EC8 Part 1 classifies concrete buildings into the following structural types:
• dual system, which may be either frame or wall equivalent
• ductile all syste
• system of large, lightly reinforced walls
• inverted pendulu syste
• torsionally flexible syste.
Apart from torsionally flexible systems, buildings may be classified as different systems in the two orthogonal directions.
Frame systems are defined as those systems where moment frames carry both vertical and lateral loads and provide resistance to 65 per cent or more of the total base shear.
onversely, buildings are designated as all syste s if alls resist 65 per cent or ore of the base shear. alls ay be classed as either ductile alls, hich are designed to respond as vertical cantilevers yielding just above a rigid foundation, or as large lightly reinforced alls. uctile alls are further subdivided into coupled or uncoupled alls. oupled alls co prise individual walls linked by coupling beams, shown in Figure 5.1, resisting lateral loads through o ent and shear reactions in the individual alls together ith an axial tensile reaction in one all balanced by an axial co pressive reaction in the other to create a global o ent reaction. he agnitude of these axial
Figure 5.1 Coupled Wall System loads is limited by the shear forces that can be transferred across the coupling beams. In order to qualify as a coupled wall system, the inclusion of coupling beams must cause at least a 25 per cent reduction in the base moments of the individual walls from that which would have occurred in the uncoupled case. As coupled walls dissipate energy, not only in yielding at the base but also in yielding of the coupling bea s, buildings ith coupled alls ay be designed for lo er inertial loads than buildings ith uncoupled alls to reflect their greater ductility and redundancy.
Large lightly reinforced walls are a category of structure introduced in EC 8 and not found in other national or international seis ic codes. hese alls are assu ed to dissipate energy, not through hysteresis in plastic hinges, but by rocking and uplift of the foundation, converting kinetic energy into potential energy of the structural ass and dissipating this through radiation da ping. The dimensions of these walls or their fixity conditions or the presence of stiff orthogonal walls effectively prevent plastic hinging at the base. These provisions are likely to find ide application in heavy concrete industrial structures. o ever, since this book is concerned pri arily ith conventional building structures, this type of structure is not considered further here.
Dual systems are structural systems in which vertical loads are carried primarily by structural frames but lateral loads are resisted by both frame and wall systems. From the earlier definitions, it is clear that, to act as a dual system, the frame and wall components must each carry more than 35 per cent but less than 65 per cent of the total base shear. When more than 50 per cent of the base shear is carried by the frames, it is designated a frame-equivalent dual system. Conversely, it is termed a wall-equivalent dual system when walls carry more than 50 per cent of the base shear.
Torsionally flexible systems are defined as those systems where the radius of gyration of the floor ass exceeds the torsional radius in one or both directions. An example of this type of system is a dual system of structural frames and walls with the stiffer walls all concentrated near the centre of the building on plan.
Inverted pendulum systems are defined as systems where 50 per cent of the total ass is concentrated in the upper third of the height of the structure or where energy dissipation is concentrated at the base of a single element. A common example would normally be one-storey frame structures. However, single storey frames are specifically excluded from this category provided the normalised axial load, Ud, does not exceed 0.3.
where NEd is the applied axial load in the seismic design situation, Ac is the area of the column and fcd is the design compressive strength of the concrete (i.e. the characteristic strength divided by the partial material factor, which can usually be taken as 1.5).
The treatment of both torsionally flexible and inverted pendulum systems within EC8 is discussed further in Section 5.4.
5.2.3 q Factors for concrete buildings
Table 5.1 shows the basic values of q factors for reinforced concrete buildings.
hese are the factors by hich the inertial loads derived fro an elastic response analysis may be reduced to account for the anticipated non-linear response of the structure, together with associated aspects such as frequency shift, increased damping, overstrength and redundancy. The factor, a Ja, represents the ratio between the lateral load at which structural instability occurs and that at which first yield occurs in any member. Default values of bettveen 1.0 and 1.3 are given in the code with an upper limit of 1.5. Higher values than the default figures may be utilised but need to be justified by pushover analysis.
For walls or wall-equivalent dual systems, the basic value of the behaviour factor then needs to be modified by a factor, k , which accounts for the prevailing failure ode of the all, the q factors being reduced on squat alls here ore brittle shear failure odes tend to govern the design.
where a0 is the prevailing aspect ratio, h Jlw, of the walls.
A lower limit of 0.5 is placed on k for walls with an aspect ratio of 0.5 or less, with the basic q factor being applied unmodified to walls with an aspect ratio of 2 or more.
The basic q0 factors tabulated are for structures that satisfy the EC8 regularity criteria, the basic factors needing to be reduced by 20 per cent for structures that are irregular in elevation according to the criteria given earlier in hapter 4.
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