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1、Introduction of a Panelized Brick Veneer Wall System and Its Building Science Evaluation Jianhai Liang1 and Ali M. Memari2 1 Project Engineer, Thornton Tomasetti, 51 Madison Ave., Floor 17, New York, NY 10010. 2 Professor, Dept. of Architectural Engineering, Pennsylvania State Univ., 104 Engineering

2、 Unit A, University Park, PA 16802. (Accepted 17 June 2010; published online 15 February 2011) Introduction topThe use of steel stud backup wall for brick veneer systems has been on the rise during the previous three decades. The reasons for the increased popularity of steel stud backup wall systems

3、 include reduced weight, cost savings, and shorter construction time. However, there are some problems with the brick veneer over steel stud (BV/SS) backup wall system. Unlike concrete masonry unit (CMU) backup walls, light-gauge steel studs used in backup systems are very flexible. Therefore, they

4、can have a large deflection under a strong wind load leading to the cracking of the brick veneer (BV). Wind-driven rain can potentially penetrate the cracked BV and corrode the metal ties and steel studs (SS). Because, in most systems, ties are the only connections between the BV and the steel stud

5、backup (SSB), corroded ties can lead to a hazardous failure of the BV under high wind load or other out-of-plane loading situations.Conventional BV over both CMU backup walls and SSB systems may also have potential problems during earthquakes. In both systems, gaps under the shelf angles serve as ho

6、rizontal movement joints and are supposed to prevent the BV from participating in the in-plane seismic load resistance. However, during recent earthquakes, some walls failed or cracked as a result of in-plane seismic forces. One major reason for this poor performance is attributable to the closure o

7、f the gaps as a result of the differential movement of the BV and the backup. This movement joint, acting as an isolation mechanism, can malfunction, and as a result, BV walls may crack or fail because of the in-plane seismic forces.These failures, together with a slow rate of construction caused by

8、 the extra time needed to lay bricks and erect scaffolding at the job site, are considered the shortcomings of a conventional BV/SS system. To improve these issues, the concept of a prefabricated and panelized BV with a steel framework backup wall system was developed at the Pennsylvania State Unive

9、rsity. For brevity, the system will be referred to as a panelized brick veneer over steel stud (PBVSS) backup wall system in this paper. The pilot research program consisted of the design and development of the system that included the consideration of the building science-related issues, a three-di

10、mensional (3D) finite-element modeling and analysis, a full-scale simulated wind-loading test, and a full-scale seismic racking test to evaluate the performance of the proposed PBVSS design. Details of the entire research program were described in Liang (200632); this paper discusses the building sc

11、ience-related research results after introducing the design features of the proposed PBVSS system.Literature Review of Major Issues with Conventional System and Overview of Panelized Systems topAnchored BV over backup wall systems can be designed more efficiently than single-wythe masonry barrier wa

12、lls to keep wind-driven rainwater out of the building and to allow the placement of insulation boards inside the wall cavity (Drysdale and Suter 199115). The BV with backup wall systems mainly serve three functions in buildings: structural functions, screen functions, and comfort functions (Drysdale

13、 and Suter 199115; Kroger 200529; Straube and Burnett 200546). To provide these functions, the following components are included in most designs of BV with backup wall systems Brick Industry Association (BIA) 19998; Devalapura et al. 199613; Drysdale and Hamid 200814; Drysdale and Sutter 199115; Gri

14、mm 199322; Hatzinikolas et al. 198525; KPFF Consulting Engineers 199828; The Masonry Standards Joint Committee (MSJC) 200235: veneer, backup wall and frame, sheathing, ties, air cavity, shelf angle, movement joints, thermal insulation, vapor retarder, air barrier, flashing, and weep holes.The failur

15、e of unreinforced masonry (URM) buildings and of some BV walls in earthquakes and tornados with life-safety hazards; as well as problems related to rainwater penetration, corrosion of masonry ties and anchor bolts, visible cracking of the brick veneer, and bowing of the wall; have been reported Broc

16、k 19969; Cowie 199012; Earthquake Engineering Research Institute (EERI) 199016, 199517, 200118; Hagel et al. 200723; Hamid et al. 198524; Jalil et al. 199326; LaBelle 200430; McGinley and Ernest 200438; Peterson and Shelton 200942; Schulatz et al. 199945. One of the primary reasons for the poor perf

