According to different panel materials, building curtain walls can be divided into three categories: glass curtain walls, metal curtain walls and stone curtain walls. No matter what kind of curtain wall, the connection between the load-bearing structure system and the main structure of the building is usually realized by embedded parts or later anchorage. Curtain wall not only bears self-weight load, but also bears the influence of wind, earthquake and other loads, so whether the connection between embedded parts and the main structure of the building is reliable and durable is directly related to the structural safety and service life of curtain wall.
I. Classification and composition of embedded parts
Embedded parts are divided into embedded parts and post-embedded parts according to their formation time sequence, among which embedded parts are divided into claw embedded parts and groove embedded parts.
1. 1 embedded parts
Embedded parts are components placed (buried) in the structure in advance, that is, components made of steel plates and anchor bars left in the structure when the structure is poured.
1. 1. 1 common claw embedded parts
Anchor bars and anchor plates are welded. Anchor bars can be made straight, curved and hooked.
1. 1.2 Embedded plate with reserved groove.
Based on the common claw-type embedded part, this kind of embedded part adds a reserved groove, which is very convenient to connect. Even in the case of large position error of embedded parts, it can be welded like ordinary embedded parts, with great flexibility.
1. 1.3 claw-shaped embedded parts (A ~ F are common ones, as shown in the figure).
1. 1.4 embedded embedded embedded parts
The metal trough can be formed by bending, casting and forging steel plates. Anchor bars and metal grooves can be made into a whole or welded. This form of embedded parts has the advantages of small size and convenient construction, and has been localized and formed a series. The length of groove embedded parts commonly used in construction is 300mm, and the length of anchor bar is 100mm or 60mm.
Comparison of advantages and disadvantages between groove embedded parts and flat embedded parts
Trough embedded parts are a common form in curtain wall construction. Compared with flat embedded parts, groove embedded parts have more advantages, so their application in curtain wall engineering is increasing gradually.
(a) Compared with plane embedded parts, advantages of groove embedded parts
1, compared from the perspective of production and processing
The processing technology of groove embedded parts is simple and the quality inspection is convenient. Generally, the efficiency of machining groove embedded parts is three times that of machining plane embedded parts.
2. Compare from the economic point of view.
Slotted embedded parts are cheap, which saves the engineering cost. The weight of an embedded embedded part is about 2kg, and with two T-bolts, the price of an embedded embedded part is about 25 yuan. The weight of a flat embedded part is about 6kg, and the price is about 60 yuan. The price of groove embedded parts is about half that of flat embedded parts.
3. Comparison from the perspective of construction difficulty
Trough embedded parts are small in size and convenient in construction. There is only one row of anchor bars in the groove embedded parts, and the channel steel of the groove embedded parts is small, so it is not easy to interfere with the main structure reinforcement, and the construction period is short, which greatly improves the construction progress. However, the flat embedded parts occupy a large volume, and the anchor bars are generally arranged in two rows and two columns, which easily interferes with the main reinforcement of the main structure. Because the constructors are not very clear about the importance of the main reinforcement, in order to bury the embedded parts, the main reinforcement of the main structure is occasionally sawed off, which will lay a safety hazard for the building. In addition, because of the small volume of the trough-shaped embedded parts, when the main structure is a thin plate structure, the trough-shaped embedded parts can only be used instead of the flat-plate embedded parts.
4. Comparison from the point of view of convenient installation of curtain wall keel.
T-bolt is used to connect the slotted embedded parts with the adapter of curtain wall keel, which is very convenient to install and does not need on-site welding. The embedded part of the groove is connected with the curtain wall keel adapter through the T-bolt, and the T-bolt can freely slide horizontally in its notch, and the adapter is connected with the T-bolt through the long hole in the vertical direction, and the adapter is connected with the curtain wall keel through the long hole in the vertical direction, so that the three-dimensional adjustment of curtain wall keel installation is realized, and the installation is very convenient. As shown in the figure. Plane embedded parts can also be adjusted in three dimensions, but they need to be welded and fixed after adjustment, which increases the difficulty of site construction and the possibility of fire on the other hand.
