Bull Moose Tube HSS Connections Manual

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All information contained herein is accurate as known at the time of publication. Bull Moose Tube reserves the right to change product specifications without notice and without incurring obligation.8/99

1Foreword................................................................................................................... ..2Introduction ............................................................................................................... ..3

Framed Connections - Bolted Type Framing ..............................................4Simple Shear Connections - HSS Column to Wide Flange Connection .................. ..5

Welds in the Center of the HSS.................................................................6Shear Tab................................................................................................7Design Procedure - Shear Tab to HSS ....................................................10The Single Angle....................................................................................13Slotted through plate ..............................................................................14Welds Located near Sidewall of the HSS .................................................14Double Angle Connections......................................................................15The Simple Tee......................................................................................16Beam Seats ...........................................................................................17ATLSS Connector ..................................................................................17

Simple Shear Connections - HSS Beams to HSS Columns..................................... 18The Double Tee Connection....................................................................18The Double Angle Connection.................................................................18Separated Double Angle Connection .......................................................19

Moment Connections - HSS to wide-flange.............................................................. 20Continuous Beams20Through Plates..21Strap Angles ..........................................................................................21Flange Diaphragms..22Column Face Reinforcement..23Moment Connection by Reinforcing the Beam Flanges .............................25

Moment Connections - HSS to HSS......................................................................... 26Stepped Connections with b/D < 0.85 ......................................................28Connections with b/D > 0.8 .....................................................................28

Reinforced Tube Connections .................................................................................. 30Plate Stiffener30Reinforcement with Haunches30Blind Fasteners ......................................................................................31

Conclusions .............................................................................................................. 32References ............................................................................................33

2Foreword

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 engineeringprinciples and is for general information only. While it is believed to be accurate, this information should not beused or relied upon for any specific application without competent professional examination and verification ofits accuracy, suitability, and applicability by a licensed professional engineer, designer, or architect. Thepublication of the material contained herein is not intended as a representation or warranty on the part of BullMoose Tube Company, that this information is suitable for any general or particular use or of freedom frominfringement of any patent or patents. Anyone making use of this information assumes all liability arising fromsuch use.

Caution must be exercised when relying upon other specifications and codes developed by other bodies andincorporated by reference herein since such material may be modified or amended from time to timesubsequent to the publishing of this edition. Bull Moose Tube Company bears no responsibility for suchmaterial other than to refer to it and incorporate it by reference at the time of the initial publication of this edition.

3Introduction

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.

4Framed 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, M2, 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

5Simple 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)

6Except 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

7Shear 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

8The 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.

tHSS ttab tbolts

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 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.

9Figure 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:

( ) ( )inchesplddLtwtwte

3.135.108.0

=

10

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

yFtb 253