17、ormance of BV wall systems is that they are generally considered as “nonstructural” walls that are not designed to participate in resisting gravity and lateral loads, whereas in reality, they participate to some degree unless property isolated. A misunderstanding of the structural function and the i

18、mportance of the load-bearing role of BV walls has led to the failure of these systems. According to Schindler (200444), inadequate attention to the nonstructural intent of the construction details for isolation purposes has been a common source of problems. Moreover, a simple serviceability problem

19、 such as water leakage through a BV wall can lead to the corrosion of ties and anchor bolts and result in a life-safety hazard during high wind or even during a moderate earthquake situation.Current earthquake design details for anchored BV walls call for horizontal movement joints under shelf angle

20、s to accommodate interstory lateral drifts. The small gap under the shelf angle is provided to accommodate the differential vertical deformation attributable to temperature, creep, and moisture between the clay BV and the structural frame. If constructed properly, this gap can also function as a hor

21、izontal isolation joint allowing story drifts without restraining the BV walls. However, in some existing buildings, this movement joint was poorly constructed, and a recent study (Memari et al. 2002a39, b40) has described the potential damage during earthquakes because of the absence of the gap or

22、because of the closure of the gap by mortar.A design assumption for out-of-plane wind-loading on BV/SS is that the BV will crack because the SSB wall is more flexible than the BV (Chen and Trestain 200410). When ties are corroded, the out-of-plane resistance of the BV under high wind loads or earthq

23、uake events will likely be jeopardized with potential fallout consequences. On the basis of Grimms literature review (199221), the recommendation by some designers is to use a heavy concrete masonry backup wall to avoid the problems associated with a conventional BV/SS system. However, such a design

24、 will lose the advantages that lightweight SSB walls can offer. Therefore, to take advantage of the weight savings of BV/SS wall systems in seismic regions, an innovative design of BV wall systems should address the potential problems under both high wind and seismic loading conditions.BV problems a

25、re not limited to performance-related issues under environmental and other loading conditions. One can still see masons on scaffolding several stories high laying bricks one-by-one. Reports of scaffolding failures attributable to various causes including excessive brick weight and scaffolding connec

26、tion failures that result in casualties are not scarce (Gonchar 200120). The construction method for BV also has room for improvement.The built-on-site character of brickwork makes its construction highly dependent on the weather and its quality control relatively difficult. One solution to such pro

27、blems is to panelize and prefabricate the brick wall construction (Tawresey 200448). Concrete wall panels with embedded thin bricks and precast concrete cladding with a face that looks like a brick wall have been commercially prefabricated (Anderson 19966). Although some wall manufacturers can cast

28、concrete panels with a variety of face shell textures including bricklike patterns, many owners and architects would still like to use real exterior clay BV walls because of their aesthetically pleasing appearance. Lindow and Jasinski (200333) described a panelized BV wall system for which the backu

29、p wall, insulation, and shelf angle were assembled as a panel at the factory, and the BV wythe was constructed at the job site. Moreover, it is possible to develop a prefabricated clay BV without a backup wall system by using vertical steel reinforcement (Palmer 199941) or by employing posttensionin

30、g (Laursen and Ingham 200031). Louis (199934) described many of the issues that should be considered in the development of prefabricated brick wall panels including veneer wall panels. Although in panelized BV walls the brick still has to be laid one-by-one, the process can always be done on the gro

31、und in the controlled environment of a fabrication plant. Workers will not have to lay bricks at a high elevation. The manufacturing process is not influenced by harsh weather like rain, snow, or extremely low temperature. Continuous production is guaranteed, and the total erection time can be decre

32、ased by up to 75% (Lindow and Jasinski 200333).The panelization of walls also makes it possible to adopt better seismic isolation connections. Conventional BV are supposed to be isolated from the seismic movements of the main frame through horizontal movement joints under the shelf angles. However,

33、the movement joints may be closed by differential movements or construction error causing the BV to be involved during in-plane seismic force resistance. To reduce the potential for such problems in conventional systems, perhaps the design professional should require a close inspection of all horizo