(B) the shortcomings of groove embedded parts compared with plane embedded parts
Compared with flat embedded parts, the most obvious disadvantage of trough embedded parts is that the bearing capacity of trough embedded parts is much smaller than that of flat embedded parts. The design tensile bearing capacity and shear bearing capacity of groove embedded parts are 32KN and 23KN respectively, while the design tensile bearing capacity and shear bearing capacity of flat embedded parts are about 140KN and 55KN respectively. Therefore, when the curtain wall span is large or the curtain wall surface is far away from the structural surface, the groove embedded parts are not used.
1.2 rear embedded parts
The spare embedded parts are flat embedded parts, which are fixed by ordinary expansion bolts, chemical anchor bolts or through bolts (studs) and welded closed steel plates.
Several construction methods of 1.2. 1 rear embedded parts
① ordinary expansion bolt fixation
② chemical anchor bolt fixation
③ Fix through bolts (studs)
(4) welding hoop steel plate (usually used for columns or beams)
(5) civil structure, and embedded parts.
⑥ Composite forms of the above forms.
1.3 Embedding method of embedded parts
According to its position on the main structure, embedded parts can be divided into upper buried type, side buried type and lower buried type, and the lower buried type is unfavorable in stress, so use it with caution.
Backfilled embedded parts can only be connected with the main structure through expansion bolts and chemical anchors. Because the installation quality of post-compensation embedded parts is greatly influenced by the on-site construction conditions and personnel, it is difficult to control, and it often fails to meet the design index, especially because the state has expressly stipulated that expansion bolts are not allowed to be used in tension parts, so try not to use post-compensation embedded parts.
Second, the embedded parts design
1. The connection strength between embedded parts and the main body directly determines the safety of the whole curtain wall and must be strictly controlled. In the design of embedded parts, attention should be paid to the following points:
(1). Set the size and position of anchor bar and embedded plate of embedded parts.
Timing should be strictly based on the Code for Glass Curtain Wall Engineering.
(JJ102-2003) and Code for Design of Concrete Structures.
(GB500 10-2002)。
(2) Pay attention to the length of anchor bar not exceeding the structural size (such as the thickness of beam) to avoid the anchor bar being exposed outside the structure.
(3) Class A and B anchor bars in claw-shaped embedded parts shall be rebar. When designing C-type and D-type anchor bars, the interference between anchor bars and structural steel bars should be considered. Class E and F embedded parts are suitable for the parts that need lightning protection.
(4) The size of the buried slab should consider the needs of the curtain wall structure when designing.
2. Pay attention to the technical requirements of embedded parts.
(1) The technical requirements of embedded parts are the special design content of curtain wall that the constructor must pay attention to. According to its stress (tensile force, shear force and bending force), the anchor plate specification, anchor bar diameter, length and weld thickness are calculated and determined, among which the minimum thickness of anchor plate and anchor bar spacing, the distance between anchor bar and anchor plate edge, the bearing capacity of embedded parts and the anchorage bearing capacity of connectors and main structure must be confirmed by calculation or entity test, which meets the specification requirements. However, the construction party often pays insufficient attention to the special design of embedded parts, even ignores the requirements of the code and hastily arranges civil engineering pre-embedding. This lack of scientific design and blind pre-embedding not only caused a large number of embedded parts to be installed, but also increased the hidden danger of curtain wall safety.
(2) The technical requirements of the rear embedded parts include not only the performance differences of various bolts, but also the performance of the base material, the mechanical properties of the anchorage connection, the type of the connection structure, and no seismic fortification requirements. Expansion bolts and reaming bolts shall not be used for the anchorage connection of structural members under tension and edge shear force (effective anchorage depth of edge c < 10 HCF anchorage) and structural members under tension-shear combined force and non-structural members of lifeline engineering. Chemical planting bars and screws that meet the anchoring depth can be applied to the post-anchorage connection of structural members and non-structural members under the action of tension, edge shear and combined stress of tension and shear with seismic fortification intensity not greater than 8 degrees.
3. The provisions of the embedded parts structure
The old code JJ102-96 Technical Code for Glass Curtain Wall Engineering copied from GBJ 10-89 Code for Design of Concrete Structures. The new code jggj102-2003 has some improvements in adapting to the characteristics of small load in curtain wall industry, such as canceling the regulation that the ratio of anchor plate thickness to anchor bar center distance is ≥ 1/8; The length of tensile anchor bar is reduced to ≥ 15d. These still can't meet the needs of setting embedded parts on small-section concrete members, and the project often faces the embarrassment that supervisors check the length of anchor bars according to the specifications. It is understood that so far, there has been no curtain wall damage accident caused by insufficient resistance of embedded parts in the curtain wall industry, which shows that the existing embedded parts are safe, but also reflects that the embedded parts are conservative to a certain extent and there is room for further improvement.
