a brief review of Eurocode 3: Design of steel structuresPart 1-8: Design of joints

Contents

1 Introduction

2 Basis of design

3 Connections made with bolts, rivets or pins

3.1 Bolts, nuts and washers3.2 Rivets3.3 Anchor bolts3.4 Categories of bolted connections3.5 Positioning of holes for bolts and rivets3.6 Design resistance of individual fasteners3.7 Group of fasteners3.8 Long joints3.9 Slip-resistant connections using 8.8 or 10.9 bolts3.10 Deductions for fastener holes3.11 Prying forces3.12 Distribution of forces between fasteners at the ultimate limit state3.13 Connections made with pins

static theorem of plastic analysis

hypotheses

- ductile material (unlimited plasticity)- no buckling

any distribution of the internal forcesand stresses which:

a) is in equilibrium

b) fulfils the resistance criterionof the material

provides a value of the structuralfailure load which is not greaterthan the real failure load

different equilibrated load distributionsmay be adopted in the computations

the acutal ductility is not unlimitedwhat is the weak link? (the bolt or the plate)a realistic force distribution involves less plastic re-arrangements at collapse

2.5 Design assumptions

Joints should be designed on the basis of a realistic assumptionof the distribution of internal forces and moments.

The following assumptions should be used to determine the distribution of forces:

(a) the internal forces and moments assumed in the analysis are in equilibriumwith the forces and moments applied to the joints,

(b) each element in the joint is capable of resisting the internal forces and moments,

(c) the deformations implied by this distribution do not exceedthe deformation capacity of the fasteners or welds and the connected parts,

(d) the assumed distribution of internal forces should be realistic

with regard to relative stiffnesses within the joint

(e) the deformations assumed in the design model are based onrigid body rotations and/or in-plane deformations which are physically possible

(f) any model used is in compliance with the evaluation of test results (see EN 1990).

1.25

1.25

high strengththey are required

for controlled tightening(preloaded bolts)

shear strength ≅ tensile strength / √2it is actually a shear-bending failure

being the geometry of bolts standardized, the maximum forcesand the tightening torques can be collected in tables

3.4 Categories of bolted connections3.4.1 Shear connections

Bolted connections loaded in shear should be designed as one of the following:

Category A: Bearing typeIn this category bolts from class 4.6 up to and including class 10.9 should be usedNo preloading and special provisions for contact surfaces are required.The design ultimate shear load should not exceed the design shear resistance,obtained from 3.6, nor the design bearing resistance, obtained from 3.6 and 3.7.

3.6 Design resistance of individual fasteners

3.7 Group of fasteners

shear plane: through the threaded or unthreaded portion of the bolt

the resistant area is the tensile area As or the gross area A respectively

the ultimate shear stress is about 50% the ultimate strength in tension

the material strength is referred to the plates grade

αd indicates who is the weakest link among:the bolt (compressed)the plate close to the hole (compressed)the global failure of the plate (tension/shear)

k1 accounts for the material confinement

direction of force

minimum distances to prevent a strength reduction because of ad and k1

direction of force

block tearing

shear resistance

different material safety factors

x 0.5for eccentric

loading

3.4 Categories of bolted connections3.4.1 Shear connections

Category B: Slip-resistant at serviceability limit stateIn this category preloaded bolts in accordance with 3.1.2(1) should be used.Slip should not occur at the serviceability limit state.The design serviceability shear load should not exceedthe design slip resistance, obtained from 3.9.The design ultimate shear load should not exceed the design shear resistance,obtained from 3.6, nor the design bearing resistance, obtained from 3.6 and 3.7.

n = number of friction surfacesµ = slip factorks accounts for the shape of the hole

controlled tightening force of high-strength bolts

shape of the hole

1.2.7 Reference Standard, Group 7:Execution of steel structures

EN 1090-2 Requirementsfor the execution of steel structures

0.45 prepared and assembledin the workshop

0.30 assembled on construction site

slip force vs shear resistance of the bolt

αv = 0.5A = As for shear through the threadsγM2 = 1.25

ks = 1.0n = 1 µ = 0.4Fp,C = 0.7 fub AsγM3 = 1.25

deleting the common parts (As, fub, γM) it turns out

Fs,Rd / Fv, Rd = 0.4 · 0.7 / 0.5 = 0.56

the ultimate shear strength of bolts is much higher than the friction.

Designing for the slip resistance under service load is more severe thandesigning for shear strength under the ultimate loads(provided that the shear collapse is not governed by the bearing resistance)

3.4 Categories of bolted connections3.4.1 Shear connections

Category C: Slip-resistant at ultimate limit stateIn this category preloaded bolts in accordance with 3.1.2(1) should be used.Slip should not occur at the ultimate limit state.The design ultimate shear load should not exceed the design slip resistance,obtained from 3.9, nor the design bearing resistance, obtained from 3.6 and 3.7.In addition for a connection in tension, the design plastic resistanceof the net cross-section at bolt holes Nnet,Rd, (see 6.2 of EN 1993-1-1),should be checked, at the ultimate limit state.

the preloading doesn't affect the ultimate strength...

the possible eccentricity between the centroidal and setting out lines

the joints and members should be designed for the resulting moments and forces

welded connections 2 main types for load bearing connections: fillet welds and butt welds

simplified designof fillet welds(loaded only // to the welding)

fillet welds

the maximum stress in fillet weldsaccording to the old Italian recommendation CNR 10011

a trimmed sphere

σ

τ τ//

σ

τ

Full penetration butt weldsThe design resistance of a full penetration butt weld should be taken as equalto the design resistance of the weaker of the parts connected,provided that the weld is made with a suitable consumable whichwill produce all-weld tensile specimens having both a minimum yield strengthand a minimum tensile strength not less than those specified for the parent metal

Partial penetration butt weldsThe design resistance of a partial penetration butt weld should be determinedusing the method for a deep penetration fillet weld.