Design of Connections

by Toby Mottram

School of Engineering, Warwick University

1st CoSACNet Meeting, Southampton

30/01/01

Len Hollaway, 1993

“Jointing has a special significance

and poses a major challenge

to the engineer”

CONTENTS

1. Introduction

(I) Composites for construction

(II) Connection types

2. Mechanical Connections

3. Design Guidance

(I) ‘Simple’

(II) ‘Rigorous’

CONTENTS

4. Research goals

5. Conclusions

1. Introduction(I) Composites for construction

• Manufacturing Processes

• Contact Moulding (non-structural)

• Filament Winding

• Resin Transfer Moulding (or RIFT, etc)

• PULTRUSION

1. Introduction(I) Composites for construction

• Fiberline Footbridge

• Standard profiles (I, channel, box)

• Cooling Towers• Strongwell

specialist market

1. Introduction(I) Composites for construction

• Creative Pultrusions Inc.

• Bridge decking

• Structural frames

1. Introduction (II) Connection types

• Mechanical Interlocking (Adhesive Bonding and (sometimes) Mechanical Fastening)

• ACCS (Maunsells)• Superdeck (Creative Pultrusions Inc.)

1. Introduction (II) Connection types

• Mechanical fastening (bolts, rivets, screws), with Adhesive Bonding

• bolting (standard connection method)

2. Mechanical Connections

Web cleated (Strongwell)

‘SIMPLE’

1. Bolted and Bonded capacity controlled by shear in heel of angle

2. Bolted capacity controlled by bearing around fastener or shear of stainless steel fasteners

3. Design Guidance

• Europe (limit states)

– 1995 - Design Manual, Fiberline Composites,

Denmark

– 1996 -EUROCOMP Design Code and Handbook

(GRPs)

3. Design Guidance

• America (Allowable stress)

– 1983 EXTREN Design manual, Strongwell

– 1993 BRP Design Guide, Bedford Reinforced

Pultrusions Inc.

– 1999 The New and Improved Pultrex Pultrusion

Design Manual of Pultrex Standard and Custom

Fiber Reinforced Polymer Structural Profiles,

Creative Pultrusions Inc. (CD-ROM)

3. Design Guidance

Warning - Design Manuals

Guidance is different and is often NOT based on ‘rigorous’ physical testing

3. Design Guidance

Why is design of connections difficult?

• Many materials, properties and members

• Many joint types and connection methods

• Lack of material ‘ductility’

• Failure can be sudden and ‘brittle’

• Need for accurate stress and failure analysis

• Lack of knowledge on durability

• Need for physical testing to verify new designs

NEED STANDARDISATION AND CO-OPERATION

3. Design Guidance

Approved design guidance!!

• Can’t have code(s) of practice without practice

• Can’t have well-established practice without codes

• Industry is fragmented and protective

• Academic research perceived to be too scientific

• Cost-competitive structures require durable material; decades to establish

• Copying practice for steel is not ‘optimum’ solution

• Need to understand client’s needs and market

3. Design Guidance(I) ‘Simple’

Assumed three basic modes of failure!!

(a) tensile (b) bearing (c) shear

3. Design Guidance(I) ‘Simple’ and (II) ‘Rigorous’

Tensile test on single bolted connection to determine design properties

Test rig (Turvey 1996) 45o failure

3. Design Guidance(I) ‘Simple’ and ‘Rigorous’

Typical test data for SLS and ULS

3. Design Guidance(I) ‘Simple’ and (II) ‘Rigorous’

• World-wide 700 plus individual test results

• Development of ‘simple’ and ‘rigorous’ design procedures for connection resistance (ULS)

• Less attention paid to any SLS

Variables making generalisation VERY difficult :-

materials (bolts and plates), joint dimensions and bolt lay-out, interface conditions (washer, torque, clearance hole), working loads and working environments

3. Design Guidance(I) ‘Simple’

EUROCOMP Design Code- Six basic load cases (1 and 2 have bolt, 4 to 6 are notched)

4 5 6

3. Design Guidance(I) ‘Simple’

EUROCOMP Design CodeProblems:

No clearance hole

Not all failure modes

Not validated

Laminates not defined

No damage tolerance

3. Design Guidance(II) ‘Rigorous’

Now to cope with general situation and involve damage tolerance

3. Design Guidance(II) ‘Rigorous’

STEPSFinite element (linear elastic) analysis

1 Source: determination of load distributions (bolts and far-field (takes account of real stiffnesses)

2 Target: determination of fastener hole stress distributions

Failure analysis3 BOLTIC FEA provides specific stress outputs to

include ‘damage tolerance’ in the design of bolted connections. The failure criterion is the well-known Point Stress Criterion

3. Design Guidance(II) ‘Rigorous’

Target: Stress analysis includes contact and friction.Failure check (using Point Stress Criterion (dk is characteristic distance and,k is design tensile strength)) must be at a number of locations.

3. Design Guidance(II) ‘Rigorous’

Progressive failure testing and analysis

Bearing test rig Local stress field (Mottram 2000) (with clearance hole)

4. Research Goals

• Establish worthiness of design guidance for

connections (bolted and other methods)

• Establish scope and limitations of aerospace

design methodologies

• Develop understanding and know-how for design

of connections to become generalised

• Provide information for preparation of approved

design guidance

• Improve confidence in using pultruded profiles in

primary load bearing structures

5. Conclusions

Large number of composite members and types

of connections are available; more to appear as

emerging technology matures

Important R&D advances have been made in

applications of primary structural connections

There is a need for standard connection details

giving easy to assemble structures that are safe,

reliable and cost-effective

5. Conclusions

Mechanical fastening will be the primary

connection method in the coming years because it

provides flexibility and is familiar to all

construction engineers

Approved design guidance (based on physical

testing and advanced numerical modelling) is

going to take time to develop; a concerted effort is

needed to transfer R&D into better practice