STEEL AND TIMBER STRUCTURES (CENG4123)...STEEL AND TIMBER STRUCTURES (CENG4123) PART THREE: DESIGN OF CONNECTIONS AND INTRODUCTION TO TIMBER STRUCTURES January 9, 2018 Addis Ababa

STEEL AND TIMBER STRUCTURES (CENG4123)

PART THREE: DESIGN OF CONNECTIONS AND INTRODUCTION TO

TIMBER STRUCTURES

January 9, 2018

Addis Ababa institute of Technology 1

Presentation

Outline

January 9, 2018Addis Ababa institute of Technology

2

Topic 1: Structural Connections

Topic 2: Plate Girders

Topic 3: Timbers Structures

Topic 1: Structural Connections


3


4

Introduction: Joints/connections in Steel structures

The performance of the structural steel members discussed in PART 2 of this course is only attained as per the design if and

only if the connections in steel structures are efficient!.

Historically, most major structural failures have been due to some form of connection failure.


5


Connections depends on:

► Type of loading

► Strength and stiffness

► Economy

► Difficulty or ease of erection


6


Can have various configuration depending on the structural members they connect:

► Beam-Beam Connections

► Beam-Column Connections

► Column to Footing Connections

are typical cases as well as column splices


7








8








9








10








11








12


The performance of the structural steel members discussed in PART 2 of this course is only attained as per the design if and

only if the connections in steel structures is efficient!.


13

Introduction: Mechanical Fasteners

Mechanical fasteners are generally realized by means of bolts, pins and rivets, which make possible the erection of the

skeleton frame in a much reduced time frame, especially when compared with the one required when site welds are employed.

They are generally composed of

a bolt a nut one or more washers,

when necessary.


14

Resistance of Bolted Connections: Introduction

Design of connections is generally based on simplified models that require in many cases only hand written calculations.

The distribution of forces in the connection may, hence, be arbitrarily determined in whatever rational way is best, provided

that:

► the assumed internal forces are balanced with the applied design forces and moments;

► each part of the connection is able to resist the applied forces and moments;

► the deformations imposed by the chosen distribution are within the deformation capacity of the fasteners, welds and the

other key parts of the connection.


15

Resistance of Bolted Connections: Introduction

Connections can be classified on the basis of the acting loads as follows:

► connections in shear; ► connections in tension;

► connections simultaneously in tension and shear.


16

Resistance of Bolted Connections: Connections in Shear

A connection is affected by shear when the plates connected via bolts are loaded by forces parallel to the contact planes.

The bolt can be considered to be a

simply supported beam loaded at its

midspan.

Different responses are expected, depending on two different modes to transfer the shear load, which make possible the

distinction between

► Bearing connections and ,

► Slip-resistant connections.


17

Resistance of Bolted Connections: Connections in Shear -

Bearing connections

It is required that the plates must be connected to each other achieving a firm contact and no tightening of the bolt is required.

Where,

► A is unthreaded area,

► Ares is threaded area

► V is the total shear force on the bolt and,

► n is the number of shear planes

ΔL

Failure of the shear connection can

be due to one of the following

mechanisms:

a) bolt failure;

b) plate bearing;

c) tension failure of the plate;

d) shear failure of the plate .


18


Bearing connections

It is required that the plates must be connected to each other achieving a firm contact and no tightening of the bolt is required.

ΔL



mechanisms:

a) bolt failure;

b) plate bearing;




19


Bearing connections



mechanisms:

a) bolt failure;

b) plate bearing;



plate bearing;

In particular, bearing pressure between bolt

and plate can be approximated with

reference to the mean value of the bearing

stress, σbear :

Where,

► V is the acting shear force per shear plane,

► t is the minimum thickness of connected plates

► d is the bolt diameter.

Typical deformation holes due to a bearing.


20


Slip Resistant Connection or Connection with Pre-Loaded Joints

Pre-loading of bolts can be explicitly required for slip resistance, seismic connections, fatigue resistance, execution purposes

or as a quality measure (e.g. for durability).

