《欧规-3 钢结构设计》基础教程 Lecture 05

《欧规-3 钢结构设计》基础教程 共25课第1-10课

Development of

A Trans-national Approach

Course: Eurocode 3

Module 2 : Frame analysis and design

Lecture 5 : Frame Classification and Joint Representation

Summary:

Pre-requisites:

Notes for Tutors:

《欧规-3 钢结构设计》基础教程 共25课第1-10课

Objectives:

References:

《欧规-3 钢结构设计》基础教程 共25课第1-10课

1. Frame classification

1.1 Braced and unbraced

When bracing is provided it is normally used to prevent, or at least to restrict, sway in multi- storey frames. Common bracing systems are trusses or shear walls (Figure 1).

Trusses

Shear w

all

Figure 1 - Common bracing systems

For a frame to be classified as a braced frame, it must possess a bracing system which is adequately stiff.

When it is justified to classify the frame as braced, it is possible to analyse the frame and the bracing system separately as follows :

5.2.5.3 The frame without the bracing system can be treated as fully supported laterally and as having to resist the action of the vertical loads only.

The bracing system resists all the horizontal loads applied to the frames it braces, 5.2.4.3 any vertical loads applied to the bracing system and the effects of the initial sway 5.2.4.4 imperfections from the frames it braces and from the bracing system itself.

5.2.5.3(6)

It should be noted that in a frame with a truss type or frame type bracing system some members participate in the bracing system in addition to being part of the frame structure (without bracing).

For frames without a bracing system and also for frames with a bracing system but which is not sufficiently stiff to allow classification of the frame as braced, the structure is classified as unbraced. In all case of unbraced frames, a single structural system, consisting of the frame and

of the bracing when present, shall be analysed for both the vertical and horizontal loads acting 5.2.4.2 together as well as for the effects of imperfections.

1.1.1 Braced and unbraced classification criteria

The existence of a bracing system in a structure does not guarantee that the frame structure is to be classified as braced. Only when the bracing system reduces the horizontal displacements by at least 80% can the frame be classified as braced.

If no bracing system is provided: the frame is unbraced. 5.2.5.3(2)

When a bracing system is provided the following applies : when br > 0,2 unbr : the frame is classified as unbraced,

《欧规-3 钢结构设计》基础教程 共25课第1-10课

when br 0,2 unbr : the frame is classified as braced,

where

br is the lateral flexibility of the structure with the bracing system. unbr is the lateral flexibility of the structure without the bracing system.

5.2.5.2(1)

5.2.1.2(1b)

5.2.1.2(2c) 5.2.6.2

5.2.5.3(7)

1.2 Sway and non-sway frames

The term non-sway frame is applicable when the frame response to in-plane horizontal forces is sufficiently stiff for it to be acceptable to neglect any additional forces or moments arising from horizontal displacements of its nodes. The global second-order effects (i.e. the P- sway effects) may be neglected for a non-sway frame.

When the global second-order effects are not negligible, the frame is said to be a sway frame.

Braced frameUnbraced frame

(may be sway if bracing is very flexible)(may be non-sway if not sensitive to horizontal loads)

Figure 2 - Braced and unbraced frame

Normally a frame with bracing is likely to be classified as non-sway, while an unbraced frame is

likely to be classified as sway. However, it is important to note that it is theoretically possible for an unbraced frame to be classified as non-sway (this is often the case of one storey portal frame buildings) while a frame with bracing may be classified as sway (possible for multi-storey buildings) (see Figure 2).

When a frame is classified as non-sway, a first-order analysis may always be used.

When a frame is classified as sway, a second-order analysis shall be used. A procedure involving iterations on a first-order elastic analysis is usually adequate for this purpose (see lecture 4 “Frame idealisation and analysis”). Furthermore, on condition that the structure meets certain conditions, a first-order analysis (without any iteration process) may be used either by making a nominal correction to member end forces to allow for the global second-order effects or by analysing for vertical loads and for sway load effects (to be magnified for design) separately (see lecture 6 “Choice of frame analysis and implications for design”).

It should be noted that bracing systems which are themselves frames (or a sub frames) must also be classified as sway or non-sway .

《欧规-3 钢结构设计》基础教程 共25课第1-10课

1.2.1 Sway and non-sway classification criteria

The classification of a frame structure (or bracing system) as sway or non-sway is based on the 5.2.5.2(3) value of the ratio of the design value of the total vertical load VSd applied to the structure to its elastic critical value Vcr producing sway instability (failure in the sway mode).

Obviously, the closer that the applied load is to the critical load, the greater is the risk of instability and the greater are the global second-order effects on the structure (the P- effects).

The classification rule is as follows :

VSd / Vcr 0,1 the structure is classified as non-sway.

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