Why did the failure patterns of brick-concrete structures in this earthquake mostly take on an “X” shape rather than along the weaker mortar joints?

It must be related to the shape of the seismic waves. During the Tangshan earthquake, individual telegraph poles remained intact, but the railway tracks were twisted into knots.

Yes, it’s related to seismic waves; earthquakes produce both P-waves and S-waves. The S-waves arrive first, followed by the P-waves!

It is by no means an exaggeration to describe the situation when an earthquake strikes as \"the earth shaking violently\" and \"mountains collapsing.\" Since buildings are attached to the Earth’s surface, the way they are damaged by earthquakes is primarily influenced by the propagation of seismic waves. Simply put, there are three ways buildings can be damaged: up and down shaking, horizontal swaying, and side-to-side twisting. Most of the time, it is still the combined effect of the three methods. 　　The modes of seismic wave propagation include P-waves, S-waves, and surface waves. Due to the complexity of the rock properties in the Earth’s surface layer, complex phenomena such as \"vortices\" in fast-flowing water can also occur during propagation. 　　Longitudinal waves cause buildings to shake up and down with great force; the buildings are not able to move in time, which results in a sudden increase in dynamic loads on the columns and walls on the ground floor. Combined with the weight of the upper parts of the building, if this exceeds the bearing capacity of the columns and walls on the ground floor, they will collapse. Once the lower layer collapsed, the weight of the upper floors acted like a hammer striking down, crushing the second floor as well and causing a chain reaction of collapses; the entire building simply collapsed, with what was originally the third floor instantly becoming the first floor. 　　Surface waves cause buildings to sway horizontally, acting as a force that pushes the building back and forth in a horizontal direction. If the strength or deformation capacity of the columns and walls at the base is insufficient, it can cause the entire building to tilt or collapse in one direction; this phenomenon is commonly seen in earthquake-prone areas. 　　The third function is torsion. The reason for torsion is that some seismic waves arrive in a rotational motion, while in other cases it is caused by the difference in time it takes for surface waves to reach the two ends of a building. This situation causes the building to twist. Buildings generally have poor torsional resistance and are prone to being damaged by twisting forces. In the earthquake-stricken area, the corners of some houses collapsed; this is the common scenario. 　　Once shaking up and down, shaking side to side, and twisting occur simultaneously, the destructive power becomes even more terrifying. In the areas close to the epicenter, these three mechanisms often act together, resulting in great destructive power. 　　Furthermore, each building has its own specific natural frequency; if this frequency is close to the frequency of seismic vibrations, it can lead to resonance-like effects, resulting in even more severe damage. 　　Another form of damage is called “liquefaction”. If the foundation of a building is made of fine sand and the house is built on it, when the ground shakes, the sand particles shift to the sides, causing the house to sink, leading to tilting or even collapse. Many houses were damaged in this way during the Tangshan earthquake. The Wenchuan earthquake did not see any occurrence of liquefaction, but there were many landslides caused by unstable mountains, and many houses built on slopes or at the foot of mountains were damaged in this way.

Regarding the issue on the first floor, the ‘X’-shaped failure of the brick wall is a typical example of shear failure, resulting mainly from horizontal seismic loads. For an explanation of the ‘X’ pattern, please refer to the section on the directions of principal stresses in Mechanics of Materials. When building brick walls, it is required that the bricks be arranged in a staggered pattern – one row straight and then one row offset –; it is absolutely not allowed for the mortar surfaces to be in direct contact with each other, as this would lead to failure due to continuous cracks. As for the issue on the fourth floor, the collapse of the ground floor often occurs when the ground floor is used as a shop. Such shops have large openings, and there are no partition walls to bear the horizontal loads, resulting in low lateral stiffness and thus a weak layer. Regarding liquefaction, please refer to the section on the principle of effective stress in Soil Mechanics. Slopes are vulnerable to two things: water and earthquakes. Avoid going to such areas during heavy rains, continuous precipitation, or earthquakes. This post was last edited by tpjj on 2009-3-27 at 19:01

To correct what was said on the 3rd floor: during an earthquake, the propagation speed of longitudinal waves is faster than that of shear waves.