Butterworth - Finite element analysis of Structural Steelwork Beam to Column Bolted Connections.pdf - MES(1) - Sygnity

Finite Element Analysis

of Structural Steelwork

Beam to Column Bolted

Connections

Jim Butterworth

Constructional Research Unit,

School of Science & Technology,

University of Teesside, UK.

Abstract

A combination of simple fabrication techniques and speedy site erection have made bolted

endplates one of the most popular methods of connecting members in structural steelwork

frames. Although simple in their use bolted endplates are extremely complex in their analysis

and behaviour. In 1995 the Steel Construction Institute (SCI) and the British Constructional

Steelwork Association (BCSA) jointly published a design guide for moment resisting

connections [1]. The Green Book design method offers increased connection capacity using

a combination of theoretical overstress in the beam compression zone and plastic bolt force

distribution. This paper reports on a PhD research program at the University of Teesside

which uses a combination of full scale testing and materially non-linear three dimensional

finite element analyses (FEA) in order to investigate extended end plate beam-to-column

connections. The FEA analyses, incorporating MYSTRO and LUSAS software [2], use

enhanced strain solid and contact gap elements to model the connection behaviour.

Introduction

An extended end plate connection consists of a plate welded in the fabrication shop to the

end of the steel beam as shown in Figure 1. The end plate is pre-drilled and then bolted at

site through corresponding holes in the column flange. The plate extends above the tension

flange in order to increase the lever arm of the bolt group and subsequently the load carrying

capacity. The connection is usually loaded by a combination of vertical shear force, axial

CS502 - Issue 1

force in the beam member and a moment as shown in the diagram of an elevation on a

beam-to-column joint in Figure 2.

Figure 1 - Extended End Plate Connection

Figure 2 - Connection Loading

Accurate analysis of the connection is difficult due to the number of connection components

and their inherit non-linear behaviour. The bolts, welds, beam and column sections,

connection geometry and the end plate itself can all have a significant effect on connection

performance. Any one of these can cause connection failure and some interact. The most

accurate method of analysis is of course to fabricate full scale connections and test these to

destruction. Unfortunately this is time consuming, expensive to undertake and has the

disadvantage of only recording strain readings at pre-defined gauge locations on the test

connection. A three dimensional materially non-linear finite element analysis approach has

therefore been developed as an alternative method of connection appraisal.

Connection Design Theory

Despite numerous years of extensive research [3], particular in the 1970’s, no fully agreed

design method exists. Many areas of connection behaviour still require investigation. More

recently Bose, Sarkar and Bahrami [4] used FEA to produce moment rotation curves,

Bose, Youngson and Wang [5] reported on 18 full scale tests to compare moment

resistance, rotational stiffness and capacity. The latest design method utilises plastic bolt

force distribution to create an increased moment connection capacity and reduced column

stiffening. In 1995 when the SCI and the BCSA produced the Green Book guide, based on

the EC3 [6] design model, the editorial committee felt a number of areas, particularly bolt

force distribution and compression flange overstress required further investigation. The

authors PhD research program is currently nearing completion at the University of Teesside

and is part of this investigation. The research has been undertaken with the financial

assistance of the SCI.

Full Scale Tests

A series of five full scale tests were completed using the self straining frame in the Heavy

Structures Laboratory at the University of Teesside. The basic arrangement of the testing rig

and a test connection can be seen in situ in Figure 3.

Figure 3 - Elevation on the Testing Frame

The beam to column joint was bolted into the frame and tested in an inverted position.

