Preliminary Analysis of Seismic Behavior of Glass Windows and Glass Curtain Walls During Extraordinary Earthquakes.

【Summary】

According to the official measurement by the Turkish Emergency Disaster Management Agency, an earthquake with a magnitude of 7.8 occurred in Turkey (38.00 degrees north latitude, 37.15 degrees east longitude) at 18:24 on February 6 (13:24 local time on the 6th) with a focal depth of 20 kilometers.

Earlier news showed that at 4:17 am on the 6th in Turkey, an earthquake with a magnitude of 7.8 occurred in the southern part of the country near the Syrian border. According to the quick report of the Turkish Emergency Disaster Management Agency, at present, this is an earthquake swarm-type earthquake, and the epicenters of the two 7.8-magnitude earthquakes are about 96 kilometers apart.

The epicenter of the earthquake was located near Gaziantep, Turkey. After the earthquake, pictures from the local news media showed crooked buildings, collapsed houses, and broken roads everywhere. Many citizens ran to the street to escape.

In a severe earthquake, buildings suffer huge losses, especially vulnerable glass buildings such as glass doors and window buildings or glass curtain wall buildings are severely damaged. The investigation of earthquake damage shows that there is a clear correlation between the damage to fixed glass windows and the damage to the main structure of the building. The high-rise glass curtain wall is intact. So far, there have been no cases of glass curtain wall buildings being damaged by earthquakes in the past.

^{Figure 1 The On-site Situation of Glass Windows and Glass Curtain Walls During the Severe Earthquake}

【Keywords】

Buildings with glass doors and windows

Glass curtain wall buildings

Seismic damage investigation

Seismic design of glass buildings

I. The overview

1.1 The adoption of shockproof glass

Generally speaking, tempered glass is also called shockproof glass. Tempered glass is a kind of prestressed glass with a certain strength, and chemical or physical means are generally used to exert pressure on the glass. Under the action of external force, the surface stress of the glass is first eliminated, thereby increasing the bearing capacity of the glass, and increasing the resistance to wind pressure, cold and heat, and impact of the glass.

The strength of tempered glass is several times higher than ordinary glass, its bending strength is 3-5 times that of ordinary glass, and its impact resistance is 5-10 times that of ordinary glass. Its bearing capacity is increased and its brittleness is improved. Even if it is broken by tempered glass, it will become non-sharp fragments, which greatly reduces the damage to the human body. Compared with ordinary glass, tempered glass has 2 to 3 times the resistance to rapid cooling and rapid heating, and can generally withstand temperature changes above 150°C, which has a significant effect on preventing thermal bursts.

In addition, shockproof glass can also be made of a laminated material called PVB (polyvinyl silicone body). It consists of several layers of separated film and glass spacer adhesive, forming an "air cushion" effect between the multi-layer film and glass, which can resist the impact of the outside world. When stressed, the PVB film stretches and shrinks, which can effectively reduce the internal pressure between two layers of glass or multiple layers of glass, so it will not damage the glass. After laminating two or more layers of glass, glass manufacturers can also use hollow glass after lamination to carry out deep processing of glass to produce laminated insulating glass, making it more shock-proof and explosion-proof. Therefore, the superiority of shock-proof glass is reflected in: it has the characteristics of explosion-proof, anti-damage, anti-aging, etc., which can effectively increase the strength of the building structure, to better protect the structure of glass doors and windows buildings or glass curtain wall buildings.

^{Figure 2 The PVB film laminated insulating glass}

1.2 The fix glass doors and windows & the glass curtain walls

Both fixed windows and glass curtain walls are maintenance structural systems composed of panels and supporting structures. The difference is that the fixed window is usually embedded in the plane of the column or wall, and must have the same lateral displacement as the main structure during an earthquake; while the structure of the curtain wall allows a slight relative displacement between the glass curtain wall and the main structure, or make the glass curtain wall itself have greater deformation capacity, which can withstand the lateral displacement of the main structure during the earthquake.

Therefore, damage to glass windows is common in previous major earthquakes. Fixed windows installed in buildings with severely cracked or damaged walls or tilted frames often have window frames distorted, glass broken, and scattered; The glass curtain wall is rarely damaged by earthquakes and is often kept intact.

