Basic knowledge of curtain wall design

Building curtain walls have achieved considerable development in our country. Some systems have reached or surpassed foreign standards, and a large number of high-quality projects and high-tech projects have emerged. However, there are also many projects that have problems, even unforgivable safety issues.

After recent years of engineering testing, laboratory testing, and discussion at technical conferences, it was discovered that these problems with the curtain wall can be avoided by design means. This article hopes to be helpful to the improvement of the design level of my country's curtain wall industry.

Glass curtain wall

Glass curtain wall is a widely used exterior wall system. The dominant position in the exterior wall of the building is unshakable, and many high-quality projects have appeared.

Direct bonding of fluorocarbon coating and structural adhesive

The bonding of some structural sealants and fluorocarbon coatings cannot meet the requirements of the curtain wall. Therefore, measures should be taken to seal the joints between the sub-frame and the glass of the hidden frame curtain wall glass assembly and the joints of the fluorocarbon coating panel to improve the adhesion. relay. There are a variety of measures to choose from: (a) Apply primer and then inject structural adhesive. However, some experts believe that this method is unreliable. It is a "two-layer skin", and there is no more convincing positive report to prove it. This method is indeed effective, so further observation and research are needed; (b) Adopting a combined profile structure, directly bonding the structural adhesive part to the other parts of the profile, and directly bonding the structural adhesive part with anodizing treatment; (c) Fluorocarbon During the spraying process, cover the area to be bonded and keep its surface still anodized; (d) Take remedial measures, remove the coating on the surface to be bonded with sandpaper, etc., relying on natural oxidation (about 5μm).

Self-tapping screw connection

The self-tapping screw connection is a general connection or positioning connection. As a structural connection, its reliability is poor.

Mixed use of steel and aluminum profiles (aluminum clad steel)

The inner surface of the square steel pipe is not easy to achieve shot peening, and it is prone to quality problems during hot-dip galvanizing, resulting in low corrosion resistance; the steel-aluminum matching gap should be relatively tight, otherwise the common force cannot be achieved, which will cause difficulties in preventing bimetallic electrochemical corrosion .

Short gland

The exposed frame curtain wall adopts pressure cover crimping, on the one hand, it is convenient to realize an equal pressure cavity, and on the other hand, it can be connected with the buckle cover. The use of discontinuous glands (short glands) can reduce costs, but problems such as uneven glass and inability to form isostatic chambers will occur.

The connecting piece between the beam and the column adopts two-point connection

Curtain wall beams often have "sagging heads". The reasons may be: (1) the bearing capacity of the beam does not meet the requirements; (2) the connection between the beam and the column is relatively weak. For example, the connection between the beam and the column uses two bolts (nails) ) Connection, due to its poor torsion resistance, causes the curtain wall beam to twist.

Large cross-section decorative strip without drip line

There will be dust accumulation on the upper surface of the large cross-section decorative strip. If the drip line is not set, it will cause more flow marks on the surface of the curtain wall. If a drip line is set at the front end of the decorative strip, it can effectively prevent water and dust from flowing on the surface of the curtain wall.

Decorative cover and live bayonet fit

The decorative cover should be connected with the bayonet of the extruded profile. This bayonet has a fixed size, high precision, and can achieve a reliable connection. The precision of the bayonet formed after the screw connection fails to meet the requirements, and the connection is unreliable.

No heat tight line is set in the opening cavity

The heat-tight wire is widely used in the design of energy-saving aluminum alloy windows, but it is less used in the opening cavity of the building curtain wall, resulting in low energy-saving effect at the opening part of the curtain wall.

Hidden frame curtain wall adopts non-fixed distance pressing plate

Hidden frame curtain wall and semi-hidden frame curtain wall usually adopt pressure plate (press block) to transmit force, and the distance between them is generally not more than 300mm. There are two kinds of fixed distance and non-fixed distance pressure plates. After the fixed distance pressing plate is fastened by the connecting bolts, the pressing gap is relatively fixed, and the pressing force on the glass panel subframe is relatively consistent, which is convenient for absorbing deformations such as structure and temperature, reducing frictional noise, and avoiding uneven pressing due to the pressing block This causes image distortion on the glass panel.

