Failure mechanisms: masonry

The failure mechanisms of interest in reinforced masonry wall elements include flexural, transverse shear, in-plane shear and in some cases, combined axial compression and flexure. Buckling failure modes of compression elements and connection failures are to be avoided. 

Limit state design methods are used in blast resistant design. These methods provide a comprehensive, reliable and realistic means of predicting failure mechanisms and structural capacities. Limit state design methods for structural steel, cold formed steel, reinforced concrete and reinforced masonry are available. However, as of now, no similar design specification is available for aluminum structures. 

Masonry, both reinforced and unrein forced, is a common construction material in petrochemical facilities. However, unreinforced masonry is inappropriate in blast resistant design due to its limited strength and its nonductile failure mechanisms. Reinforced masonry walls with independent structural framing for vertical loads arc commonly used in blast resistant design. 

The macro-blocks are portions of a structure with similar material properties and structural behavior, to which the mechanical properties of the material can be assigned or, by simplification, they can be assumed to be infinitely rigid. The interfaces in a macro-modelling represent, in general, the cracks associated to the failure mechanisms. However, in a micro-modelling strategy applied to masonry, in which the units and the joints are individually considered, the interfaces simulate the behavior of the joints. Furthermore, different criteria for the strength parameters of macro-blocks and interfaces can be considered. 

The equivalent strut method is an oversimplification of the actual behavior of an infill wall and fails to capture some key failure mechanisms, such as the one depicted in Fig. 2c. A strut model will not account for the possible shear failure of a column that could be induced by the frame-wall interaction. There is no simple solution to overcome this problem. A study by Stavridis (2009) based on detailed nonlinear finite element models has demonstrated that the compressive stress field in a masonry infill wall may not be accurately represented by a single diagonal strut and that a strut model ignores the shear transfer between the beam and the infill. Hence, replacing a wall by a diagonal strut will not lead to a realistic representation of the load transfer from the frame to the wall. Moreover, as mentioned previously, it is not possible to have a single strut width to capture both the initial stiffness and load capacity of an infilled frame. 

Apart from the effect of masonry components on its strength, the type and morphology of the masonry (one or multiple leaf system) plays an important role to failure mechanism. For example, in three leaf masonry walls, separation of leaves is often observed (Binda et al. 2007). Moreover, if the unit-mortar interface has been weakened, shear or tensile type failure occurs easily. 

The incompatibility of resins to masonry structure has an impact to the bond of FRP on masonry surface. Moreover, FRP may create secondary side effects that make the benefits of reversibility questionable. This occurs because of the lack of vapor permeability that leads to moisture accumulation inside the masoruy mass which favors deterioration phenomena, particularly in the case of large masonry area rapping with FRP. Furthermore, FRP or AR fiber glass textiles exhibit brittle failure mechanism when their bonding is based on adhesion and friction. In fiber grids with cement-based matrix, these effects are mitigated. 

For shear failure mechanisms of brick masonry walls, when mortar bed and head stone failure is involved, they proposed to adopt a Mohr-Coulomb friction model for the wall, namely ... 

Some authors [11] reports included assays masonry walls of different thicknesses and materials. All information of the trials gave way to other documents and publications. The results were translated into tables [12] in which there was discussion about the impossibility of establishing methods for calculating fire resistance of masonry walls before knowing the properties of the materials at high temperatures and the accurate mechanism of failure (effect of loss of strength, increase of the slenderness of the wall, different thermal expansion between the two sides of the wall and external links). 

The next edition of ASCE 41 - ASCE/SEI 41-13 (ASCE 2014) - has just been published and expands the remit of ASCE 41 to include the multitiered seismic assessment approach previously found in ASCE/SEI 31-03 (ASCE 2003). The nonlinear response history analysis procedures have been expanded somewhat, reflecting significantly increased application of this method and the availability of research efforts such as Deierlein et al. 2010 NEHRP Consultants Joint Venture 2010, since the publication of ASCE/SEI 41-06. Nonlinear behavior of force-controlled actions is now permitted to allow explicit modeled of post-failure redistribution following brittle failure. Finally, and importantly for this entry, the unreinforced masonry (URM) wall provisions have been updated major changes are that bed-joint sliding is now considered a deformation-controlled action, while the pier rocking mechanism is now limited to lower axial load ratios and for piers at least 6" in thickness. 

Shear Behavior Along the Bed Joints The influence of mortar joints acting as a plane of weakness on the composite behavior of masonry is particularly relevant in case of strong unit-weak mortar joint combinations. Two basic failure modes can occur at the level of the unit-mortar interface tensile failure (mode I) associated with stresses acting normal to joints and leading to the separation of the interface and shear failure (mode II) corresponding to a sliding mechanism of the units or shear failure of the mortar joint. The preponderance of one failure mode over another or the combination of various failure modes is essentially related to the orientation of the bed joints with respect to the principal stresses and to the ratio between the principal stresses. 

The limit analysis with macro-blocks is a simple tool for evaluating the maximum load capacity of masonry structures. The possible mechanisms are proposed and then the respective load factors are determined. The mechanism that presents the minimum load factor corresponds to the coUapse, and its load factor is assumed as the failure load. Thus, a careful evaluation of the possible mechanisms is needed, aiming at not excluding any important mechanism and predicting correctly the maximum load capacity of the structure. 

Masonry infills, like other masonry elements already discussed, tend to fail in plane or out of plane (Fig. 16) following similar mechanisms as other masonry elements (diagonal thrust or tension, comer compression, sliding at joints through brick or mortar, out-of-plane collapse, etc.). Such a full or partial failure of the infills will lead to local failures of the confining elements due to the formation of unintentional short column effects or due to a shear failure of the top of the column or the beam-column joint. For these reasons, in the PBD assessment approach of existing stmc-tures, the infills need to be accounted for in seismic analysis. 

Dissipative Anchor Devices Given the difficulty of ensuring ductile failures of anchors embedded in masonry, D Ayala and Paganoni (2014) in collaboration with CINTFC International developed two prototypes of dissipative anchor devices to address the problem of out-of-plane mechanisms. 

Fire endurance models These models focus specifically on the mechanical cndmance of structures during fire, and so are generally finite element models which calculate changes in stress in response to temperature change within particular structural components of different materials, namely steels, masonries or polymer composites. They can be written in isolation or combined with either zone or field models to produce a comprehensive description of the fire scenario in terms of heat and mass flows as well as component stresses. Thus, such models can be used to predict the moment of struetural failure in a member with variable accuracy. One important specific result of such models is a temperature-time profile through the thickness of a structural component such as an I-beam for example. 

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