34、ntal movement joints as part of the approval process. For panelized BV with backup walls, a special seismic isolation mechanism can be included in the connections between the wall panels and the main structural frame. The connections may also be used to isolate the panels from in-plane wind load tra

35、nsferred from the rest of the building and from movements of the main structural frame.Some other advantages of panelized BV systems include the omission of scaffolding or swing stage, better brickwork quality, uniformity, and less site space required for construction (Palmer 199941). Issues with th

36、e performance of the panelized products currently available, as well as limitations on the usage of the panels, have also been discussed by Louis (199934). Some of the problems are typical for all precast members; others are just for the BV panels. The major issues discussed include the relative dif

37、ficulty of transportation, the limit on the minimum size of a project, the design of the joints between panels, and the limited research and design guidelines currently available.Conceptual Design of the Proposed PBVSS topGiven the background of the potential deficiencies of conventional BV/SS wall

38、systems, it is desirable to minimize both the BV cracking possibility and the crack width under high wind loads and to have an in-plane seismic isolation of the BV wall from the primary structural system. One can add to this the desirability of avoiding the use of scaffolding and its related potenti

39、al hazards. To address these issues, a PBVSS system was recently developed at the Pennsylvania State University by prefabricating the wall system as a panel. Regarding the cracking of BV under high wind loads, it should be noted that, according to the commentary in Building Code Requirements for Mas

40、onry Structures (MSJC 200235), the design of BV wall systems asserts and supposes the following guidelines and assumptions: (1) the veneer may crack in flexure under service load; (2) the deflection of the backup should be limited to control crack width in the veneer and to provide veneer stability;

41、 and (3) water penetration through the BV is expected and the wall system should be designed, detailed, and constructed to prevent water penetration into the building. The proposed PBVSS is expected to better control the number and width of cracks compared to conventional BV/SS walls so that a desir

42、able performance of the BV walls can be achieved. Cracks may still form in the BV component, but the overall performance will be improved in terms of moisture penetration.The panelized BV wall system is enhanced by means of a structural steel framework that will support the weight of the BV and SSB

43、wall during transportation and erection. Fig.1 shows the detail of a vertical section of the entire wall panel as installed. The structural steel frame consists of a lower beam, an upper beam, and two vertical load carrying members. The lower member, which performs the function of a conventional she

44、lf angle, consists of a channel and an angle bolted together at three pointsat the two ends of the member and at midspan. However, whereas shelf angles in conventional designs support only the BV, the lower member supports both the BV and the SSB wall. The angle supports the BV; the channel sitting

45、on the floor slab supports the SS. The upper member consists of a channel and a steel plate bolted together, where the channel is positioned under the floor above (i.e., the bottom of the slab or the spandrel beam of the floor above) separated by movement joints. The steel plate attached to the chan

46、nel needs to be extended all the way to near the top of the slab to provide the out-of-plane support for the BV. The two vertical members are constructed of steel channels sitting between the top and bottom channels and are designed for the gravity loads of the wall panel when lifted by a crane. The

47、 vertical channels are orientated so that the webs of the channels will face the interior of the panel. Fig.2 shows several photos of the PBVSS mock-up taken during construction.Fig 1. Elevation of the PBVSSView first occurrence of Fig. 1 in article.Fig 2. PBVSS mock-up during constructionView first

48、 occurrence of Fig. 2 in article.Typically, 18gauge studs at 400600mm center-to-center spacing (more often at 400mm) are used for the SSB in conventional walls (BIA 19998; Suter et al. 199047). To increase the flexural out-of-plane stiffness, heavier gauge SS (e.g., 12gauge) framing or structural st

49、eel channels can also be used (McGinley 200037). For the proposed PBVSS system, because a larger out-of-plane stiffness was desired, 12gauge studs were used in the SSB frame. The more commonly used stud spacing of 400mm was chosen for the PBVSS so that only the gauge of the studs were different from the conventional BV/SS. The studs could normally be connected to the BV with various types of ties such as V-Tie ties, Z-Tie ties, corrugated metal ties, or ladder shape ties (Drysdale and Hamid 200814). Choi and LeFave (2004