4- 1 cross-sectional area of anchor bar
The new code stipulates the minimum cross-sectional area of anchor and provides the calculation formula of the minimum cross-sectional area of anchor. According to my experience, due to the small load acting on the embedded parts of general curtain wall, the cross-sectional area of anchor bar determined according to the structure can meet the requirements of the specification, so it is generally unnecessary to check the cross-sectional area of anchor bar.
4-2 Materials of embedded parts
According to the specification, Q235 grade steel should be used for the anchor plate of embedded parts. Anchor bars shall be hot-rolled steel bars of HPB235, HRB335 or HRB400 grade, and cold-worked steel bars are strictly prohibited. According to the design of curtain wall, the following instructions are made:
The code (1) only requires the anchor plate material to be Q235 grade steel, and does not specify which one of A, B and C grades. Curtain wall industry, too high is not desirable, to the bottom. As long as it can meet the use requirements, it is more economical and competitive.
(2) The anchor bar can be any common building steel bar. It is most suitable to use HRB335 grade steel bar as anchor bar. HPB235 steel bar is smooth, and the end must be hooked, so it is difficult to make and install special-shaped steel bar. However, HRB400 steel bar is expensive and difficult to process.
(3) Steel bars can be divided into hot-rolled steel bars, heat-treated steel bars and cold-drawn steel bars according to their production methods. Both HPB235 steel bars and HRB335 steel bars widely used in construction projects are hot rolled steel bars. Cold-drawn steel bars, also known as cold-worked steel bars, improve the yield limit and strength of materials through cold-drawn technology. The disadvantages are reduced plasticity, brittle materials and poor cold bending performance. It is correct to stipulate that cold-worked steel bars are strictly prohibited for anchor bars. Cold working, such as cold bending, is allowed for hot rolled steel bars and is widely used in buildings. It is considered that hot-rolled steel bars cannot be cold-worked and hot-rolled steel bar anchorage cannot be bent, which confuses the two different concepts of cold-worked steel bars and cold-worked steel bars.
4-3 Specification for Anchor Length Anchor length refers to the minimum structural length allowed when the tensile strength of anchor is fully utilized.
(1) When the tensile strength of the anchor rod is fully utilized in the calculation, the anchorage length shall be calculated as follows:
Among them, circular steel bar (such as HPB235 steel bar): α = 0.16; Ribbed reinforcement (such as HRB335 reinforcement): α=0. 14.
Design strength of reinforcement: HPB235 reinforcement = 210n/mm; HRB335 reinforcement =300N/mm concrete strength grade:
Tensile strength of concrete corresponding to C20C25C30C35C40 (n/mm):1.1.271.431.571.
D is the diameter of the steel bar.
4.4 The thickness of anchor plate shall be calculated and determined according to its stress.
The calculation diagram is a point-supported flat plate, and the distance between anchor bars is the main factor to determine the thickness of the plate. This is the reason why the ratio of anchor plate thickness to anchor bar center distance in Code for Design of Concrete Structures is ≥ 1/8. According to the characteristics of curtain wall, the new code does not adopt this provision, and the thickness of anchor plate is limited widely. At present, the thickness of the designed anchor plate is arbitrary, and some anchor plates have larger area and smaller thickness, while others have smaller area and larger thickness. I think, in general, when the side length of the anchor plate of curtain wall 4 is less than 250mm, the plate thickness is 8 mm; When 250mm≤ side length < < 350mm, the plate thickness is10 mm; ; When the side length is ≥350mm, the appropriate thickness is12 mm.
4.5 Connection between anchor bars and anchor plates Generally, anchor bars and anchor plates are connected by T-welding. When the diameter of anchor bar is greater than 20mm, perforated plug welding should be adopted, and the weld grade is Grade III. When connecting steels with different strengths, covered electrode suitable for low-strength steels should be adopted. E43X(0~5) covered electrode is used in the project, and the weld height is mm, which can meet the welding requirements of common curtain wall embedded parts.
