Portable structure tester may bring better-built homes, shopping malls, skyscrapers

Insurance adjusters who reenter an area the day after a hurricane never know specifically how a structure was destroyed. Did a window pane shatter, creating a momentary pressure surge inside that pushed the roof off like a pop-gun cork? Or did a gust of wind pry up the eaves of a house and peel away the roof like a sardine can lid?

Could the builder have done something to prevent the destruction — used better nails, stronger materials, special braces, or improved design features, for instance?

Historically, next-day knowledge of the specific cause-and-effect relationships that lead to structural damage from high winds, earthquakes, or floods has been unattainable and, thus, irrelevant in settling claims or setting policy premiums. But high on the insurance industry’s wish list is a better understanding of why buildings fail.

A portable instrument package being developed by engineers in Engineering Sciences Center 9100 may one day help gather real-world data about the response of typical buildings to high winds and other structural distress. Such data may help construction contractors, insurance companies, regulatory agencies, and homeowners create safer, stronger, more secure buildings that might withstand most storms.

Better-built buildings would lower the costs of damage claims stemming from natural disasters, often in the billions of dollars for a single storm. Understanding which design features beget the most insult-tolerant buildings might allow insurers to offer some policy-holders better rates, as well.

And stronger structures would help buildings do what they are intended to do — protect their occupants from even the most severe elements, says Tom Baca, Manager of Experimental Structural Dynamics Dept. 9119. The device was developed as part of Sandia’s Architectural Surety ^SM program (Lab News, Aug. 28, 1998).

"The intent eventually is to be able to use sensor data to certify a building design, much like the way we certify weapons components and systems today," he says.

Putting real buildings to the test

The idea to create Sandia’s portable building tester, called the Autonomous Structural Event Recorder (ASER), grew out of discussions with officials of the Institute for Business and Home Safety (IBHS), a nonprofit coalition of insurance companies aimed at reducing losses from natural disasters by encouraging homeowners and builders to make and buy more resilient homes and businesses.

In their efforts to find ways to buffer buildings, IBHS insurers found plenty of companies eager to recommend their own disaster-mitigation products — from pressure-treated wood and metal joist straps to "magic nails" and nylon nets that can be cast over homes before a hurricane. They also found very little data to support or contradict the product claims.

Truth is, says Tom, most information about which disaster-mitigation approaches work and which don’t are conjecture. Differences among local building codes skew any data that is available. And storms can be selective; based on the buildings’ orientations, construction quality, or other factors, a hurricane might destroy all the homes on one side of a street and leave the other side intact. Adequately localized wind readings are rare too; the nearest airport or weather station can be miles from the damage.

And often nobody is around during a storm to report exactly how the damage occurred. Did the roof fly off first, or did a particular joist give way? Did the failure of a window pane lead to other damage? Insurance adjusters can only make educated guesses about what happened based on what’s left of the evacuated buildings the next day.

One major insurance company has embarked on a costly effort to study wind speed, building orientations, construction standards, and other factors buried in millions of old insurance claim documents to create better actuarial data that might shed light on what conditions tend to cause the greatest losses.

"To evaluate the true performances of structures, you’d need localized data taken on real buildings during real storms," Tom says. "That’s what we hope ASER can provide."

The stakes are high. In the last 10 years the Federal Emergency Management Administration (FEMA) doled out more than $20 billion in federal disaster assistance. Insurance companies paid out hundreds of billions more in claims. (See "Interest in structural performance is growing" below.)

"Using real data would represent a very large departure from the insurance industry’s normal way of doing business," he says.

ASERs for a variety of natural disasters

To address this need a team in Center 9100, which has for decades been Sandia’s go-to group for structural testing of weapons systems and weapons components in severe environments, conceptualized an inexpensive suite of sensing instruments that could be widely deployed in selected buildings within a historically storm-affected region.

Original team members Pat Barney, Todd Simmermacher (both 9119), Johnny Hurtado (9611), and Bill Sullivan (1401) proposed an ASER setup configured for hurricane-type weather, and Pat and David Kelton (9119) programmed and assembled a prototype with support from the Laboratory-Directed Research and Development program.

The prototype employs roof-mounted wind speed and wind direction sensors called anemometers, including a developmental anemometer specially designed by Sandia for winds in excess of 80 mph.

Pressure sensors deployed at various locations throughout the building — under the eaves and in the attic, for instance — help measure pressure differences inside and outside the building. Abrupt changes in this pressure differential can help pinpoint the exact moment a window gave way or a roof was destroyed.

Strain gauges measure stresses on structural elements, window sensors record window breakage and vibration, and a triaxial accelerometer mounted within the roof’s structural members detect structurally significant building shudders.

All the sensing instruments are linked to a ruggedly packaged, laptop-based computer system that includes backup power, high-capacity disk storage, and data-compression software that allow storage of up to 10 hours of data. The system doesn’t begin recording storm data until a triggering event sets it off — a gust of wind exceeding a predefined threshold or a building movement of predetermined magnitude, for instance.

To test the concept, the prototype system was used to gather data about the structural response of a large sheet metal shed in Area 1 during February and March, the windiest months of the year in New Mexico. From the data the researchers can now correlate wind speed readings with structural displacement measurements to develop cause-and-effect relationships. The results will be used to improve the system’s software.

In addition, Sandia has used the ASER idea to help the Air Force test a large tracking antenna.

Following a real storm, researchers theoretically could draw correlations among speed and pressure, pressure and motion, motion cycles, strain-causing stress, and other indicators. All data are recorded with respect to time so that cause-and-effect issues can be analyzed.

Each ASER system for hurricane weather should cost less than $5,000 including the computer, says Todd. The low cost would allow data to be gathered, potentially, at hundreds of sites in a storm-affected region. An ASER could be set up on a home in less than 48 hours, if needed.

ASERs also could be configured to monitor building performance in a variety of other natural disasters — earthquakes, bomb blasts, floods, tornadoes, and fires.

Investing wisely

In the long run, Tom says, the goal is to provide the insurance and construction industries with a new tool that can help them gather data about the performance of the generic home, office building, shopping center, or skyscraper and lead to prioritized improvements to building design, construction standards, and retrofits.

"We can’t subject every house in Florida to a hurricane, but we could model the generic house and come to conclusions about what designs withstand storms and which don’t, then develop some solutions," he says.

That could help home and business owners make informed decisions about which construction features are worth investing in, and help regulators know which building code changes make sense. It might also allow insurance companies to develop incentive programs that encourage building owners to adopt the approaches that are proven to work.

"People want to know, ‘If I modify my home, when am I going to get some of that investment back?’ " says Tom. "They’ll do it if they see it making a difference in their monthly insurance bill."

Eventually, he says, enough data might be gathered and fed into high-performance computing codes that building performance might be simulated and different designs or building materials modeled before construction begins.

Buildings of the future might also have built-in sensing systems to continuously monitor their own state of health, he says.

"Multipurpose sensor systems might be integrated into smart homes like security systems are today," he says. "It might become a safety feature people are willing to pay for, like airbags in cars," he says.