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Causes of imaging deformation of glass curtain wall façade and design countermeasures

With the increase in the frequency of using glass curtain walls, we have also accumulated experience in the process of glass curtain wall form design, structural details, material selection, and installation effect supervision. Among them, in the selection of hanging samples, we will pay special attention to the flatness of the glass by checking the reflection imaging of the glass.

However, during the construction of a recent project, it was found that the reflection imaging restoration of the glass curtain wall was not satisfactory.

The projection distance from the main entrance of a commercial project is 16 meters from the façade of an office building, but the deformation of the projection engineering crane on the other side of the same building is not so obvious. What is the reason for this? Do other projects have similar problems? With this question in mind, we investigated several projects and found that all of them had varying degrees of imaging distortion.

What is the cause of such widespread image distortion?

We visited the glass factory and curtain wall construction unit, and consulted relevant materials and literature, and found that the possible reasons are as follows:

01 Temperedglass flatness

At present, the glass we use on the building curtain wall is roughly divided into two types: hollow fully tempered glass and laminated semi-tempered glass.

Tempering is the process of secondary heat treatment of the original sheet, and it is well known that glass will be deformed after tempering. Semi-tempered glass, the wind pressure in the quenching process is lower than the process requirements of tempered glass, so the flatness is higher than that of tempered glass.

The industry flatness standard for tempered glass is divided into two types: waveform deformation and bow deformation:

Insulating glass is composed of two or more pieces of glass spaced and sealed, and each glass surface will produce reflected light, if the glass surface is not completely parallel, the reflected light does not coincide completely, and it will produce ghosting.

02 Installation of the extrusion of the glass by the stressed member

Frame-type glass curtain wall often has two forms of open frame and semi-hidden frame, its glass is generally fixed on the inner keel by mechanical screws by aluminum alloy pressing plate or briquetting, when the pressing plate and briquetting are arranged in segmented spacing type, if there is an installation gap between the inner side of the glass and the keel, or the pressing plate does not adopt the fixed distance type, a mounting load will be applied to the glass here when the pressing plate is fixed, resulting in the deformation of the glass in the fixed part of the pressing plate:

03 Environmental effects of temperature during use

Insulating glass is a product in which two or more pieces of glass are evenly separated by effective support, and the perimeter is bonded and sealed, so that a dry gas space is formed between the glasses.

Due to the change of ambient temperature, the internal and external pressure of the insulating glass will increase, causing the concave and convex deformation of the glass.

When the air sealed in the hollow glass cavity is heated and expands in the hot summer environment, the glass expands into a convex mirror, and the lines will become "thin waist shape", and the shape of the reflected object becomes smaller.

In the low-temperature winter environment, the air sealed in the hollow glass cavity is cold and shrinks, and the glass is absorbed into a concave mirror, and the lines will become "bulging belly", and the shape of the reflected object becomes larger.

04 The shape and distance of the target to be imaged, etc

Image the shape of the target

When there is no imaging target, it is impossible to see whether the glass is deformed; When the imaging target is a non-linear object such as a tree or cloud, it is impossible to see whether the glass is deformed; When the imaging target is a linear object such as eaves, straight pipes, and linear contour buildings, it is clear whether the glass is deformed.

Observation distance

When the position of the glass and the position of the imaging target remain the same, the farther the observation distance, the more obvious the image distortion observed.

Therefore, according to the above reasons for imaging deformation, we try to summarize the measures to deal with glass deformation, so as to obtain better glass surface effect in later projects:

The deformation rate of semi-tempered glass is less than that of tempered glass, and semi-tempered laminated glass is used as much as possible in the case of cost support.

Under the controllable cost, the thickness of the outer glass is increased, the deformation rate is alleviated, and the deformation of the outer glass is reduced due to the change of air pressure inside the insulating glass.

Before putting on the wall, different batches of glass are sampled to strictly control the deformation rate.

Effectively control the extrusion of the stressed member to the glass, improve the flatness of the glass keel installation, the hidden frame position adopts a fixed distance briquetting to avoid the extrusion of the briquetting on the glass, and the open frame position adopts a long pressure plate and a reasonable design of the open frame keel structure to prevent the generation of point deformation.

Control the imaging environment

Attention should be paid to the façade, focusing on the imaging environment around the glass curtain wall in the perspective of the main entrances and exits: the façade model should increase the imaging effect of glass reflection, simulate the main viewing angle, deliberate the influence of the imaging environment on the glass surface, evaluate the influence of the imaging environment (especially the linear environment) on the viewing angle, and control the size and integrity of the glass surface.

It is expected that our later projects can gradually solve the problem of glass curtain wall imaging deformation.