126.96.36.199 Unreinforced Masonry Parapets
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Unreinforced masonry (URM) parapets, cornices, and appendages pose a significant falling hazard and have caused numerous injuries and required costly repairs in past earthquakes. While the function of parapets is “nonstructural,” i.e., to prevent fire spread, create a safety railing, or conceal roof-mounted equipment, they are a structural concern that requires engineering expertise to address.
- Typical Causes of Damage
- Seismic Mitigation Considerations
Building Code Provisions
Parapets are subject to the force requirements of Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) Chapter 13, Nonstructural Components. In general, unreinforced masonry construction is not permitted in areas of moderate and high seismicity (Seismic Design Categories B and higher).
Retrofit Standard Provisions
Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06) classifies unreinforced masonry parapets as force-controlled. Compliance with the requirements of the standard is requiredfor all performance levels in areas of high, moderate and low seismicity.If the performance level is Hazards Reduced and the unreinforced masonry parapet is located in areas of public occupancy or egress, it must meet the Life Safety performance level. If the unreinforced masonry parapet is not over 4 feet in height, it may be retrofitted using prescriptivedesign concepts.
Typical Causes of Damage
- Heavy unbraced parapets typically fail out-of-plane at the roofline and may take part of the building wall with it as they fall. Even small pieces of masonry falling several stories may cause serious bodily injury.
- Appendages may crack or spall; connections may be damaged. Pounding between adjacent buildings often results in damage to brittle masonry parapets, cornices, and appendages.
- Failed parapets may fall either inwards or outward. When parapets collapse inward they can damage the roof and have the potential to fall through light roof construction posing a safety hazard to occupants below. If they collapse outward they can fall to the street or onto the roof of an adjacent property.
Figure 184.108.40.206-1 URM parapet fell and smashed two cars in the 1992 magnitude-7.2 Petrolia Earthquake. A parapet at same location in Ferndale, California failed in 1906 and killed two cows (NGDC, 2009).
Figure 220.127.116.11-3 Damage to roof framing caused by failure of overhanging brick masonry during the 1962 magnitude-5.8 Cache Valley, Utah earthquake (Photo courtesy of PEER Steinbrugge Collection, No. S828).
- Some jurisdictions have parapet ordinances requiring seismic bracing for URM parapets along a public right of way; check the local jurisdiction for requirements.
- Parapet and roof conditions may vary widely. An engineered design accounting for specific as-built construction details is needed to provide reliable earthquake performance. Flashing and weatherproofing must be provided for any roof-mounted connections.
- The reliability of bolts in unreinforced masonryis heavily dependent on the quality of masonry and the amount of overburden (weight above the anchor). With poor quality masonry or little overburden, oversized plate washers or a continuous plate may be needed on the outside face of the wall to enhance the effectiveness of the anchorage.
- Connection details for terra cotta cornices and appendages are similar to those for anchored veneer. See Section 18.104.22.168 and check the internet for various types of masonry, stone and veneer anchors.
Figure 22.214.171.124-7 Unreinforced masonry parapet (ER).