One of the first and most effective steps taken to make old unreinforced masonry (URM) buildings more seismic-resistant is to anchor wood floor and roof diaphragms to URM walls. Unanchored walls and diaphragms can separate during a quake, resulting in collapse of one or both. Anchors keep walls and diaphragms tied together and reduce the chance of failure. The three anchoring types in common use are tension anchors, shear anchors, and combination anchors. The type used depends on the specific application, the engineer's requirements, and the local building codes. TENSION ANCHORS Tension anchors are used to keep walls that are perpendicular to seismic acceleration from pulling away from diaphragms, causing the diaphragms to drop. The tension anchor is simply a through bolt. A hole is drilled completely through the masonry and the bolt is installed. On the outside, it's anchored by a large washer and nut. On the inside, it's fastened to the diaphragm. SHEAR ANCHORS Shear anchors help prevent diaphragms from sliding inside URM walls and help transfer seismic force from the diaphragms to the shear walls. The shear anchor is not a through bolt. A hole is bored at least 8 inches deep but not completely through the masonry. The anchor is centered in the hole and mortared into place. COMBINATION ANCHORS Combination anchors combine the functions of both tension and shear anchors into one unit. All three types of combination anchors are mortar packed to accept lateral forces like a shear anchor. One type uses a through bolt for tension anchoring. One uses a beveled washer to square the anchor with the wall face. The other employs a prebent anchor bolt.