The Seven Steps
Secure Concrete or Masonry Walls to Roof or Floor

The Problem

When earthquakes shake the ground, the various parts of buildings may move in different directions. If the connections (anchorage) between concrete or reinforced masonry walls, wood floors, and roof are weak, walls can pull away. (See figures a, b and c.) And the building, or a portion of it, may collapse. Until the mid-1970s, California building codes did not require new buildings to have wall anchorage that was adequate to prevent separation between the walls and the roof.

The Northridge earthquake showed that some types of wall anchorage installed even after 1975 were not adequate to support the walls.

Poor wall anchorage is also common in unreinforced masonry buildings. (See figure d)

How to Identify It

Hire a qualified civil or structural engineer to:

  • Check buildings with precast (tiltup) concrete or reinforced masonry walls that were built before 1975 for wall anchorage.
  • It is also a good idea to check all anchorages built before 1995 to ensure the meet post-Northridge Earthquake standards.
  • Check unreinforced masonry building wall anchorages.

Remember

  • A good time to check and fix wall anchorage is when you replace or patch your roof.

Failures of Wall Anchors During Earthquakes (a – d)

  1. The roofs and floors of many concrete tiltup and masonry buildings rest on ledgers bolted into the wall.
  2. When an earthquake occurs, the building’s movement may rip out the nails holding the roof in place,
  3. or split the ledger along the wood grain allowing the roof to collapse.
  4. “Dog ties,” also known as government anchors, will not always prevent unreinforced masonry walls from separating from floors or roofs.
Source: California Seismic Safety Commission

Aspects to Consider

Strengthening Wall Anchors – One of several ways to strengthen the connections between the roof and floor joists and the walls is to install brackets and rods that go through the walls and attach to the joists.
Source: California Seismic Safety Commission
If you suspect your building has poor wall anchorage, consider hiring a qualified engineer or architect to determine the most cost-effective way to strengthen it.Technical information for engineers to use when designing strengthened wall anchors can be found in Appendix Chapter A2 in the International Existing Building Code, published by the International Code Council.

Contractors can add new anchorage and continuous ties across the roof. Work can be done inside above the ceiling (see drawing right) or on the roof, at relatively low cost.

Wall Anchors Can Prevent Failures Like This – Even newer buildings are not immune to earthquakes. This reinforced masonry strip mall was built in the 1980’s but did not comply with the building code. The wall anchorage failed in the 1992 Landers Earthquake. Its front wall fell into the parking lot.
Source: California Seismic Safety Commission

Possible Solution

To anchor the walls to the roof/floor.

Recommendation

Joists Parallel to Wall
Source: Noson, Perbix, SSD
Install LTT anchors.

Supplies Required

  • LTT anchors (manufactured by Simpson or equivalent)
  • Thru-bolt—3/4″ diameter
  • Washer—3/8″ x 6″ diameter
  • 2x___ blocking (depth to match existing joist depth)

Installation

Wall Section
Source: Noson, Perbix, SSD
  1. Install LTT anchors approximately every 4′ on center.
  2. Where existing joists are perpendicular to the wall, attached anchors to the sides at the top or bottom of the joists.
  3. Where existing joists are parallel to the wall, install blocking to two joist spaces at the LTT anchor locations. Install anchors above or below blocking.

Note

If washers are not desired, as in historic buildings or terra cotta exteriors, epoxy an all-thread rod inserted into the wall a minimum 8″.

Larger imageJoists Perpendicular to Wall

Source: Noson, Perbix, SSD

Source: Noson, Perbix, SSD

© 2024 Statewide California Earthquake Center (SCEC), headquartered at the University of Southern California (USC)