4.6 Quality standards for built-in parts
(1) The variety, model, specification, size, performance, plate thickness and surface treatment of embedded parts shall meet the design requirements, and shall have the factory certificate.
(2) For the welding treatment of embedded parts, check whether the varieties of reinforced steel plates meet the design requirements and mandatory standards.
(3) The anchor plates (flat and grooved) of embedded parts are made of Q235B grade steel, and the anchor bars are made of HRB335 or HRB400 grade (ribbed) hot rolled steel bars.
(4) T-shaped welding is adopted for the straight anchor bar and the anchor plate, and pressure arc welding is adopted when the diameter of the anchor bar is less than 20mm; When the diameter of anchor bar is greater than 20mm, perforated plug welding is adopted; It is not allowed to bend the anchor bar into an L shape and weld it with the anchor plate.
(5) When the surface of embedded parts is hot-dip galvanized, the thickness of zinc film should be greater than 45 microns.
(6) When making embedded parts, the allowable deviation of anchor plate and anchor bar and the verticality between anchor bar and anchor plate surface should be controlled according to the specifications, and the length of anchor bar is not allowed to be negative deviation.
Third, the building curtain wall embedded parts construction requirements
(a) the standard jgj 102-2003 article 5.5 related provisions:
1. The main structure or structural member should be able to bear the load and function transmitted by the curtain wall. The design value of the anchorage bearing capacity of the connector and the main structure should be greater than the design value of the connector itself;
2. The glass curtain wall column should be connected with the main concrete structure by embedded parts, which should be embedded in the concrete construction of the main structure, and the position is accurate; When there is no condition to use embedded parts connectors, reliable measures should be taken and its bearing capacity should be determined through tests.
3. The stress embedded parts composed of anchor plates and symmetrically arranged anchor bars can be designed according to the provisions in Appendix C of this specification.
4. The embedded steel plates and other connection measures of the trunking embedded parts shall be designed according to the current national standard Code for Design of Steel Structures (GB500 17), and its bearing capacity shall be determined through tests.
5. When the glass curtain wall frame is connected with the main structure by adding anchor bolts, it shall meet the following requirements:
(1) The product shall have a certificate of conformity; (Test report on chemical composition and mechanical properties of steel, design method data and factory certificate).
(2) Carbon steel anchor bolts should be treated with anticorrosion;
(3) After adding bolts, the pull-out test of single root and node (group) must be carried out on site, and the load added in the test should reach 1.5 times of the load design value, and there is no obvious slip. When necessary, the ultimate pull-out test shall be carried out in the test unit.
(4) After each connection point, there should be no less than two bolts, the bolt spacing and the distance from the bolt to the edge of the component should not be less than 70mm, and it should be designed as the node where the bolt is sheared;
(5) The bolt diameter shall be determined by bearing capacity calculation, and shall not be less than l0mm；;
(6) Welding operation shall not be carried out on the connector contacting with the chemical anchor bolt;
(7) The design value of the bearing capacity of the anchor bolt should not be greater than 50% of its ultimate bearing capacity.
6. When the curtain wall is connected with the masonry structure, it is advisable to add reinforced concrete or steel structure beams and columns to the main structure of the connection part. The concrete strength grade of the connected main structure shall not be lower than C30. Curtain walls should not be connected with masonry, let alone lightweight walls.
(II) Construction requirements of rear anchor pieces
Post-anchorage is widely used in curtain wall construction, especially in a large number of curtain wall projects in the reconstruction and expansion of old buildings, and even all of them are used. A large number or even all of the rear anchors are used in curtain wall engineering, and the construction quality control is not good, which will bring security risks to the use of curtain wall.
Technical Specification for Post-anchorage of Concrete Structures (JGJ 145—2004) clearly stipulates the construction requirements of post-anchorage.
1. The rear anchor includes expansion bolts, reaming bolts, chemical rebar planting and other types of bolts.
When using the rear anchor, besides the performance differences of various bolts, other factors should also be considered, such as the nature of the base material, the mechanical properties of the anchorage connection, the type of the connection structure, and whether there are seismic fortification requirements.
