Final answer:
Retrofit techniques for URM structures with a flexible diaphragm involve the use of anchors, ties, supplemental framing, or stiffening materials to improve the connection between the diaphragm and URM walls, enhancing seismic resistance.
Step-by-step explanation:
Retrofit techniques for Unreinforced Masonry (URM) structures with a flexible diaphragm are engineering methods used to enhance the seismic performance of older masonry buildings. These techniques aim to improve the overall structural integrity and ensure that the diaphragm - typically the roof or floor system - can adequately distribute seismic forces to the vertical supporting elements without causing a disproportionate collapse. One common method is to tie the diaphragm to the URM walls with anchors or ties, which helps keep the walls from separating from the roof or floor during an earthquake. Other methods involve the use of supplemental framing or trusses to provide additional support to the diaphragm, and the stiffening of the diaphragm itself with new materials or reinforcement. The ultimate goal is to create a more ductile connection between the diaphragm and the walls, allowing for some movement without catastrophic failure.