In the 2.6 earthquakes in Turkey, there were 33 large and medium-sized cities within 300 kilometers of the epicenter. Gaziantep itself is the sixth largest city in Turkey with a population of more than 2 million; the earthquake also caused damage to Syria, Lebanon, Iraq, Israel, and other countries affected to varying degrees. The earthquake caused the collapse of a large number of buildings in the Aleppo province of Syria, and many people were trapped under the rubble. Witnesses said that in Damascus, Syria, Beirut, the Lebanese capital, and Tripoli, people worried about the collapse of buildings gathered on the streets to avoid danger. In areas and cities that have experienced strong earthquake shocks, the seismic performance of glass doors, windows, and glass curtain walls show the same regularity as previous major earthquakes.

^{Figure 3 The fix glass doors and windows & the glass curtain walls}

1.3 Estimation of lateral displacement

Generally speaking, it is impossible for us to actually measure the plane inward displacement of glass windows and glass curtain walls in buildings during earthquakes. However, by observing the structural damage of the plane where the glass windows and curtain walls are located, it is possible to estimate the large horizontal displacement they experienced during the earthquake.

The damaged condition of the brick wall is the most applicable measure. Not only because brick walls are the most commonly used, but also because the degree of damage of brick walls has a good correspondence with the horizontal displacement it suffers. Over the years, a large number of performance test studies of masonry structures have been carried out.

For unreinforced brick masonry, the basic laws are as follows:

• The horizontal displacement angle is less than 1/800 and the brick wall is intact
• The horizontal displacement angle 1/800～1/500 Slight crack
• The horizontal displacement angle 1/500～1/300 Severe cracking
• The horizontal displacement angle 1/300～1/150 Serious damage, falling bricks, partial collapse
• The horizontal displacement angle is greater than 1/150 collapsed

^{Figure 4 The horizontal displacement angle of glass curtain walls}

Therefore, we can estimate the horizontal lateral displacement of the plane by referring to the damage degree of the brick wall in the plane where the glass doors, windows, and curtain walls are located, to determine the working performance of the architectural glass doors, windows, and glass curtain walls under this displacement.

2. The performance of fixed windows in earthquakes

In different surviving buildings, the degree of earthquake damage is different, and the performance of glass windows is also different accordingly:

2.1 Buildings with no earthquake damage to the main structure

If the main structure of glass doors and windows is intact, and the load-bearing brick wall or infill brick wall has no visible earthquake damage, it can be considered that its horizontal lateral displacement is not greater than 1/800. At this time, the glass doors and windows will generally not be damaged.

^{Figure 5 The glass windows in Turkey earthquake are also intact}

Figure 5 The teaching building in a certain area of Turkey is intact, and its glass windows are also intact. The earthquake intensity in this area is relatively high. In the picture, some glass doors and windows behind the lens were damaged, but the brick wall was not damaged. It can be judged that the area was not greatly affected by the earthquake, and the glass door and window frames and the main structure of the glass doors and windows are still intact.

2.2 Buildings with severely cracked brick walls

When a large number of coarse cracks and even very thick crossed oblique cracks appeared on the brick wall, it indicates that the building suffered repeated horizontal movements during the earthquake, and the displacement reached 1/300. In this case, the window glass will be broken in large numbers.

^{Figure 6 Although the brick walls were broken, but more glass left behind}

Figure 6 shows the buildings preserved by the earthquake in Turkey. Although the brick walls were broken, it may be due to the relatively high rigidity, small lateral movement, and more glass left behind. Similar examples are isolated. After a strong earthquake, the fixed windows of the remaining buildings are generally damaged more seriously. We can also see that the brick walls of two adjacent buildings have different degrees of damage, which means that their lateral displacements are different. Glass windows The damage is also very different.

2.3 Buildings with severely damaged brick walls and frame structures with large lateral displacement

When the lateral displacement of the brick wall exceeds 1/300 of the story height, it will cause major damage, bricks falling, and even partial collapse. At this time, the window frame buckles protrude and even throws out, the glass is basically broken and scattered, and often only the window opening is left on the wall.

A four-story frame-filled brick wall structure was used in a certain area of Syria. During the earthquake, the frame had a large sideways movement, the frame columns were tilted, the frame beams were bent in a wave shape, and the filled brick walls had cross-wide cracks, and some even collapsed; the glass windows were seriously damaged.

^{Figure 7 The cell-filling walls collapsed, and the glass windows flew out}

The beam-column section of the frame structure is small, and the horizontal displacement in the earthquake is generally relatively large. Figure 7 shows a local multi-story glass frame structure house in Turkey. Under the action of huge horizontal thrust, the frame tilted greatly, many cell-filling walls collapsed, and the glass windows flew out.