Thus, once the bolt is tightened, the joint is loaded by self-balanced stresses associated with the bolt in tension and the

compression in the plates and with the torsion of the bolt and plate/bolt friction.

TC

ΔL≈0


21



Tightening increases joint performance, mainly with reference to serviceability limit states. Furthermore, it should be noted

that:.

► in shear joints, tightening prevents plate slippage and, therefore, inelastic settlements in the structure;

► in tension joints, tightening prevents plate separation (reducing corrosion dangers) and significantly improves fatigue

resistance.

However, tightening must not exceed a certain limit, to avoid attaining joint ultimate capacity.

The load increases from zero but no relative

displacement is observed; force transmission is

due to friction between the plates until friction

limit of the joint is reached, which depends on

the degree of preload.

Curve (c) is related to a connection with a

pre-load degree greater than the one of

case (b);


22



The value of the force at which slippage occurs depends upon:.

► bolt tightening,

► surface treatment, and

► number of surfaces in contact (nf ).

The maximum value of the force transferred by friction, FLim , can be estimated as:.

Where,

► is the friction coefficient.


23


Slip Resistant Connection or Connection with Pre-Loaded Joints- Stress Distribution

As the deformation capacity of plates is generally much higher than the deformation capacity of the bolts, it is strongly

recommended to design the connection such that yielding of the plates in bearing occurs before yielding of the bolts in shear,

in order to guarantee a ductile failure rather than a brittle failure.

Plastic redistribution at failure occurs with a uniform stress distribution and this justifies the use in design of a mean value of

stress, assumed for sake of simplicity constant in elastic range and conventionally considered equal to:

Distribution of the stress in the

plate of a bearing connection in

elastic (a) and plastic (b) range.

Where,

► V is the shear force.

► An is the net area of the cross-section of the plate

(i.e. gross area reduced for the presence of the hole.


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In connections with more than one bolt, a correct evaluation of the resistant area for the plates could become complex,

depending on the ultimate load for tension and shear as a function of the possible failure path

To minimize the weakness of cross-section for the presence of holes, it is

possible to increase the number of the holes from the end to the center of

the connection, as shown.

It is worth noting that this causes an increase in the dimension

of the joint.

NO

TE

1:

the main rules for estimating an appropriate value of the

reduced area have already been introduced for tension

member verification


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As it happens in some practical cases dealing with joints, the design load Fv can be eccentric with reference to the centroid of

the fasteners, the result of this is the connection is subject to shear and torsion.

The actual response of this connection is quite

difficult to be predicted.

NO

TE

2:

Using the superimposition principle

Where,

► nf is the number of shear resisting plane per bolt

and n is the number of the bolts.

► ai is the distance between the centroid of all the

bolts and that of the single i-bolt.


26

Resistance of Bolted Connections: Connections in Tension

Tension occurs when the plates connected via bolts are loaded by a force normal to the contact plane; that is parallel to the

bolt axis.

If the flange is sufficiently stiff

► its deformation can be

disregarded,

► The bolts can be assumed

to be in pure tension

► Connection failure is

expected to be due to

failure of the bolts.

As in case of bearing connection, the response of a connection in tension is quite difficult to predict.

if the flange is more flexible,

► the presence of prying forces, Q.

► increases the value of the axial

load transferred via bolts.

► Connection failure may be due

to bolts, flange or to both

components.


27


In order to better appraise the tightening effects, reference can be made to the response of the tension connection presented

below, which is realized by one bolt.

Relationships between the applied external load

N to the connection and bolt elongation ΔL

curve a - related to the case of non-tightened bolt

curve b - related to the case of tightened bolt

Relationships between the applied external load

N is plotted versus the axial force acting in

the bolt shank Nb


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In case of tension force applied on the centroid of the bolts, it is assumed that the design load is balanced by forces equal on

each bolt. Otherwise, if a bending moment also acts, the evaluation of the bolt forces is usually based on the assumption of

stiff plate.

