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Unreinforced Masonry Buildings: A Hidden Danger

July 23, 2014

Did you know that almost 1,000 buildings in Seattle are constructed of unreinforced masonry (URM)? At first glance this sounds like a large number of buildings and a random fact, but once we understand the seismic dangers of unreinforced masonry this statistic suddenly becomes pretty frightening.

About 929 buildings in the Pioneer Square neighborhood and all around Seattle are unreinforced, and only about 15% of those buildings have had the necessary seismic upgrades to make them earthquake safe. So what’s the big deal?

A Little Bit of Background

During the early to mid-19th century, wood was plentiful and cities all over America were booming, especially on the West Coast. URM SeattleGold fever struck and Seattle became a major stopping point for those heading to Alaska, while Californians were discovering gold all up and down their state. Many cities needed a cheap, easy-to-acquire material to accommodate huge amounts of growth as quickly as possible. Forests were cleared and the wood was the obvious choice for building. This produced the results one might expect, and by the end of the 19th and beginning of the 20th century, many large cities in America experienced huge conflagrations, sometimes multiple, that decimated city centers. Even Seattle had its own massive fire, joining Chicago and other cities – including Washington State cities Ellensburg and Spokane – who found they had to rebuild.

Masonry, whether it’s concrete, adobe, brick, stone, or otherwise, is relatively noncombustible, especially in comparison to wood. While the contents and combustible flooring within these buildings are susceptible to fire, the external structures themselves tend not to be. During the early 20th century, after most wooden cities had burned down, a majority of buildings were built of joisted masonry construction.  These buildings are still susceptible to fire due to combustible roofing (and combustible floors in multi-story structures), but they’re designed to fall in on themselves when they collapse and don’t spread fire to other buildings quite so easily. Masonry also helps keep a fire contained within a structure for longer, whereas fire easily spreads between wooden buildings. Many cities, after experiencing devastating losses from fires, decided to rebuild using masonry materials to help prevent future conflagrations. Seattle created ordinances requiring buildings in commercial districts to be built of stone or masonry materials, and this worked wonders in many communities as giant cities burning to the ground became a thing of the past.

Problem solved, right? Not so much.URM2 Many people know California is a hot bed of earthquakes, but Seattle is equally as likely to experience a devastating earthquake at any moment. Unreinforced masonry is notoriously brittle and lacks tensile strength. Tensile strength is a measure of how much stretching or pulling a material can withstand before it falls apart. Earthquakes, on the other hand, move the earth in all different kinds of waves causing structures standing on top of it to oscillate; that is, earthquakes cause structures to move and bend. When you combine a rigid, brittle structure with a moving, fluid foundation, you have a recipe for disaster.

It’s quickly becoming clearer why URM buildings in an earthquake-prone area are a huge problem. If we look back at a long string of earthquakes from the 1906 San Francisco earthquake all the way up to the 1994 Northridge quake, we can find examples of URM buildings suffering from complete collapse. In this picture, from the Long Beach California earthquake in 1933, you can see what happened to the Continental Baking Company building. It has completely collapsed and most likely anyone who may have been inside did not survive.

URM3

Photo Courtesy of Colorado.edu

An estimated 82% of URM brick buildings experienced more than minor damage, and 7% collapsed after the 1886 earthquake in Charleston, South Carolina. But all of these examples were a long time ago and far away from Seattle, right? How about we move a little closer to home: in 2001, after the Nisqually Earthquake (which was centered near Olympia, WA):

…buildings built before 1950 exhibited the poorest behavior. The most common damage included shedding of brick from parapets and chimneys. Other URM buildings exhibited diagonal ‘stair-step’ cracking in walls and piers, damage to walls in the upper stories, vertical cracking in walls, damage to masonry arches, and damage to walls as a result of pounding. In many cases, fallen brick resulted in damage to objects, such as cars and canopies, outside the building.” (Source)

The Nisqually Earthquake was a magnitude 6.8 with an epicenter located near Anderson Island, about an hour and a half south of the city. Imagine the damage had this earthquake been located closer to Seattle! If you look at the buildings after earthquakes in third world countries, including the one a few years ago in Haiti, you can see what utter devastation URM buildings cause.

What can we do? Going back to cities built from wood is not practical, and URM buildings are obviously unsafe. The solution, it ends up, is reinforcing buildings.

The Case for Reinforced Masonry Buildings

The entire west coast of the United States, and some states across the South, require that buildings are built using reinforcing structures (check out this publication by FEMA to learn more). Buildings can be reinforced using pre- or post-stressed concrete (sometimes referred to as pre- and post-tensioned) during the construction process. Existing URM structures can be retrofitted to give them the strength and stability they need.

