Ground Motions and Structures - Earthquake Engineering for Structural Design - page 303

Ground Motions and Structures 303

- Overturning , illustrated in Figure 8.6b, occurs as a result of the overturning loads
arising from the seismic ground motion. Here the building tilts in such a way that the
P-delta effect is sufficient to keep the building moving after the initial movement due to
seismic loads.
- Shear failure may occur when the shear stresses along the sliding surface exceed the
shear strength of the soil, producing the formation of a slip mechanism (Fig. 8.6c). It is
the more general form of the sliding case and again can arise from a combination of the
maximum lateral load with the minimum vertical load, but may also occur due to the
combination of maximum lateral load and maximum vertical load. Liquefaction is a
major problem for this failure mode, especially for low seismicity levels.
The most significant among these failure modes for the ultimate bearing capacity is
the dynamic shear failure due to seismic loads. Figure 8.6c shows the shear failure
kinematical mechanism, composed by three rigid zones in limit equilibrium: the first
one below the foundation being the active zone, the second one in the form of fan,
being an intermediate zone, and the last one, the passive zone. The dimensions of these
zones depend on the angle of internal friction. Considering that the effect of cyclic loads
produces the reduction of this angle, the shape of failure mode is different in
comparison with the static loading. Due to the reducing of the passive zone, the
corresponding bearing capacity decreases drastically. Some numerical examples, using
pseudo-static approach, performed by Choudhury and Subba Rao (2006), show a
reduction of more than 50%, especially if the foundation is embedded in slope ground.

8.2.3Mat Foundations

The mat foundation is a continuous footing, used when the soil conditions at the site are
poor, in order to distribute the heavy loads transmitted by columns and walls across the
entire building area, ensuring a relative uniform load transfer and lowering the contact
pressure compared to the conventional spread foundations (Fig. 8.7). For the seismic
approach, the mat foundation can be assumed to be a rigid body. Due to this
assumption, the main structural problem is the evaluation of the ultimate load due to the
foundation rocking or overturning on the deformable base, when the supporting soil is
soft and weak.
The problem is becoming of increasing engineering interest after the 1995 Kobe
earthquake and especially both the 1999 Kocaeli (Adapazari) and Chi-Chi (Taiwan)
earthquakes (Gazetas et al, 2003). Large settlements, permanent tilting and complete
overturning of numerous buildings, which otherwise retained their integrity, captured
the attention of the world geotechnical and earthquake engineering community. The
liquefaction of shallow sediment and soil layers was evident in the ground surface.
Significant tilting and overturning were observed only in slender buildings, with aspect
ratio H/B > 2, free from other buildings on one of their sides. Small or no rotations were
detected in wider buildings.
The behavior of mat foundations is influenced by the condition of contact between