2.4 Damage caused by out-of-plane displacement

In addition to a large amount of damage caused by in-plane deformation, some damage is also caused by out-of-plane displacement. The roof of some glass curtain wall buildings slides outward, pushing the glass windows out of the plane and deforming them, causing the glass windows on the top floor to shatter and fly away, while the glass windows on other floors are intact.

2.5 The movable window sash is slightly damaged

The casement window sash is often in the open state during an earthquake. At this time, the sash is not in the plane of the wall, and it is rarely squeezed by the deformation of the wall, so the casement window sash during opening is not damaged much.

The push-pull sash can slide horizontally. Even when the building deforms greatly, it can have a certain degree of relative displacement relative to the window frame. The actual deformation is smaller than the window frame, so the degree of damage is smaller. It can be seen from Figure 6 that the window frame has become a parallelogram under the extrusion of the wall, but the push-pull sash remains rectangular and stuck in the window frame intact.

2.6 Deformation tolerance of fixed window

Judging from the performance of glass windows in the 6th Feb. earthquakes in Turkey, fixed windows can withstand 1/500 of the inter-story displacement angle without damage; even individual glass construction projects may have glass cracks, and glass doors and windows were broken The number will also be very small.

The reason for this is that during construction and installation, the doors and windows are usually left on the wall first, and then the doors and windows are installed. There will be a gap between the window frame and the opening, and there will be a gap between the sash and the window frame. They can absorb some deformation and can slightly improve the earthquake resistance of the window.

3. The performance of the glass curtain wall in the 6th Feb. earthquakes in Turkey

3.1 The deformation performance of glass curtain wall

Due to the structural characteristics of the curtain wall itself, it can withstand large in-plane deformation without damage. The in-plane deformation tests of glass curtain walls carried out in many glass construction projects show that glass curtain walls that meet the design requirements of the code can withstand in-plane horizontal displacements of 1/100 story height without damage, and can even withstand larger displacements.

As a professional manufacturer of glass deep-processing equipment and a supplier of insulating glass testing equipment, LIJIANG Glass has conducted many shaking table tests on glass curtain walls, using glass, stone, aluminum panels, ceramic panels, glass-ceramics, stone powder panels, and other wood-based panels as panels. When the acceleration of the table top is 0.7-0.9g (equivalent to an intensity of about 9-0 degrees), the displacement angle reaches 1/70-1/50, and they can still be kept intact without any damage. These test results show that the glass curtain wall has a much larger in-plane deformation capacity than the fixed glass doors and windows, and can be protected from earthquake damage in high-intensity earthquakes.

3.2 The performance of glass curtain wall in an earthquake

So far, LIJIANG Glass has not received any reports of earthquake damage to glass curtain walls on uncollapsed houses in the epicenter of the 6th Feb, earthquakes in Turkey. The reason is very simple. When the glass curtain wall reaches its tolerable in-plane displacement angle limit of 1/70 to 1/50, most of the main structure collapsed. Therefore, it can be said that as long as the house does not fall, the glass curtain wall will not be damaged, and at most, there will be some slight damage in individual cases.

3.3 Comparison of seismic performance of curtain wall with other building components

If there are curtain walls and other building components in a glass building and they are subjected to the same earthquake, then the seismic performance of the curtain walls and other components will be comparable. From the examples in Figure 5 to Figure 7, it can be seen that when the brick walls, fixed windows, and facing bricks have been significantly damaged by the earthquake, the curtain wall of the same building is intact.

4. The summary

Judging from the current preliminary understanding, especially the current situation of the 6th Feb. earthquakes in Turkey and the earthquake in Syria, there is still a big difference in the earthquake resistance of fixed glass doors and windows compared with glass curtain walls.

In the strong earthquake area, the fixed windows in the buildings that did not collapse, the degree of glass damage varies greatly, depending on the lateral displacement of the main structure of the glass building. When the lateral displacement angle in the plane is not greater than 1/500, the door and window glass is expected to remain intact; when the displacement angle is greater than 1/300, the damage to the door and window glass is very serious.

The displacement resistance of the glass curtain wall is much greater than that of the fixed door and window glass. As long as the main structure has not collapsed during an earthquake, the curtain wall can generally remain intact. The performance of the building curtain wall in the Turkey 2.6 earthquake fully reflects this phenomenon.

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