M

Figure 5.2: Simplified bi-lineardesign moment-rotation characteristic

Figure 5.2: Simplified bi-lineardesign moment-rotation characteristic

5.1.5 Global analysis of lattice girders

(1) The provisions given in 5.1.5 apply only to structures whose joints are verified according to section 7.

(2) The distribution of axial forces in a lattice girder may be determined on the assumption that the members are connected by pinned joints (see also 2.7).

(3) Secondary moments at the joints, caused by the rotational stiffnesses of the joints, may be neglected both in the design of the members and in the design of the joints, provided that both of the following conditions are satisfied:

- the joint geometry is within the range of validity specified in Table 7.1, Table 7.8, Table 7.9 or Table 7.20 as appropriate;

- the ratio of the system length to the depth of the member in the plane of the lattice girder is not less than the appropriate minimum value. For building structures, the appropriate minimum value may be assumed to be 6. Larger values may apply in other parts of EN 1993.

(4) The moments resulting from transverse loads (whether in-plane or out-of-plane) that are applied between panel points, should be taken into account in the design of the members to which they are applied. Provided that the conditions given in 5.1.5(3) are satisfied:

- the brace members may be considered as pin-connected to the chords, so moments resulting from transverse loads applied to chord members need not be distributed into brace members, and vice versa;

- the chords may be considered as continuous beams, with simple supports at panel points.

(5) Moments resulting from eccentricities may be neglected in the design of tension chord members and brace members. They may also be neglected in the design of connections if the eccentricities are within the following limits:

-0,55 d0 <e < 0,25 d0 -0,55 U0 < e < 0,25 U0

where:

e is the eccentricity defined in Figure 5.3;

d0 is the diameter of the chord;

U0 is the depth of the chord, in the plane of the lattice girder.

(6) When the eccentricities are within the limits given in 5.1.5(5), the moments resulting from the eccentricities should be taken into account in the design of compression chord members. In this case the moments produced by the eccentricity should be distributed between the compression chord members on each side of the joint, on the basis of their relative stiffness coefficients I/L , where L is the system length of the member, measured between panel points.

(7) When the eccentricities are outside the limits given in 5.1.5(5), the moments resulting from the eccentricities should be taken into account in the design of the connections and the compression chord members. In this case the moments produced by the eccentricity should be distributed between all the members meeting at the joint, on the basis of their relative stiffness coefficients I/L .

(8) The stresses in a chord resulting from moments taken into account in the design of the chord, should also be taken into account in determining the factors Xm , Xn and Xp used in the design of the connections, see Table 7.2 to Table 7.5, Table 7.10 and Table 7.12 to Table 7.14.

(9) The cases where moments should be taken into account are summarized in Table 5.3.

Figure 5.3: Eccentricity ofjoints Table 5.3 Allowance for bending moments

Type of component

Source of the bending moment

Secondary effects

Transverse loading

Eccentricity

Compression chord

Not if 5.1.5(3) is satisfied

Yes

Yes

Tension chord

No

Brace member

No

Connection

Not if 5.1.5(5) is satisfied

5.2 Classification of joints

5.2.1 General

(1) The details of all joints shall fulfil the assumptions made in the relevant design method, without adversely affecting any other part of the structure.

(2) Joints may be classified by their stiffness (see 5.2.2) and by their strength (see 5.2.3).

5.2.2 Classification by stiffness

5.2.2.1 General

(1) A joint may be classified as rigid, nominally pinned or semi-rigid according to its rotational stiffness, by comparing its initial rotational stiffness 5j,ini with the classification boundaries given in 5.2.2.5.

NOTE: Rules for the determination of 5j,ini for joints connecting H or I sections are given in 6.3.1. Rules for the determination of 5j,ini for joints connecting hollow sections are not given in this Standard.

(2) A joint may be classified on the basis of experimental evidence, experience of previous satisfactory performance in similar cases or by calculations based on test evidence.

5.2.2.2 Nominally pinned joints

(1) A nominally pinned joint shall be capable of transmitting the internal forces, without developing significant moments which might adversely affect the members or the structure as a whole.

(2) A nominally pinned joint shall be capable of accepting the resulting rotations under the design loads. 5.2.23 Rigid joints

(1) Joints classified as rigid may be assumed to have sufficient rotational stiffness to justify analysis based on full continuity.

5.2.2.4 Semi-rigid joints

(1) A joint which does not meet the criteria for a rigid joint or a nominally pinned joint should be classified as a semi-rigid joint.

NOTE: Semi-rigid joints provide a predictable degree of interaction between members, based on the design moment-rotation characteristics of the joints.

(2) Semi-rigid joints should be capable of transmitting the internal forces and moments.

5.2.2.5 Classification boundaries

(1) Classification boundaries for joints other than column bases are given in 5.2.2.1(1) and Figure 5.4.

(2) Column bases may be classified as rigid provided the following conditions are satisfied:

- in frames where the bracing system reduces the horizontal displacement by at least 80 % and where the effects of deformation may be neglected if X < 0,5; ... (5.2a)

and Sj,tai >7(2 ^ - 1 ) EIc / Lc and 5j,ini > 48 EIc / Lc otherwise if 5j,ini >30 EIc / Lc.

where:

A0 is the slenderness of a column in which both ends are assumed to be pinned;

Ic, Lc are as given in Figure 5.4.

Key:

Xb = 8 for frames where the bracing system reduces the horizontal displacement by at least 80 % Xb = 25 for other frames, provided that in every storey Kb/Kc > 0,1 *

Zone 2: semi-rigid

All joints in zone 2 should be classified as semi-rigid. Joints in zones 1 or 3 may optionally also be treated as semi-rigid.

Zone 3: nominally pinned, if Sjini < 0,5 EIb / Lb

For frames where Kb/Kc < 0,1 the joints should be classified as semi-rigid.

Kb

is

the

Fc

is

the

lb

is

the

lc

is

the

Lb

is

the

Gc

is

the

Figure 5.4: Classification ofjoints by stiffness

5.2.3 Classification by strength 5.2.3.1 General c "-0

(1) A joint may be classified as full-strength, nominally pinned or partial strength by comparing its design moment resistance Mj,Rd with the design moment resistances of the members that it connects. When classifying joints, the design resistance of a member should be taken as that member adjacent to the joint.

5.2.3.2 Nominally pinned joints

(1) A nominally pinned joint shall be capable of transmitting the internal forces, without developing significant moments which might adversely affect the members or the structure as a whole.

(2) A nominally pinned joint shall be capable of accepting the resulting rotations under the design loads.

(3) A joint may be classified as nominally pinned if its design moment resistance Mj,Rd is not greater than 0,25 times the design moment resistance required for a full-strength joint, provided that it also has sufficient rotation capacity.

5.2.3.3 Full-strength joints

(1) The design resistance of a full strength joint shall be not less than that of the connected members.

(2) A joint may be classified as full-strength if it meets the criteria given in Figure 5.5.

5.2.3.4 Partial-strength joints

(1) A joint which does not meet the criteria for a full-strength joint or a nominally pinned joint should be classified as a partial-strength joint.

a) Top of column b) Within column height r)

Mj,Sd

Either

Mj,Rd

>

0 0

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