NO

TE

1:

Angle legs on the plane a

► subjected to shear force

and torsion moment

Angle legs on the plane b

► subjected to shear

force and bending

moment (tension force)

Equilibrium condition dictates


29

Resistance of Bolted Connections: Connections in Shear

and Tension

The approaches previously introduced for the case of sole shear force and sole tension force on the connection can be

combined to each other in order to be used for the more general case of shear and tension.

More details about the requirements for verification are presented in the following parts, in accordance with European Norms.

Summary

Thank you for your kind attention!

End of Class Seventeen! Questions?

January 9, 2018

30

Addis Ababa institute of Technology

Design in Accordance with European Practice: Introduction to-European Practice for Fastener Assemblages


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Chapter 8 (Mechanical fastenings) of the EN 1090-2 (Execution of steel structures and aluminium structures – Part 2:

Technical requirements for steel structures) deals with mechanical fasteners, giving important and useful information on

assembly techniques of site bolted connections.

Bolts Nuts Washers► The minimum nominal fastener diameter

used for structural bolting is 12 mm

(M12 bolt), except for thin gauge

components and sheeting.

► For non-preloaded bolts, at least one full

thread (in addition to the thread run out)

are required to remain clear between

the bearing surface of the nut and the

unthreaded part of the shank.

► For preloaded bolts according to EN

14399-3 and EN 14399-7, at least four

full threads (in addition to the thread run

out) have to remain clear between the

bearing surface of the nut and the

unthreaded part of the shank.

► It is required that nuts run freely on their

partnering bolt, which is easily checked

during hand assembly.

► Nuts have to be assembled so that their

designation markings are visible for

inspection afterwards.

► Washers are not required when non-

preloaded bolts are used in normal

round holes, but recommended

anyway to avoid damage to steel

painting.

► If used, it must be specified as to

whether washers must be placed

under the nut or the bolt head

(whichever is rotated) or both.

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications - Bolt Designations


32

The yield strength fyb and the ultimate tensile strength fub for bolt classes 4.6 to 10.9 are given in Table 3.1. of EN:1993-1-1.

These values should be adopted as characteristic values in design calculations.

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications - Clearances for Bolts and Pins


33

The definition of the nominal hole diameter combined with the nominal diameter of the bolt to be used in the hole determines

whether the hole is ‘normal’ or ‘oversize’.

The terms ‘short’ and ‘long’ applied to slotted holes refer to the two types of holes used for the structural design of preloaded

bolts, which may be used also to designate clearances for non-preloaded bolts.

Nominal clearances for bolts and pins (values in millimetres).

No indications are given in EN 1993-1-8 on the nominal clearance for bolts and pins, which are reported in the table below

and are derived directly from EN 1090-2, where this topic is dealt with.

Connections Loaded in Shear

Category A – Bearing type:

No preloading required.

Bolt classes from 4.6 to 10.9 may be used.

Category B – Slip resistant at serviceability limit states:

The design serviceability shear load should not exceed the design slip resistance.

The design ultimate shear load should not exceed either the design shear resistance or the design bearing resistance

Category C – Slip resistant at ultimate limit states:

The design ultimate shear load should not exceed either the design slip resistance or the design bearing resistance.

Connections Loaded in Tension

Category D – tension connection non-preloaded:

This category must not be used where the connections are frequently subjected to variations of the tensile force.

However, they may be used in connections designed to resist normal wind loads.

Category E – tension connection preloaded:

preloaded 8.8 and 10.9 bolts with controlled tightening are used.

This category shall be used in connections designed to resist to seismic loads.

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications – Category of Bolt Connections


34

Part 1–8 of Eurocode 3, that is EN 1993-1-8 (Design of Steel Structures – Part 1–8: Design of Joints) deals with connections

that are divided in two groups depending on the type of loading:

► Shear and

► Tension connections.

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications - Partial Material Factors


35

Part 1–8 of Eurocode 3, that is EN 1993-1-8 (Design of Steel Structures – Part 1–8: Design of Joints) provides the following

partial safety factors.