False exposed frame hidden frame is not designed according to hidden frame curtain wall

The false exposed frame is usually installed with a decorative strip at the joint of the hidden frame curtain wall to achieve the decorative effect of the exposed frame curtain wall. This structure should adopt the design method of the hidden frame curtain wall to design the insulating glass and structural glue, that is, the second sealant should be sealed with silicone structural glue. If polysulfide glue is used as the second seal of insulating glass, although it may not be damaged under ultraviolet radiation, there are still unsafe factors.

No pallet at the bottom of the empty glass in the hidden frame

The insulating glass structural glue bears the shearing force for a long time, which is detrimental to the service life of the structural glue. Therefore, JGJ02 requires that a pallet should be provided under the glass. The direct connection between the pallet and the beam is reasonable, and it can be designed as a snap connection or a bolt connection; the use of bolts and the subframe connection of the glass component may affect the injection of the structural glue, and there are quality defects. It is recommended to use it with caution.

Insulation strip bears shear force

Insulation strips play the role of structural force transmission and reduce heat transfer in the insulation profile, and are widely used by curtain wall profiles. With a strip-through structure, a composite production line is used to force heat insulation strips and aluminum alloy profiles. Therefore, there is a cold work hardening phenomenon in the pressed part of the heat insulation strip and the aluminum alloy profile, and even some micro-crack defects. If the beam of the curtain wall adopts heat-insulating profiles, structural measures should be taken to avoid the shearing force of the heat-insulating strips and prevent damage to the connecting parts of the heat-insulating strips and aluminum alloy profiles. Generally, a pallet or a stronger aluminum alloy profile is used to bear the weight of the glass.

There are defects in the anti-dropping design of the hook-type opening fan

The curtain wall opening window usually adopts a top-hung structure, but due to design defects, the phenomenon of fan falling often occurs in the project, and the probability of falling in the closed state of individual projects is higher. The main reason is that these projects have no anti-dropout design, or the hook anti-dropout design is unreasonable, or the overlap depth of the hook is not enough, or the wall thickness of the profile at the hook is too thin.

Arc crimping is not used at obtuse angles

The use of traditional fixed distance clamps cannot meet the requirements of crimping, and a connection structure with adjustable angle should be adopted.

Immutable glass notch

When designing the profile, it is necessary to consider the operability during construction, the use of variable notches can be fine-tuned, and installation is convenient, so fixed glass notches should be avoided as much as possible.

Unlimited horizontal seam unit

The small unit panel is widely used, and its insertion depth should meet the requirements. There are accidents of falling fans during the project. For the small slotted unit, because there is no sealant for positioning, structural measures should be taken for positioning, otherwise the frame fan The indirect connection may fail, posing safety risks.

The secondary sealant for insulating glass leaking from the edge is not used for structural glue

Insulating glass should be sealed with double sealant, and the second sealant of hidden frame, semi-hidden frame, dummy frame and point-supported insulating glass panel should be sealed with silicone structural glue, so as to be able to transmit force reliably and improve the UV resistance of insulating glass Irradiation capacity, its width should be determined through structural calculations. Polysulfide glue has poor resistance to ultraviolet radiation, so the hollow glass that uses polysulfide glue for the second seal cannot be used for the above-mentioned insulating glass. Due to the use of polysulfide glue as the second sealing material in some projects, large quantities of outer glass fell, which became a serious safety hazard for curtain wall projects.

Hollow glass size piece

Insulating glass uses large and small pieces of structure, which has some advantages in some applications, especially it can provide more space for profile design, but there are also many shortcomings: (1) It is inconvenient to use mechanical glue injection; (2) The force transmission method is unreasonable , It may even cause the relative displacement between the glass, and finally cause the insulating glass to leak and fail; (3) There are also some engineering large and small pieces of insulating glass that do not use structural glue. There are also some disputes about the calculation of large and small pieces, mainly in terms of load distribution, and more attention should be paid to the design.

Open fan hollow glass "big cap"

"Large cap" is an extreme form of large and small pieces of hollow glass, which is used in some open fan designs. Once the large piece of glass is broken, the small piece of glass will lose connection and fall off.