How is it done? To start, they use steel and concrete. Steel is created when carbon is added to iron, and concrete is made when sand or gravel is added to cement and water. Steel can withstand a large amount of tensile stress, comparatively, meaning it bends and flexes rather easily. URM5Concrete can withstand compressive stress (meaning it can withstand force pushing down on it) and the combination of these two elements make both stronger than they would be alone. By using both in the construction of large buildings, the steel can withstand tensile stresses while the concrete withstands the compressive stress. These structures can also withstand weather and fire because of the concrete, and the concrete helps protect the steel from rust and heat.

What’s the difference between pre- and post-stressed concrete? In short, a pre-stressed concrete beam has tension put on it before it leaves the manufacturing plant. There’s a slight, but noticeable, arch to the beam. A “prestressing strand” made of steel is stretched across a casting bed. Generally about 30,000 pounds of tension is then applied to the cable and then concrete is poured on. After the concrete hardens and dries, the strands are cut. Why do they stretch and tension it beforehand? In an excellent demonstration by PBS, imagine a rubber band loosely held between two fingers.  Stretch the fingers apart and see how taut and pressure resistant the band becomes. The same idea holds true for the steel.  Pre-tensioning is normally done at a factory and then trucked to a jobsite.

Post-tensioned concrete slabs tend to be about 8 inches thick in residential construction and are created by laying out steel cables in a grid pattern inside tubes or ducts. Concrete is then poured around the steel cables and allowed to dry and harden to a certain specified strength. Once appropriately hardened, the steel is then tensioned and anchored to the outer edges of the concrete. The benefit of post-tensioning concrete is that it can be done at the job site, making it ideal for much larger structures.

While these are simplified examples of how it’s done, and no matter what method is used, adding the steel cables to concrete increases the overall strength of both materials in the structure. As we’ve stated this increases its fire resistiveness and ability to withstand earthquakes and helps protect the steel from rust and heat.

By increasing masonry’s tensile strength with steel, and by strengthening steel’s compressive stress abilities, modern reinforced masonry buildings are proving to be safer and more reliable in earthquakes while helping to prevent massive conflagrations in most major US cities.

Retrofitting – A Costly, But Worthwhile, Endeavour

Fortunately these URM buildings all over Washington State, and the country, are not just sitting ducks. Retrofitting, even 100 years after the building was constructed, is possible and it works. Many major US cities are undergoing retrofitting programs or are working with building owners to address these issues.

URM6The process is slow and requires careful consideration because retrofitting a building is, many times, prohibitively expensive. After the Nisqually Earthquake in 2001, the City of Seattle passed a ballot measure to retrofit 32 of the city’s fire stations that were not properly reinforced. Over $197 million in tax dollars were collected to upgrade these structures. Some building owners in Seattle have said that it may cost as much as $1.5 million to update a single building. No retrofit requirements are currently in force in the city, but that could change.

Why spend the money to retrofit? Unreinforced masonry buildings are prone to massive damage or collapse after an earthquake. If buildings are not reinforced prior to a quake, much of it may need to be rebuilt after. More importantly, however, is life safety. Retrofitting a building helps prevent damage or collapse, and keeping the structure sound means the people living or working inside have a much higher chance of surviving the shake. Find out the intention of the architect or engineer prior to starting on a retrofitting project. Life safety and maintaining the structure to help avoid costly rebuilds after an earthquake should be the priority.

What Does This Mean For Your Insurance?

Earthquake coverage is not included on a standard insurance policy in Washington State and must be bought separately. If you live in or own a business in a URM building and an earthquake completely or even partially collapses your business, home, or apartment, your regular policy will not cover you or your belongings for the earthquake damage. That’s pretty scary. Talk with your agent about purchasing earthquake insurance and make sure you understand the ins and outs. For example: fire-following is a common clause in most insurance policies which states that if your home is damaged or destroyed by a non-covered cause of loss, but a fire results directly from the non-covered cause of loss (example: an earthquake severs the gas line running into your home and causes your home to catch on fire) the damage from the fire is generally covered. Another example: relatives of mine bought specific earthquake insurance, and the deductible is 10%. If $500,000 of damage is caused to their home by an earthquake, they will have to pay $50,000 out of pocket before the insurance kicks in, and only the structure of their home is covered, not the contents. Talk to your agent. It’s always best to understand the coverage you’re buying and what it means before disaster strikes.

For more information on URMs please check out the following sites!

Article by: Kristen Skinner

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