Loading on the connection was provided with a 20 tonne jack situated on the top of the

straining frame and activated by hand from a pull ram positioned on the laboratory floor. The

jack was connected to the beam with a 25mm dia. high tensile steel bar and shackle

arrangement. The shackle arrangement allowed adequate rotation to be obtained to ensure a

truly vertical pull was always applied. To measure the force a load cell is placed between the

jack and a steel block positioned on the top of the straining frame. The load cell is then

connected to the statimeter. All connections used M20 grade 8.8 bolts which were torqued

up to 110 Nm. 110 Nm is considered to represent a typical tightening force obtained using a

steelwork erectors podger spanner. Test E1 used a lever arm of 1000mm, unfortunately this

was found to be too small to produce failure with the loading equipment available. Therefore

in subsequent tests the lever arm was increased to 1900mm. Connection details and

dimensions were taken from ref. [1] with the exception of Test E2 which used an end plate

thickness of 15mm. The Green Book recommended an end plate thickness of 20mm.

All tests used the same arrangement for the location of the strain and dial gauges. Three

strain gauges were applied to both beam compression and tension flanges. Six gauges were

applied to the beam web, local to the tension and compression areas. Dial gauges were

situated under the tension flange to measure rotation. The bolt strains were measured by

bonding Kyowa type KFG-3-120-C20-11 gauges into the bolts. The 11 mm long circular

strain gauges were inserted into a 2 mm dia. hole drilled into the centre of each bolt head.

The bolts were previously all individually calibrated in a specially fabricated bolt testing

assembly to obtain a bolt force to strain calibration factor. Strain readings were taken by

connection of the gauges to two Vishay portable strain indicators and readings at each load

increment noted.

The arrangement of test strain/dial gauges are shown in Figure 4. Details of a bolt strain

gauge are shown in Figure 5.

Figure 4 - Strain / Dial gauge locations

Figure 5 - Strain gauge in situ detail and enlarged detail of gauge

Details of the five section sizes tested are given in the following table:

Test

Ref

Beam Size

(GR355)

Column Size

(GR355)

Column

Stiffene

End Plate

WxThkxL

No. of M20

Gr 8.8

Bolts

Beam Welds

Top-Web-

Btm

356 x 127 x 33UB

254UC73

Yes

200 x 20 x 460

12-6-6

356 x 127 x 33UB

254UC73

Yes

200 x 15 x 460

8-6-6

356 x 171 x 51UB

254UC73

Yes

200 x 20 x 460

10-6-6

254 x 146 x 37UB

203UC60

200 x 20 x 370

8-6-6

457 x 191 x 67UB

203UC60

200 x 20 x 570

10-6-6

Table 1 - Full Scale Test Details

Full Scale Test Results

Test E1 had to be unfortunately halted at 215 kNm due to the capacity of the jack. Test E2

failed at 220 kNm when the compression flange buckled. Test E2 at its ultimate load of 220

kNm had a flange stress of 607 N/mm2 when the compression flange buckled. At this time

the flange was overstressed by 70%. The Green Book design allows a compression flange

to be overstressed by 40% with 20% of this apportioned to material strain hardening and

the remaining 20% to dispersal into the beam web. Test E3 failed at a connection moment of

290 kNm due to thread stripping of both the bolts and nuts local to the tension area. At the

time of failure considerable bending of the end plate local to the tension flange was clearly

visible. Tests E4 and E5 both failed as expected due to column flange bending. Table 2

shows details of the test results compared with the Green Book theoretical capacities and

from these the relevant safety factors.

Test

Ref

Green

Book

Capacity

Test

Failure

Load

Safety Factor /

Green Book

Mode of Test Failure

160 kN

215 kN

1.34

Test halted at 215 kNm due to jack

capacity

159 kN

220 kN

1.38

Beam compression flange buckled

222 kN

290 kN

1.31

Upper rows bolt failure (thread stripping)

101 kN

135 kN

1.34

Column flange bending

147 kN

253 kN

1.72

Column flange bending

Table 2 - Full Scale Test Results

Finite Element Model

MYSTRO and LUSAS FEA software was used for the finite element analysis. The FEA

models were created using command files rather than the CAD interface tools even though

this method was longer and initially tedious. The command file could simply be copied and

edited. The command file also was more logical in order than command files produced by

the software after a model has been created. The command file was also well described by

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