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications-Design Resistance


36

► Table 3.4 of EN 1993-1-8 gives the different checks required for individual fasteners subjected to shear and/or tension.

► Checks need to be carried out for a number of possible failure modes:

▪ Tension Resistance

▪ Shear resistance per shear plate

▪ Bearing Resistance

▪ Combined shear and tension .



37

Bolted Joints-Tension Resistance

The tension resistance per bolt at ultimate limit states, Ft,Rd , is defined as:

Where,

► As is the tensile stress area of the bolt.

► γM2 = 1.25

► fub is the ultimate tensile strength of the bolt

► k2 = 0.63 for countersunk bolt, otherwise k2 = 0.9

Punching shear resistance Bp,Rd of the plate is defined as:

Where,

► fu is the ultimate tensile strength.

► tp is the thickness of the plate

► dm is the minimum between the nut diameter and the mean

value of the bolt head.



38

Bolted Joints-Shear Resistance per Shear Plane

Shear resistance per shear plane for ordinary bolts where the shear plane passes through the threaded portion of the bolt:

Where,


► γM2 = 1.25


► αv

Where,


► γM2 = 1.25


Shear resistance per shear plane for ordinary bolts where the shear plane passes through the unthreaded portion of the

bolt:

Where,

► d is the bolt diameter .

► γM2 = 1.25

► t is the thickness

► fu is the ultimate tensile strength

► αb In case of bolts in the direction of load: for edge bolts:

► αb for internal bolts:

► k1 In case of bolts in the direction of load: for edge bolts:

► k1 for internal bolts:



39

Bolted Joints-Bearing Resistance

Bearing resistance for ordinary bolts::

A reduction of the bearing resistance, Fb,Rd , has to be considered

in the following cases:

► Bolts in oversized holes, for which a bearing resistance of

0.8Fb,Rd (reduction of 20% with reference to the case of

normal holes) has to be considered;

► Bolts in slotted holes, where the longitudinal axis of the slotted

hole is perpendicular to the direction of the force transfer, for

which a bearing resistance of 0.6Fb,Rd (reduction of 40% with

reference to the case of normal holes) has to be considered.



40

Bolted Joints-Combined Shear and Tension Resistance

If the bolt is subjected to combined design shear, Fv,Ed and tension, Ft,Ed , the resistance of the bolt is defined as:

Where,

► Fv,Rd is the design shear resistance per bolt, and

► Ft,Rd is design tension resistance per bolt.



41

Bolted Joints-Slip-Resistant Connection

In case of slip-resistant connection, the design pre-loading force for high strength class 8.8 or 10.9 bolt, Fp,Cd , has to be

taken, as recommended in EN 1090-2, as: Where,

► µ is the friction coefficient

► γM3 = 1.1

► γM7 = 1.1

► ks accounts for the type of holes and assumes

the following values:

= 1 for bolts in normal holes

= 0.85 for bolts in either oversized holes or short

slotted holes with the axis of the slot

perpendicular to the direction of load transfer

= 0.7 for bolts in long slotted holes with the axis of

the slot perpendicular to the direction of load

transfer

= 0.76 for bolts in short slotted holes with the axis of

the slot parallel to the direction of load transfer

= 0.63 for bolts in long slotted holes with the axis of

the slot parallel to the direction of load transfer.

By h

ow

much

to t

hig

hte

n:

The design slip resistance, FS,Rd , of a preloaded class 8.8 or 10.9 bolt

is:

How

much s

lip

forc

e it

can

resis

t:

The design pre-loading force Fp,C , (in equation

above) is defined as:



42

Bolted Joints-Long Joints

Where the distance Lj between the centres of the end fasteners in a joint, measured in the direction of force transfer, is

more than 15d, the design shear resistance Fv,Rd of all the fasteners has to be reduced by multiplying it by a reduction

factor βLf defined as:

with the limitation βLf ≤ 1.0 and βLf ≥ 0.75.



