Design Model for Welded Joints in Trusses Using Structural Hollow Sections

NCCI: Design model for welded joints in trusses using structural hollow sections SN040a-EN-EU

NCCI: Design model for welded joints in trusses using structural hollow sections

This NCCI deals with the verification procedure for welded joints in trusses using structural hollow sections alone or in combination with open section profiles. Uniplanar, unreinforced joints are considered.

Contents

1. Introduction 2

2. Scope 2

3. General design guidance 4

4. Parameters affecting joint resistance 5

5. Joint design guidance 7

6. Welds 9

7. Design aids 10

8. References 10

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1. Introduction In trusses with structural hollow sections, the members are generally welded directly to each other. The selection of member sizes has a direct effect on the cost of fabrication and on the joint resistance. Therefore, it is important for the architect and the design engineer to recognise at the outset, the effects that their design decisions will have on the joint resistance, the fabrication, assembly and erection of the structure.

2. Scope The rules given in this document apply to structures whose joints are verified according to EN1993-1-8 §7. This NCCI explains the verification procedure for the following joints:

Joints involving circular hollow section chords

Joints involving rectangular or square hollow section chords

Joints involving I or H section chords

This NCCI covers uniplanar, unreinforced joints under predominantly static axial forces and/or bending moments.

Information on the influence of the main parameters on the joint resistance is also provided.

2.1 Joint types The joint types covered are shown in the figure below:

X joints T and Y joints

K and N joints with gap K and N joints with overlap

Figure 2.1 Typical joint types

2.2 Validity range Provided that the geometric ranges given in section 5 are satisfied for each type of joint, only the verification of the failure modes given in the relevant table is required. Outside these ranges all the failure modes, some of them not covered by the design formulae, may become critical and should be verified accordingly.

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2.3 Joint symbols and parameters A cross-sectional area of member i (i = 0, 1, 2 or 3); i

N design value of the internal axial force in member i (i = 0, 1, 2 or 3); i,Ed

N design value of the resistance of the joint, expressed in terms of the internal axial force in member i (i = 0, 1, 2 or 3);

i,Rd

M design value of the in-plane internal moment in member i (i = 0, 1, 2 or 3); ip,i,Ed

M design value of the resistance of the joint, expressed in terms of the in-plane internal moment in member i (i = 0, 1, 2 or 3);

ip,i,Rd

M design value of the out-of-plane internal moment in member i (i = 0, 1, 2 or 3); op,i,Ed

M design value of the resistance of the joint, expressed in terms of the out-of-plane internal moment in member i (i = 0, 1, 2 or 3);

op,i,Rd

d overall diameter of circular hollow section member i (i = 0, 1, 2 or 3); i

e joint eccentricity (sign criteria defined in Figure 2.2)

f yield strength of member i (i = 0, 1, 2 or 3); yi

g gap between the brace members in a K or N joint (negative values represent an overlap q)

t wall thickness of member i (i = 0, 1, 2 or 3); i

θ included angle between brace member i and the chord (i = 1, 2 or 3); i

β ratio of the mean diameter or width of the brace members, to that of the chord:

0

1

00

;bbor

bd

dd ii (for T, Y and X joints)

0

2121

0

21

0

21

42;

2 bhhbbor

bdd

ddd +++++ (for K and N joints)

0

321321

0

321

0

321

63;

3 bhhhdddor

bddd

dddd +++++++++ (for KT joints)

λov overlap ratio, expressed as a percentage (λov = (q/p) x 100%)

a) Gap joint with positive eccentricity b) Overlap joint with negative eccentricity

Figure 2.2 Definition of joint eccentricity

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Figure 2.3 Main joint parameters for hollow section chords and brace members

g

q

p

g

Figure 2.4 Gap and overlap

3. General design guidance 3.1 Structural analysis Trusses are usually designed as pin-jointed frames with members meeting at a common point at the centre of each joint and loads applied at that point, resulting in the members being subjected to tension or compression only. The assumption of centre line loading and pinned connections enables a good approximation of the axial forces in the members. Figure 3.1 shows the way in which members are arranged at the joint, i.e. with the centre lines of the bracing members intersecting with the centre line of the chord.

Figure 3.1 Noding joints in trusses

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3.2 Joint eccentricity For economic fabrication, it is generally much easier and therefore cheaper to assemble and weld with a gap between the bracing members (see Figure 2.2(a)). This may result in eccentricity at the joint which should be accounted for as given in section 3.3

3.3 Primary and secondary moments The assumption of pinned connections together with load application at the nodes provides a good approximation of the axial forces in the members. However, there are three factors that may introduce bending moments in the real structure. These factors are: stiffness of the joint, transversal loads (if any) and the eccentricity (often needed when gap or overlap arrangements are specified). EN1993-1-8 covers these effects in the following way:

Provided that the geometrical limits given in EN1993-1-8 §5.1.5(3) are satisfied, the secondary moments due to joint stiffness do not influence the resistance of the joint since it is assumed to have sufficient deformation capacity.

