18, p. 209-226 xii Hertech, A., Sherman, D, " Beam Connections to Rectangular Tubular Columns," AISC Proceedings, 1988 xiii Korol, R. M. , and Brady, F. J., "Unequal width connections of square hollow sections in vierendeel trussess," Canadian Journal of Civil Engineering, 4(2): p. 190-201
* This manual was compiled by the Bull Moose Tube Technical Support Department.
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