Damage to Structures - Fundamentals of Earthquake Engineering - page 333

B.4.2 Requisites of Masonry Structures
Masonry structures exhibit high vulnerability to seismic forces. To prevent the damage patterns outlined
in Section B.2.2, it is necessary to account for the following:
(i) Tight quality controls should be performed on construction materials, especially of mortar and
masonry units in adobe and stone- masonry buildings.
(ii) Reinforced and confi ned masonry are preferable to URM. The effect of reinforcement is to
limit the amount of diagonal cracks and prevent toppling, particularly in perimeter walls. In
systems with confi ned masonry, this reinforcement should be anchored into the surrounding
frame.
(iii) Structural and non -structural walls should possess limited slenderness to prevent global buck-
ling. Connections between orthogonal structural and non-structural walls are adequate to avoid
overturning due to out- of - plane seismic forces.
(iv) Adequate connections should be provided between structural walls and slabs at each fl oor.
Slabs act as horizontal diaphragms and distribute horizontal seismic forces among vertical
structural walls as illustrated in Section A.1. Diaphragmatic actions should, however, always
be checked and are signifi cantly reduced by the presence of large openings. Bond beams should
be located at each fl oor along perimeter walls to achieve monolithic behaviour of masonry
structures.
(v) Low values of length -to-width ratios in piers of structural walls should be avoided. This type
of geometric layout can give rise to severe brittle shear failures as also discussed in Section
A.2.2 .
(vi) Large openings should be limited in structural masonry walls. They signifi cantly lower the
strength capacity under earthquake loads. Additionally, diagonal cracks often originate at the
corner of large openings.
(vii) Adequate building layout is a fundamental requisite to survive moderate and severe earthquakes
( see Appendix A). Simple and symmetrical confi gurations along each principal axis with a
suffi cient number of structural walls, and with approximately the same cross- sectional area and
stiffness should be provided in each direction of the building.
B.4.3 Requisites of Steel and Composite Structures
Steel and composite structures have shown generally adequate seismic performance under moderate
and severe earthquakes. Their energy dissipation capacity is endangered if the requisites summarized
below are not satisfi ed:
(i) Brittle failure modes, such as weld cracks and fracture, bolt fracture in tension or shear, should
be avoided, even in response to a major seismic event.
(ii) Local buckling and global buckling can be avoided by adopting adequate width-to-thickness
ratios and member slenderness.
(iii) Excessive column panel zone deformations in beam -to-column connections should be pre-
vented. These deformations may signifi cantly increase lateral drifts of unbraced framed struc-
tures and impair their global stability (increased P - Δ effects).
(iv) Overstrength due to the presence of composite slabs should be accounted for in the evaluation
of the inelastic seismic demands of capacity-designed components, e.g. beams in MRFs, diago-
nal braces in CBFs and links in EBFs.