Transverse loads: primary bending moments caused by transverse loading should be considered in the design of the joints. In most cases, since transverse loads are not applied to brace members, they are not subject to bending moments. For bending moments from transverse loads on the chords, the compressive bending stress should be added to the axial compression in the chord.

Eccentricity: if the geometrical limits given in EN1993-1-8 §5.1.5(5) are satisfied, its effect on the joint is automatically taken into account in the joint design formulae in this NCCI. However, it is always good practice to keep eccentricities to a minimum.

How the effects of these moments should be taken into account is summarized in Table 3.1.

Table 3.1 Allowance for bending moments

Source of the bending moment Type of component

Joint stiffness Transverse loading Eccentricity

Compression chord Yes

Tension chord No

Brace member No Ignore if EN1993-1-8 Yes §5.1.5(3) is satisfied Joint EN1993-1-8 Ignore if

§5.1.5(5) is satisfied

4. Parameters affecting joint resistance Table 4.1 to Table 4.3 give information on how the joint parameters should be modified in order to increase the joint resistance. The joint resistance is taken as the maximum axial resistance of the brace members. These recommendations are valid only for cases where the joint resistance is smaller than the resistance of the members.

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Table 4.1 Single braced joints (T, Y and X joints)

Parameter value Effect on joint resistance Joint Parameter

Chord width to thickness ratio 0000 tdortb reduced increased

Bracing to chord width ratio 0101 bbordd (1) increased increased

θBracing angle reduced increased

00

11

tftf

y

yBracing to chord strength ratio reduced increased

(1) provided that chord side wall buckling (square or rectangular hollow sections) does not become critical, when β > 0.85

Table 4.2 K and N gap joints

Joint Parameter Parameter value Effect on joint resistance


Bracing to chord width ratio 0101 bbordd (1) increased increased

θBracing angle reduced increased

00y

11y

tftfBracing to chord strength

ratio reduced increased

g Gap between bracings reduced increased (2)

(1) provided that chord side wall buckling (square or rectangular hollow sections) does not become critical, when β > 0.85 (2) only true for circular hollow section chord joints

Table 4.3 K and N overlap joints

Effect on joint resistance Joint Parameter Parameter value


Overlapped bracing width to thickness ratio jj tb (1) reduced increased

Bracing to chord width ratio 0101 bbordd increased increased (2)

θBracing angle reduced increased (3)

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00y

jyj

tftfOverlapped bracing to

chord strength factor reduced increased

jyj

iyi

tftfBracing to bracing

strength factor reduced increased

vO reduced increased Overlap of bracings

(1) only true for rectangular or square hollow section joints (2) provided that rectangular or square hollow section chord side wall buckling does not become critical, when β > 0.85 (3) only true for circular hollow section chord joints

5. Joint design guidance The tables presented in this section provide guidance on the Eurocode tables and clauses (EN 1993-1-8) to be used in each case for the design of joints involving hollow sections alone or in combination with open section profiles. The tables are presented according to the section used for the chord members. As previously stated in section 2.2, provided that the joint geometry falls within the geometrical range given, only the axial and moment design resistances in the corresponding tables need to be checked. Otherwise, all the failure modes presented in EN 1993-1-8 §7.2.2 should be considered.

The design resistance of the joint is expressed in terms of maximum design axial and/or moment resistance for the brace members.

For brace member connections subject to axial forces only, (i.e. the most common case), the following should be satisfied:

RdiEdi NN ,, <

However, for combined bending and axial force, the relevant interaction formulae stated in the tables below should be used. The typical application of joints subject to in-plane bending moments is the T joint of Vierendeel girder. Out-of-plane bending moments are not very common in planar structures. These moments occur due to out-of-plane loads such as wind loads.

