Structural Capacity of Anchorage Ties in Masonry Veneer Walls Subjected to Earthquake: The implications of Eurocode 8 and Eurocode 6 on a typica lNorwegian veneer wall
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The brick masonry veneer wall is a common type of cladding in Norwegian buildings. The veneer wall system is a composite structure, which in essence consists of an exterior masonry wall and an interior backup (bracing) wall, separated by an air cavity with regularly spaced metal ties connecting the exterior masonry wall to the interior backup wall. Backup walls can be made of masonry, concrete, wood or steel, in the form of panels or frames. A common type in Norway is the wooden backup wall, with vertical wooden studs covered by external and internal sheets. Previously, anchorage ties were essentially designed to withstand wind loading. The common practice in Norway is to support the veneer wall vertically, at its base only. This makes it especially important to ensure that the joint action of the inner and outer wall, which relies on the connection ties, is capable of resisting horizontal loadings from wind and earthquake excitations. From April 1st 2010, the Norwegian national standards in series NS34xx have been withdrawn by Standards Norway and replaced by the Eurocodes, which are now the valid Norwegian Standards for structural design. The main objective of the study is to investigate the structural capacity of anchorage ties in masonry veneer walls subjected to earthquake excitations. In particular, to assess the capacity of a typical and modern Norwegian wall system in view of the newly implemented Eurocodes for structural design. The anchorage connection depends on the tensile strength and the buckling strength of the tie, as well as of the pull-out strength from mortar joint and backup wall. For common types of ties, the pull-out strength from joints is decisive. Background information from earlier studies and development of masonry veneers that have been tested and analyzed under simulated earthquake and wind pressure loads, are summarized. Accidents and damage experiences mainly due to earthquake are presented. Requirements for anchored veneers from guidelines in Norway are discussed. Relevant code requirements in the USA are introduced and compared with the requirements in Norway. The out-of-plane seismic performance of anchored brick veneer wall systems supported by wooden backup walls, representing prescriptive design requirements and current construction practice, is investigated. Prescriptive requirements for the design and construction of anchored brick veneers are currently given by the Eurocodes. The nature of earthquake and the seismicity in Norway is discussed, with special attention on the applicability of the code for the veneer wall as a non-structural element. The analysis methods allowed by Eurocode 8 are dealt with. Artificially generated accelerograms, representative for the area under consideration, are made and modified by the necessary base line correction technique. Despite Norway being considered a country with low seismic activity, there are regions where earthquake resistance is the governing design criteria for veneer ties. The simplified method suggested in Eurocode 8, clause 184.108.40.206(2), for design check of non-structural elements, is not applicable for typical veneer walls in Norway, where they are vertically supported only at their base. Experimental test results are used to validate FEM analysis. Simplified 2D FEM analyses are performed to assess the capacity of anchorage ties due to seismic excitations at selected locations in Norway. The analyses results indicate that the resistance of typical veneer walls, connected to a backup wall in wood with 4 ties/m2, seems to be inadequate, except for areas with low seismicity. It should however be noticed that this consideration is based on the assumption of a tie capacity of 0.6 kN/tie, as recommended by the Masonry Catalogue (2005) and the Building Research Institute (2009), whereas testing performed by Madso (1980) indicates a capacity in the range 1.55-2.0 kN/tie. If even the lower limit of Madso’s measurements are taken as basis, the 2D FEM results suggest that 4 ties/m2 provides sufficient earthquake resistance of typical veneer walls in all areas of Norway. Design guides, codes, and current construction practice is evaluated in light of the overall findings of the current study. Supporting the veneer wall by brackets attached to the main structure at floors’ levels, as in the USA is a solution when increasing the number of ties per m2 is not reducing ties forces to the allowable limit. This solution has the cost of introducing thermal bridges to the building. It is recommended that veneer walls together with their ties should be verified for the seismic design situation wherever seismic check is mandatory according to Eurocode 8. A detailed seismic analysis (time history analysis) should be performed in order to check and determine the number and layout anchorage ties. A quality assurance system is necessary, with respect to installation of ties. Experimental seismic/dynamic tests for pull-out capacity of different types of anchorage ties from timber as well as from mortar are required.