43

Summary



44

Summary

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications-Position of Holes


45

Bolted Joints-Position of Holes

Table 3.3 of EN 1993-1-8 gives the maximum

and minimum spacing, end and edge

distances

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications-Position of Holes


46

Bolted Joints-Position of Holes

Table 3.3 of EN 1993-1-8 gives the maximum and minimum spacing, end and edge distances

normal holes staggered holes

staggered holes for compression members tension member

slotted holes

Worked Example: Example on Verification of a Bearing Connection

According to EC3


47

Example 6.1. Verify, according to EC3, the connection shown in the figure below (all dimensions are in millimetres). It

is a single lap bearing type joint with one shear plane. Ultimate design load, NSd , is 140 kN. Bolts have a 16 mm

diameter, class 8.8, not preloaded and the threaded portion of the shank is located in the bearing length. Holes have

18 mm (0.709 in.) diameter. Plates to be connected by bolts are 150mm wide and 5mm thick. The steel of the plates is

S235

Step1: Check the positioning of the holes (spacing and end and edge distances);

Step2: Evaluation of shear design force for each shear plane of each bolt (VEd )

The verification of this bearing type bolted connection goes through the following steps:

Procedure

Step3: Evaluation of the design shear resistance for each shear plane (Fv.Rd )

Step4: Evaluation of the design bearing resistance (Fb.Rd )

Step5: Evaluation of design ultimate tensile resistance of the connected plate net cross-

section at holes (Nu,Rd )

Worked Example: Example on Evaluation of the Resistance of a Slip-

Resistant Connection Subjected to Shear Force According to EC3


48

Example 6.2. Evaluate shear design resistance, according to EC3, of connection illustrated in the Figure below (all

dimensions are in millimetres). The connection is category C slip-resistant at an ultimate limit state. Bolts have a 20

mm diameter, class 10.9, preloaded. Holes have a 22 mm diameter. Each bolt has two friction surfaces. Elements to

be connected and cover plates are made of S235 steel. Friction surfaces are in class A (surfaces blasted with shot or

grit with loose rust removed, not pitted).

Step1: Check of positioning of the holes (spacing and end and edge distances);

Step2: Evaluation of minimum bolt preloading force (Fp.C )

Connection design resistance is computed with the following steps::

Procedure

Step3: Evaluation of design slip resistance at ultimate limit states (Fs,Rd )

Step4: Evaluation of the design bearing resistance (Fb,Rd )

Step5: Evaluation of design ultimate tensile resistance of cover plate net cross-section at

holes (Fd,u,Rd )


End of Class Eighteen! Questions?

January 9, 2018

49



50

Introduction: Welded Connections

► Welding is an assembling process that allows us to permanently join two

metallic elements causing fusion of the adjoining parts.

► When comparing welded connections to bolted, nailed or riveted ones, it is

apparent that the former are inherently monolithic and are at the same time

stiffer and less complicated, allowing more freedom to the designer.

► These advantages are balanced by the need of additional detailing and

fabrication requirements, especially for that which concerns the

assurance and verification of the quality of welded joints, in order to

prevent potential partial loss of strength or stiffness, or possibly brittle

fractures.

► This is the reason why the welding process should always be performed

by qualified welders.

► In welded connections the connected elements are identified as base

material, while the weld material, when applicable, refers to the material

that is added to the joint in its liquid state during the welding process..


51


► A classification of welding processes can be made from:

I. Autogenous processes: the base metal participates to the formation of the

joint by fusion or crystallization with the weld metal, if present.

The most common processes are: oxyacetylene (oxyfuel) welding, arc

welding, submerged arc welding (SAW), shielded metal arc welding (SMAW),

gas metal arc welding (GMAW), also known as metal inert gas welding

(MIG), metal active gas welding (MAG), gas tungsten arcwelding (GTAW),

also known as tungsten inert gas welding (TIG), and electroslag welding

(ESW), used mostly for automatic applications for large welds.

II. Heterogeneous processes: in these processes, only the base material is the

weld material used at a temperature lower than the melting temperature of the

base material. A classic example is provided by soldering or brazing

processes.