Expansion bolts and reaming bolts shall not be used for the anchorage connection of structural members under tension and edge shear force (effective anchorage depth of edge c < 10 HCF anchorage) and structural members under tension-shear combined force and non-structural members of lifeline engineering. (Non-structural components of the building include: enclosure wall, partition wall, curtain wall, ceiling, billboard, etc.). )
Chemical planting bars and screws that meet the anchoring depth can be applied to the post-anchorage connection of structural members and non-structural members under the action of tension, edge shear and combined stress of tension and shear with seismic fortification intensity not greater than 8 degrees.
2. Pay attention to the construction quality of anchor bolts. The Technical Specification for Post-Anchoring of Concrete Structures (JGJG 145-2004) stipulates the construction of anchor bolts:
(1) Requirements for drilling anchor bolts: See the table below.
Allowable deviation of aperture
Allowable deviation of hole depth
Permissible deviation of verticality
Allowable deviation of position
≤0.5 mm
Expansion and reaming bolts 0 10/0mm
Chemical rebar planting: 0.20mm.
≤50
5 mm
Fourth, the construction quality of embedded parts
(1). Design and calculation problems
Some curtain wall projects, especially small and medium-sized curtain wall projects, do not pay enough attention to the professional design of curtain wall. Some designs have only a few simple design drawings, no embedded parts location map, no structural mechanics calculation book, and some have calculation books, but there is no embedded parts calculation and no audit.
(2) Welding quality of flat embedded parts
1. The common form of embedded parts is composed of welded anchor bars on the anchor plate. (Cold-rolled steel bars shall not be used for anchors. When the diameter of anchor bar is ≥ 10mm, secondary rebar is used, including crescent-shaped and threaded rebar. See JSJT-203 Embedded Parts for Reinforced Concrete Structures. ) In the early days, the steel bar was bent and welded directly to the anchor plate. At present, the methods of drilling holes and plug welding in anchor plates are basically adopted. The latter is more reliable. The welding quality of anchor plate and anchor bar is the key to the quality of embedded parts. In order to ensure welding quality, welding operators must be trained and certified. The acceptance of embedded parts is also the key. It is necessary to check not only the appearance quality to prevent virtual welding from breaking, but also the welding strength between anchor plate and steel bar according to regulations.
2. Most of the embedded parts deviate from the design position, which makes them unusable.
The reason is:
(1) Embedded parts have been buried during civil construction, but they cannot be used due to the design change of curtain wall.
(2) The embedded parts are not firmly bound, and the embedded parts are displaced and offset when the concrete is cast in the construction.
Article 10.2.3 of Technical Code for Glass Curtain Wall Engineering (JJ102—2003): The embedded parts connected between the glass curtain wall and the main structure shall be buried according to the design requirements during the construction of the main structure, and the position deviation of the embedded parts shall not be greater than 20 mm
3. Quality problems of post-anchorage construction.
(1) Install the rear anchor on the lamp wall.
In some projects, the floor span is large, and the deflection calculation or strength calculation of the column fails, so some designers add a fulcrum between the upper and lower beams. If it is on the reinforced concrete (or steel structure) structural beam (column), this fulcrum is effective. In some frame structures, lightweight infilled walls are usually used for masonry. If the fulcrum is added to the light-filled wall, even if steel plates and through-wall bolts are used, it will not play an effective supporting role. Therefore, the specification stipulates that curtain walls shall not be connected with masonry, let alone light walls.
(2) The anchoring matrix is unreliable. If the strength of the concrete matrix is not enough and the margin is not enough, the concrete matrix will crack and the anchoring will fail.
(3) The rear anchor is offset. When drilling, deviation and deflection often occur when steel bars are encountered, and the hole is not cleaned up, which leads to the loosening of the anchor during use.
According to the specification, the single pull-out test and node (group) pull-out test must be carried out on the spot for the added bolts, and the increased load in the test should reach 1.5 times of the load design value, and there is no obvious slip. If necessary, the final pull-out test shall be carried out in the test device. The test results should be checked with the design calculation, and the design value of anchor bolt bearing capacity should not be greater than 50% of its ultimate bearing capacity.
