8. THE EARTHQUAKE RESISTANCE OF DHAJJI DEWARI CONSTRUCTION
In the 2005 EERI report, Professors Rai and Murty commented more extensively on how dhajji dewari construction was affected by the earthquake: “In Kashmir traditional timber-brick masonry construction consists of burnt clay bricks filling in a framework of timber to create a patchwork of masonry, which is confined in small panels by the surrounding timber elements. This timber lacing of masonry, which is locally referred as dhajji-dewari has excellent earthquake resistant features. The resulting masonry is quite different from typical brick masonry and its performance in this earthquake has once again been shown to be superior with no or very little damage. No collapse was observed for such masonry even in the areas of higher shaking”. They go on to explain the reason for this good behaviour: The presence of timber studs, which subdivides the infill, arrests the loss of the portion or all of several masonry panels and resists progressive destruction of the rest of the wall. Moreover, the closely spaced studs prevent propagation of diagonal shear cracks within any single panel, and reduce the possibility of out-of-plane failure of masonry of thin half-brick walls even in the higher storeys and the gable portion of the walls. Dhajji dewari is timber frame construction rather than masonry bearing wall construction. Thus the vertical loads are transferred to the ground primarily, but not exclusively, through the frame. However, the masonry does form an integral part of the structural system, sharing the vertical load path with the timber frame. As has already been explained, this infill masonry serves a primary role in the case of lateral earthquake loads. In the damage district of Kashmir on both sides of the Line of Control, there were enough buildings of dhajji construction to observe the effects of the earthquake on the construction system. In the mountains on the Pakistan side, the infill material is more commonly rubble stone set in mud mortar, while on the Indian side, particularly in the Vale of Kashmir where clay is abundant, the use of fired and unfired clay brick is more common. For the same reasons as explained above for taq construction, the mud or weak lime mortar encourages sliding along the bed joints instead of cracking through the bricks when the masonry panels deform. This sliding also serves to dissipate energy and reduce the incompatibility between rigid masonry panels and the flexible timber frame. The basic principle in this weak and flexible frame with masonry infill construction is that there are no strong and stiff elements to attract the full lateral force of the earthquake. The buildings thus survive the earthquake by not fully engaging with it. This “working” during an earthquake can continue for a long period before the degradation advances to a destructive level. Thus the engineering principle behind the earthquake performance of the dhajji walls is a simple one. The subdivision of the walls into many smaller panels with studs and horizontal members, combined with the use of low-strength mortar, prevents the formation of large cracks that can lead to the collapse of the entire infill wall, while the redundancy provided by the many interior and exterior walls that exist in a standard residential building reduces the likelihood of catastrophic failure of the frame.