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5.1 Welded joints involving circular hollow section chords Table 5.1 EN 1993-1-8 tables and clauses to be used in the design of joints involving circular

hollow section chords

Brace member Geometric range Axial design resistance

Moment design resistance

Interaction formula

Circular hollow sections

T 7.1 T 7.2 T 7.5 §7.4.2

Gusset plate T 7.1, T 7.3 T 7.3 T 7.3 §7.4.2

I or H T 7.1, T 7.4 T 7.4 T 7.4 §7.4.2

Regular or square hollow sections

T 7.1, T 7.4 T 7.4 T 7.4 §7.4.2

Special circular hollow section

T 7.1 T 7.6 -------- T 7.6

5.2 Welded joints involving rectangular or square hollow section chords Table 5.2 EN1993-1-8 tables and clauses to be used in the design of joints involving square

Hollow Section chords


Interaction formula

Circular hollow section

T 7.8, T 7.9 T 7.10 §7.5.2

Square Hollow section

T 7.8, T 7.9 T 7.10 §7.5.2

Table 5.3 EN 1993-1-8 tables and clauses to be used in the design of joints involving square or rectangular hollow section chords



Interaction formula

Circular hollow section

T 7.8 T 7.11, T. 7.12 --------- §7.5.2.1

Square or rectangular hollow section

T 7.8 T 7.11, T. 7.12 T 7.14 §7.5.2.1

(1)Gusset plate T 7.8, T 7.13 T 7.13 §7.5.2.1

I or H T 7.8, T 7.13 T 7.13 T 7.13 §7.5.2.1

Special rectangular hollow sections

T 7.8 T 7.15, T 7.16 ---------- T 7.16

(1) See Table 5.4 below

For the particular case of square or rectangular hollow section chords with gusset plates, the following table may be used for the determination of the moment resistance of such connections, [2]:

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Table 5.4 Moment resistance of square or rectangular hollow section chords with gusset plates

Transverse gusset plate Longitudinal gusset plate

In-plane moment: In-plane moment: iRdRdip tNM ,1,1, 5.0= RdiRdip NhM ,1,1, 5.0=

Out of plane moment: Out of plane moment: iRdRdop bNM ,1,1, 5.0= iRdRdop tNM ,1,1, 5.0=

5.3 Welded joints involving I or H section chord Table 5.5 EN 1993-1-8 tables and clauses to be used in the design of welded joints involving I

or H section chords



Interaction formula

-------------Circular hollow section

T 7.20 T 7.21 §7.6

Square or rectangular hollow section

T 7.20 T 7.21 T 7.22 §7.6

6. Welds The weld should normally be made around the whole perimeter of the bracing member by means of a butt weld, a fillet weld or a combination of the two.

Appropriate execution details for welding are given in ENV 1090-4.

The welds should be designed taking into account the requirements of EN 1993-1-8 §7.3 and the throat thickness calculated according to EN 1993-1-8 §4.

The design resistance of a fillet weld should not normally be less than the design resistance of the bracing member. This requirement will be satisfied provided

⎥⎦

⎤⎢⎣

⎡≥

M0u

M2y2γγβ

ff

ta w

When γ = 1,0 and γ = 1,25: M0 M2

a/t ≥ 0,96 for S275 steel and a/t ≥ 1,11 for S355 steel

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7. Design aids The software CIDJOINT applies the calculation and verification methods of joints formed by structural hollow sections subject to predominantly static loads. These methods have been developed thanks to the research performed by the Comité International pour le Développement et l’Etude de la Construction Tubulaire (CIDECT) and the Sub-commission XV-E of the International Institute of Welding (IIW). This software is consistent with the methods given in ENV1993-1-8.

More information is available at www.cidect.org.

8. References 1 Hollow Sections in Structural Applications, by J. Wardenier. Comité International pour le

Développement et l’Etude de la Construction Tubulaire (CIDECT). This publication may be freely downloaded from www.cidect.org

2 Design of SHS welded joints, Corus Tubes. This document is freely downloadable from www.corus.com.

3 ENV1090-4 Execution of steel structures – Part 4: Supplementary rules for hollow section lattice structures.

4 ENV1993-1-1:1992 “Eurocode 3. Design of steel structures. General rules and rules for buildings.

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http://www.cidect.org/

http://www.cidect.org/

http://www.corus.com/



Quality Record RESOURCE TITLE NCCI: Design models for welded joints in lattice structures

Reference(s)

ORIGINAL DOCUMENT

Name Company Date

Created by Francisco Rey LABEIN

Technical content checked by Jose A. Chica LABEIN

Editorial content checked by

Technical content endorsed by the following STEEL Partners:

1. UK G W Owens SCI 7/4/06

2. France A Bureau CTICM 7/4/06

3. Sweden A Olsson SBI 7/4/06

4. Germany C Müller RWTH 7/4/06

5. Spain J Chica Labein 7/4/06

Resource approved by Technical Coordinator

G W Owens SCI 12/7/06

TRANSLATED DOCUMENT

This Translation made and checked by:

Translated resource approved by:

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Documents

Design Model for Welded Joints in Trusses Using Structural Hollow Sections