The hollow layer of hollow glass is unreasonable, and there are phenomena such as sticking and interference

For large-area hollow glass, adsorption may occur when a 9mm hollow layer is used. Therefore, the size of the hollow layer should be determined comprehensively according to structural requirements and thermal requirements.

Frosted tempered glass

The frosted tempered glass, before or after tempering, will destroy the stress distribution on the glass surface and easily induce self-exposure of the glass. The frosted point support glass is more dangerous. Long narrow glass should not be supported by short sides.

Forced glass installation

The bending strength of glass will decrease over time. The reason is that the microcracks on the glass surface will continue to expand. Therefore, when designing the curtain wall, the glass should be made to work in a free state. However, in actual projects, some glass does work under unnecessary permanent loads, such as forced installation, crimping and sealing. A certain project in Beijing adopts a crimped and sealed structure, and the probability of glass breakage is relatively high, which is worth learning.

Unreasonable deformation joint design

The design of deformation joints is a difficult point. Architects cannot accept the design principle that some components or panels can be destroyed after deformation. Therefore, the deformation joints should be able to absorb deformation (including the deformation of the supporting structure, load, temperature and earthquake). Reduce the physical properties of the part, such as air tightness, water tightness, wind pressure resistance and heat preservation performance.

No window cleaner connection design

Window cleaners are needed for building cleaning, but unfortunately the window cleaners of many projects do not really work. On the one hand, it may be a management problem. After all, it is easier to ask a professional team to clean the curtain wall. On the other hand, window cleaners exist. A certain risk, especially when the wind is relatively strong, cannot be relatively fixed with the curtain wall, that is, there is no window cleaner connection design. In the Great Wall Hotel, my country's first curtain wall project, there is a permanent dovetail slot for the window cleaner, which is safe and convenient.

Laminated glass is not used for floor-to-ceiling curtain wall floors below 800mm

The "General Principles of Civil Building Design" GB50352 and the "Code for Residential Design" GB50096 stipulate how to use railings for front windows. For curtain walls, beams are generally installed at a position of 800mm. The use of laminated glass between the beam and the floor can pass the review.

Fire protection between layers and direct glass sealing

In GB50210 "Code for Acceptance of Building Decoration Engineering" and JGJ/T139 "Standard for Inspection Methods of Glass Curtain Walls", there are clear regulations on the fire-proof sealing of curtain walls. When the glass crosses the inter-layer sealing, there will be inter-layer fire-proof sealing and glass The design of direct contact is not allowed by the specification, and there are problems in practice. The glass may burst at about 250°C, and the flame directly threatens the upper curtain wall. Therefore, in the design, avoid the sealing of the glass across the layers, and ensure that the glass bursts and the flame can not go up; the sealing should be tight and prevent smoke.

Super high-rise curtain wall without indoor disassembly design

Due to the inevitable self-exposure of tempered glass, the phenomenon of glass replacement will become more common. However, for super high-rise buildings or buildings that are difficult to replace, it is difficult to replace them according to conventional operating methods. If the curtain wall structure design adopts the structure that can replace the panel indoors, it will undoubtedly improve the safety of the replacement operation and ensure the quality of the curtain wall.

Welding of rear embedded parts

Welding operations near the chemical plugs will greatly weaken the bearing capacity of the chemical plugs. Therefore, welding operations should be avoided as much as possible or appropriate welding procedures should be adopted to avoid greater impact on the chemical plugs.

The strong exhaust of the internally ventilated double-layer curtain wall is inconsistent with the air conditioning

The internally ventilated double-layer curtain wall is equipped with an independent forced exhaust system, which should be designed in conjunction with central air conditioning. If there is an inconsistency, it will be difficult to deal with. Of course, the forced exhaust system cannot be replaced by an air conditioning system.

Short-circuit air flow on double-layer curtain wall

There are many ventilation methods for the externally ventilated double-layer curtain wall, but there should be no air short circuit phenomenon, that is, the air flow discharged from the next hot aisle should not directly enter the previous hot aisle.

Double-layer curtain wall is not equipped with filtering device or insect screen

One of the main features of the double-layer curtain wall is that it has a thermal channel. The orderly flow of air in the thermal channel is designed to achieve excellent thermal performance. In order to ensure the cleanness of the air, a sponge filter should be installed on the inner ventilated double curtain wall, and an insect-proof net should be installed on the outer ventilated curtain wall.

