Frames - The Behaviour and Design of Steel Structures to EC3 - page 350

8.3 Two-dimensional flexural frames
8.3.1 General
The structural behaviour of a frame may be classified as two-dimensional when
thereareanumberofindependenttwo-dimensionalframeswithin-planeloading,
orwhenthisisapproximatelyso.Theprimaryactionsinatwo-dimensionalframe
in which the members support transverse loads are usually flexural, and are often
accompanied by significant axial actions. The structural behaviour of a flexural
frame is influenced by the behaviour of the member joints, which are usually
considered to be either simple, semi-rigid (or semi-continuous), or rigid (or con-
tinuous),accordingtotheirabilitytotransmitmoment.TheEC3classificationsof
joints are given in [1].
In the following sub-sections, the behaviour, analysis, and design of two-
dimensionalflexuralframeswithsimple, semi-rigid, orrigidjointsarediscussed,
together with the corresponding EC3 requirements. An introduction to the EC3
requirements is given in [2].
8.3.2 Frames with simple joints
Simple joints may be defined as being those that will not develop restraining
moments which adversely affect the members and the structure as a whole,
inwhichcasethestructuremaybedesignedasifpin-jointed.Itisusuallyassumed,
therefore,thatnomomentistransmittedthroughasimplejoint,andthatthemem-
bers connected to the joint may rotate. The joint should therefore have sufficient
rotationcapacitytopermitmemberendrotationstooccurwithoutcausingfailure
of the joint or its elements.An example of a typical simple joint between a beam
and column is shown in Figure 9.3a.
If there are a sufficient number of pin-joints to make the structure statically
determinate, then each member will act independently of the others, and may
be designed as an isolated tension member (Chapter 2), compression member
(Chapter 3), beam (Chapters 5 and 6), or beam-column (Chapter 7). However,
if the pin-jointed structure is indeterminate, then some part of it may act as a
rigid-jointed frame. The behaviour, analysis, and design of rigid-jointed frames
are discussed in Sections 8.3.4, 8.3.5, and 8.3.6.
One of the most common methods of designing frames with simple joints is
often used for rectangular frames with vertical (column) and horizontal (beam)
membersundertheactionofverticalloadsonly.Thecolumnsinsuchaframeare
assumedtoactasifeccentricallyloaded.Aminimumeccentricityof100mmfrom
the face of the column was specified in [3], but there is no specific guidance in
EC3.Approximateproceduresfordistributingthemomentcausedbytheeccentric
beam reaction between the column lengths above and below the connection are
given in [1]. It should be noted that such a pin-jointed frame is usually incapable
ofresistingtransverseforces,andmustthereforebeprovidedwithanindependent
bracing or shear wall system.