► As a consequence of the metallurgical phenomena (creation of the

weld, solidification of the weld pool and thermal effects in the base

material surrounding the weld region, known as heat affected zone –

HAZ), there can be defects in the welded connection. These are

classified into metallurgical and geometric defects..

Metallurgical defects Geometric defects

Cracks (hot or cold) Excess of weld metal

Lamellar tearing Lack of penetration (lack

of fusion)

Inclusions Lack of alignment


52


Metallurgical defects Geometric defects

Cracks (hot or cold) Excess of weld metal

Lamellar tearing Lack of penetration (lack

of fusion)

Inclusions Lack of alignment

Cracks

Lamellar tearing

Lack of penetration Lack of alignment

In most instances, welded connections must be inspected in order to

ascertain the presence of defects. Such NDT include:

Visual inspections, dye penetrant testing, magnetic particle testing,

ultrasonic testing, testing with radiation imaging systems, radiographic

testing and eddy current testing.


53

Types of Welded Connections: Introduction

The load-resisting elements of a welded joint are the welds.

Based on the relative position of the

elements to be joined, there can be

Based on the position of the weld and on the

direction of the force to be transferred, there can be:

Based on the type of weld, there can be:

► Groove welds,

► Fillet welds,

► Slot welds,

► Plug welds

Butt joints Edge joints

Corner joints T-joints

L-joints

Lap joints

longitudinal welds transverse welds inclined welds


54

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications-Butt Welds

According to the ES-EN:1993-1-8, there can be the following types of join as depicted in EN12345 :

► Fillet welds,

► Fillet weld all round,

► Flare groove welds,

► Plug welds, and

► Butt welds

For Butt welds, for most common cases the design strength can be taken to be equal to the design strength of the weaker

of the connecting members.


55

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Structural Verifications-Fillet Welds

For fillet welds, after Eurocode 3, the design strength per unit length Fw,Rd can be calculated based on either of the following

methods:

► Directional method,

► Simplified method,


56


The directional method requires the determination of the state of stress in the

effective throat area without rotations, and thus the stresses σ and τ are the

normal and shear stresses in the plane of the effective throat area, respectively.

Where,

► fu is the nominal tensile strength of the weakest element in the joint

► γM2 = 1.25

► βw is an appropriate correlation coefficient as shown

σ⊥, normal stress, acting perpendicularly to the

effective area;τ⊥, shearing stress, perpendicular to the

longitudinal axis of the fillet;

τ//, shearing stress, parallel to the longitudinal

axis of the fillet;


57


The simplified method establishes that the design strength of a fillet weld should be taken, independently on the orientation

of the weld, as:

Where,

► Fw,Ed is the design value of the weld force per unit length

► Fw,Rd is the design resistance per unit length

► fvw,d is the design shear strength of the weld

► a is the effective throat thickness

► fu is the nominal tensile strength of the weakest element in the joint

► γM2 = 1.25

► βw is an appropriate correlation coefficient as shown


58

Design in Accordance with European Practice: Introduction to EN:1993-1-8 - Summary

Summary

Worked Example: Example on Welded Connection According to EC3

for a Tension Member


59

Example 6.3. Verify in accordance with EC3, the welded connection shown between a plate 250 × 20 mm in tension

and a column flange, realized by one fillet weld orthogonal to the force axes.

Worked Example: Example on Welded Connection According to EC3

for a Member in Bending and Shear


60

Example 6.4. Verify in accordance with the EC3 provisions, the welded connection between a UPN 240 profile in

bending, connected to a gusset plate by two fillet welds of same length


End of Class Nineteen! Questions?

January 9, 2018

61


Documents

STEEL AND TIMBER STRUCTURES (CENG4123)...STEEL AND TIMBER STRUCTURES (CENG4123) PART THREE: DESIGN OF CONNECTIONS AND INTRODUCTION TO TIMBER STRUCTURES January 9, 2018 Addis Ababa