Misuse of external shading system

External shading is not suitable for areas with large wind and sand.

Insulating glass built-in photovoltaic modules

Under sunlight, the temperature inside the hollow glass can reach 80°C, and the power generation efficiency of photovoltaic modules, especially crystalline silicon photovoltaic modules, will be greatly reduced in an environment above 80°C.

The photovoltaic system runs in the glue seam between the glass components

The wiring of the photovoltaic module should be laid in accordance with the requirements of the relevant building electrical engineering regulations, and should be easy to maintain and repair, and should not be routed in the glue joint.

Unclear requirements for photovoltaic system marking

Accessories and equipment such as photovoltaic modules, junction boxes, inverters, accumulators, and grid-connected equipment do not have live warning signs and do not meet the mandatory requirements of the standard.

Misuse of ordinary EVA

EVA has large thermal deformation and poor durability. It should be avoided in curtain walls. It is currently used in some photovoltaic modules. However, curtain wall glass components are different from other non-building glass components, and curtain walls have higher requirements for durability. Therefore, the use of EVA in curtain wall glass components should be avoided.

Unit curtain wall

Hanging point without horizontal positioning

The unit curtain wall hanging point is the basis of the force transmission of the curtain wall structure, so it cannot be taken lightly. There are usually three design defects: (1) The strength design of the hanging point is poor, especially the resistance to negative wind pressure cannot meet the needs. In the experiment, it was found that some pendants were broken under negative wind pressure, and the bearing capacity could not meet the requirements; (2) All the hanging points could be slid, and the whole unit had no lateral positioning; (3) The hanging depth was not enough and there was a danger of slotting. The design principles that should be mastered about the hanging point: (1) The hanging strength should be able to meet the force transmission requirements; (2) It can be adjusted in three dimensions. After adjustment, one point can be relatively fixed to the main structure, and the other point can slide horizontally. There is accurate positioning, and the deformation caused by the sliding expansion and contraction structure, temperature and other reasons can be absorbed; (3) The amount of adjustment should be sufficient, not less than 20mm in all directions; (4) The hook depth is generally not less than 15mm; (5) Effectively absorb deformation during normal work without generating noise

Airtight lines are not coplanar

The unit curtain wall is designed with the principle of equal pressure (rain screen principle or rain screen principle), and there is a cavity between the air tight line and the water tight line, which is called an equal pressure cavity. For a unit, the isobaric cavities around it may be connected, and individual horizontal sliding structures are sealed in the horizontal direction of the unit by means of glue, so at least three isobaric cavities are connected. The airtight line is the last line of defense. If it is disconnected, it will cause leakage. Therefore, if the airtight lines of the horizontal and vertical profiles of the unit are not coplanar, there will be permanent holes, which will cause water and gas leakage.

The internal panel and frame of the unit plate are directly bonded with structural adhesive

"Cannot inject silicone structural adhesive on site" is the consensus of everyone, and it is also a mandatory requirement of the specification. The direct use of structural adhesive to bond the internal panels of the unit plate and the frame can be completed in a qualified indoor glue environment without problems. However, if the glass plate needs to be replaced in the project, this structure must be injected with structural glue on site. If the environmental conditions such as temperature and humidity are not available, the quality of the glue joint cannot be guaranteed, so this problem needs to be solved from the structural design.

Airtight line and watertight line adopt butt joint rubber strip

The unit curtain wall system that adopts rubber strip butt joint and rubber strip insertion for sealing has poor sealing effect. This type of system has higher requirements for curtain wall construction quality: (1) It needs to be installed more accurately; (2) The butt joint needs to be pressed tightly. Otherwise, if the pressure is not enough or the civil construction error is too large, the seal will not be achieved; and the inserting rubber strip should be centered, otherwise it will cause leakage problems; (3) It is necessary to set an independent force transmission member to transmit the load. The test shows that the curtain wall of this kind of structure is more likely to leak, so it should be used with caution in engineering.

Watertight line is fully enclosed

Unless vertical materials are used to achieve internal drainage, the watertight line shall not be fully sealed, and drainage holes should be provided, and the drainage holes should be blocked with sponges to prevent rainwater from flowing back.

Large span profiles adopt open