4. The quality of chemical anchor bolt is not high.
Chemical bolts are widely used in the rear embedded parts of curtain wall industry. In the 1990s, chemical bolt products were introduced from abroad and applied to construction projects. In recent years, many domestic manufacturers have followed suit and produced in large quantities. The market price ranges from a dozen yuan to two or three yuan, which can be described as heterogeneous brands with different quality. The anchoring glue of chemical bolt plays the role of bonding concrete base material and anchor bar. At present, there are many kinds of chemical anchoring agents in the market, such as modified epoxy resin, ethylene acrylic resin and unsaturated resin. The physical and chemical properties of anchoring glue directly affect the anchoring effect. In addition to the publicity materials of some well-known imported brands, such as the durability, temperature resistance and freeze-thaw resistance of anchor glue, most manufacturers' product introductions only have vague statements such as acid and alkali resistance, heat and fire resistance and low temperature sensitivity.
Although the on-site pull-out force test meets the design requirements, the durability of anchoring glue is only predicted in the laboratory at present, and the influence of high welding temperature on anchoring agent is not clearly stated. No wonder people in the industry question the durability of anchoring glue and worry about the large-scale use of some cheap products.
Stainless steel bolts with rear embedded parts shall have certificate of approval, material mechanical performance report and mechanical performance re-inspection.
During the review of the National Architectural Decoration Award (building curtain wall category), it was found that there were still some problems in the construction and field pull-out test of some post-installed anchors.
(1) Some projects have no embedded parts, and chemical bolts of various specifications are used as post-treatment pretreatment. In the observable parts, the exposed length of bolts varies, and some obviously feel that the effective contact length between bolts and concrete matrix is not enough.
In the reconstruction of old buildings, there is a paint layer (generally 20mm) on the wall, and the effective embedding depth of bolts should also consider the thickness of the paint layer. If there is an old building renovation project, the original wall is tiled. In order to make up for the non-vertical structure, the method of increasing the thickness of wall paint layer is adopted to make its maximum thickness reach 7-10 mm. If chemical bolts are used as post-embedded parts in this project, we should be very cautious, and we should use through-wall screws and anchor plates or adopt other reliable connection methods.
(2) In some projects, the pull-out force is only tested on the test blocks used in the laboratory, and the pull-out force is not tested on site, or only 1-2 bolts are tested. For example, a project used four chemical bolts of different specifications, but only two bolts were tested.
(3) The number of on-site bolt tension tests is not enough, and only one set (3 pieces) of symbolic tests are carried out for some projects.
According to the regulations, the on-site drawing force test of bolts should be composed of parts with the same model, specification and basically the same, and the drawing batch should be 1‰ of the total number of bolts in each batch, and each batch should be no less than 3.
(4) The test results are not checked with the design calculation. Ensure that the design value of the bearing capacity of the anchor bolt should not be greater than 50% of its ultimate bearing capacity.
5. The problem of groove embedded parts
Trough embedded parts have the advantages of good adjustability, flexible connection, no need for welding, easy embedding and so on, and have been widely used in building curtain wall engineering. However, the trough-shaped embedded parts, like other embedded parts, are prone to offset and tilt when buried and enter the structural wall.
Five, the deviation of embedded parts processing opinions
1. The position of embedded parts of flat plate deviates from the design position.
When the deviation of embedded parts occurs, the method of adding (or lengthening) embedded anchor plates can be used to remedy it. After lengthening the anchor plate, fix it with bolts.
2. The embedded parts are skewed
When deflection occurs, the angle of the adapter can be changed to adapt to the deflection caused by the embedding of the adapter, or a new anchor plate can be used instead.
3. The cavity appears under the embedded anchor plate.
When there is a hole under the embedded parts, it should be filled with cement mortar.
Although the proportion of embedded parts in curtain wall investment is very small, it plays a vital role. It is the foundation of curtain wall components, the bridge connecting the main structure and the key to engineering safety. It has a strong node in the whole curtain wall project. Due to various reasons, such as' lack of experience',' lagging design' and' poor audit', the curtain wall bidding often lags behind the main construction bidding, so that the civil work has started and the curtain wall design is still unclear. Missed the key node of synchronous construction of embedded parts and main structure, and hastily entrusted civil works to embed them according to the curtain wall with rough architectural design, which resulted in excessive deviation of embedded parts and serious waste. Part of the main body has been capped, and the curtain wall construction has just begun, so all the rear embedded parts have to be adopted, which not only doubles the project cost, but also has a series of problems such as structural damage and unstable quality.
Therefore, it is not only related to the investment cost of the construction project, but also related to the later construction and quality safety of the curtain wall project to master the node law of the curtain wall project and scientifically select the curtain wall embedded parts.
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