Page 236 - Forensic Structural Engineering Handbook

7.20 CAUSES OF FAILURES

the Reynolds number effects on pressures and forces. Also, the response characteristics of the
pressure-sensing instruments must be matched properly to the recording requirements. 1
Tunnels that are suitable for modeling wind response typically have apertures 6 to 12 ft
(2 to 4 m) wide and 6 to 10 ft (2 to 3 m) high. Wind normally is drawn at 25 to 100 mi/h
(10 to 45 m/s) over a length of 50 to 100 ft (15 to 30 m) before reaching the model. Suitable
wind tunnels may be either open circuit or recirculating.
Several types of models can be constructed, depending on the nature of the data that need
to be acquired. A rigid pressure model is the simplest, and it can be used to measure local peak
pressures for analysis of cladding and mean pressures for overall mean loads. A rigid high fre-
quency base balance model can be used to measure overall fluctuations of loads for analysis
of dynamic response. Aeroelastic models are necessary to investigate the potential for struc-
tural motions themselves to affect wind loading on the structure of interest. Aeroelastic mod-
els require accurate, scaled representations of the flexibility of the modeled structure. 1

EARTHQUAKE LOADS

Nature and Consequences of Earthquake Loads
Earthquake loads on structures originate not from externally applied forces or pressures,
but from dynamic structural distortions that occur as the ground below a structure is dis-
placed. In the beginning moments of a seismic event, ground movements induce founda-
tion motions. Immediately, structural stiffnesses from shear walls, braced frames, moment
frames, or incidental structural and architectural elements develop shear forces which are
transmitted from one building level to another. Almost immediately, the entire building
sways dynamically in response to the ongoing ground movements. It is the interplay among
structural masses at each level and interstory stiffnesses and relative distortions that gener-
ate member forces that must be supported by the structure.
Ground movements are both horizontal and vertical. However, most commonly it is the
horizontal movements that cause the greatest distress in buildings. Often, the accelerations
and motion amplitudes are larger in the horizontal direction than in the vertical direction.
Also, horizontal ground motions tend to cause forces that are more difficult for conven-
tional structures to support. Substantial frame or shear wall strength and ductility are
needed to prevent lateral failures during severe earthquakes. However, the inherent strength
designed into structures to support gravity loads usually has sufficient reserve to support
loads from vertical components of seismic ground motions. When axial loads induce com-
pression failures in columns, the sources of those vertical loads usually are overturning
moments caused by horizontal excitation.
Clearly, vertical excitations cannot be ignored entirely. The 1994 Northridge earth-
quake demonstrated that significant vertical excitation can cause damage in structures.
For most common structures in regions of severe earthquake activity, there are few
practical ways to design lateral load systems to remain elastic during seismic events. The
interstory forces that would develop during elastic response exceed capacities that can be
provided practically and economically. Therefore, conventional design philosophy accepts
that building frames will deform inelastically. The inelastic distortion effectively dissipates
the kinetic energy that is imparted to the structure by ground motions, and “softens” the
response of the structure to the excitation.
It follows that structures that perform as designed during severe earthquakes will
sustain damage. This anticipated damage can include large permanent deformations
(Fig. 7.13); serious cracking of walls, columns, and beams; and partial disintegration of
joints (Fig. 7.14)—particularly in concrete structures. Sometimes the expected damage
is difficult or impossible to repair economically (Fig. 7.15). However, it is this expected
damage that saves the building from collapse.