1/30/2008

Wind, Rising Water or Both?

With an issue this critical, inspectors must get it right.

By Tim Marshall

The question whether wind, rising water, or both damaged a building becomes important when different insurance coverages are involved. Damage assessment is easier, obviously, if the building remains intact and on its foundation. But, if the building is gone, determining the degree of damage becomes more difficult. Buildings located in low-lying areas along or near the coast are vulnerable to both wind and rising water damage in a hurricane.

An inspector must know about building construction, building materials and environmental factors, as well as have an understanding of wind and water forces in order to make an accurate damage assessment. Here are some tips on how to accurately assess wind and rising water damage to a building.

Wind vs. Water
First, remember that wind and water forces attack a building differently. In general, wind forces are greatest at roof level while water forces attack the base of the building. Logically, the roof is particularly susceptible to wind damage since it is the highest building component above the ground. Wind pressures on a building are not uniform but increase with height above the ground and thus, are greatest at roof corners, rakes and ridges on the windward side. Wind damage generally begins at roof level and progresses downward and inward as wind velocities increase.

Look at items such as television antennas, satellite dishes and chimneys for wind damage. Some building items “catch” the wind and are more susceptible to damage like awnings and carports. Then, as wind velocity increases, “claddings” or items on the building become susceptible to wind damage such as three-tab roof shingles, unanchored roof tiles and vinyl siding. Further increases in wind velocity can lead to loss of the roof deck, roof structure, or perimeter walls.

The direction of wind is an important consideration in any damage inspection. For example, if the east gable end blew inward, this means the damage occurred from an east wind. Since wind direction changes as the hurricane passes, we can estimate the time that damage occurred when studying local wind data. However, when looking at wind data, be careful to consider the type, height, location and exposure of the wind measuring equipment relative to the building site. Often, wind data needs correcting. Plus, forecasts and computer simulations of wind speed/direction are only estimates which can vary greatly from what actually occurred at the building site.

Water forces are greatest at the base of the building. These have a tendency to destroy the support walls and collapse part or all of the building. So, a building elevated above the water can be destroyed by moving water if its supports are removed. In these cases, the building collapses on the oceanfront side first, appearing to be “pitched down” toward the water. Low-level forces from moving water pushes the bases of walls landward creating hinge lines along the top of the walls. In contrast, wind forces create hinge lines along the base of the walls. Thus, wind and water forces have opposite hinge lines on walls oriented perpendicular to the flow.

One huge difference between moving air and moving water is that water possesses a much greater force than air at the same velocity. For example, a 1-foot tall wave traveling at 10 miles per hour possesses as much kinetic energy as a 280 mph wind. Moving water can lift poorly anchored wooden buildings since they are buoyant and will float. These buoyant wooden buildings can travel hundreds, even thousands of feet inland.

However, buildings with brick veneer construction tend to rise and sink within the brick veneer shell. A brick veneer building invariably does not come back to the same position causing distortion of its wooden-frame. Moving water also frequently guts the interior of the building removing or dissolving gypsum board and transporting contents such as furniture and appliances. Look for scrape marks on ceilings and walls above the water line, indicating that these items floated.

When inspecting the site of a building that is gone, first determine the elevation of the foundation and whether the building survived prior hurricanes. This can establish a history of wind and water forces that the building had experienced.

It is a good idea to examine other buildings nearby that survived. Check for water lines that appear like bathtub rings in these buildings and compare them to the height of the water level at your building. Also, check the degree of wind damage to trees, buildings and other objects around the site. This comparative analysis may help you better delineate the damage.

Buildings on Concrete Slabs
Even when a building has been reduced to a concrete slab, evidence still remains. Indications that moving water destroyed the building involve finding pieces of the roof structure remaining upright on the ground with the roof shingles still intact. Look for items remaining on the slab, such as bent plumbing, nails, or bolts may give some indication of the direction and magnitude of the low-level forces that were applied to the base of the building.

Look for unbroken debris deposited nearby such as glass doors, windows, mirrors, dishes and lights. We even have found ceiling fans with the blades and glass globes still attached. The lack of damage to such brittle items resulted when the building was dismantled slowly by wave action and these items fell into the water. Once submerged, such items remained protected.

If wind destroyed the building, there should be evidence of high velocity impacts on remaining items such as bathtubs, commodes, chimneys, etc. Roof sections that are transported downwind shatter upon impact. Look for pieces of the roof covering, decking, or structure impaled in trees or other objects. Attic insulation and other lightweight items litter nearby trees, above the water line. Trees that survive downwind can be debarked by flying debris above the water level.

When encountering a building which has been transported intact away from its foundation, look for items still hanging on the walls and glassware standing upright in cabinets. This would indicate the building moved slowly (low velocity) and came to rest slowly (low impact). Wind would have broken such items if the building moved rapidly (high velocity) and came to rest suddenly (high impact).

Buildings on Timber Pilings
The extent of rising water damage to a building elevated on timber pilings depends on factors such as the height of the building above the water, the depth of the pilings and its exposure to wave action. If the building is located on a sandy beach, sand can be scoured from around the pilings which can cause the pilings to rotate or collapse.

Concrete slabs are typically poured on-grade around the pilings and scouring of sand beneath them by wave action frequently leaves the slabs elevated or causes them to collapse. As the water level rises, the first story becomes submerged and moving water removes the breakaway walls and first story contents. Sometimes abrasions or scar marks appear on the floor slab or pilings where floating debris repeatedly impacted.

Moving water and wave action create both lateral and upward forces on the second story floors. Wood girders are bent landward or uplifted from the tops of the pilings breaking out of their bolted connections. Sometimes scrape marks appear on the girders where floor joists have bobbed up and down in the water rubbing up against and abrading the wood. Look for the presence of floating debris draped over the girders. Rolling waves can lift the subfloor causing fasteners to back out of the wood.

This process does not occur all at once but progressively as each wave strikes the floor bottom. Moving water frequently rotates or removes the wood blocking between the floor joists. Floor joists oriented perpendicular to the flow can be pushed inland by wave action and stacked together on the landward side of the building. The loss of the floor usually results in the partial or complete collapse of the building. Generally, the connections between the floor joists and girders are weaker than between girders and pilings. Thus, in many instances, all that remains of the building are the girders bolted to the pilings.

If wind removes the roof, the tops of the windward walls are no longer supported and the walls can be pushed inward rotating about a hinge line at the floor. The key difference between wind and rising water damage to an elevated building is that wind dismantles the building from the top down leaving the floor platform in place. However, rising water and wave action dismantles the building from below removing the floor platform.

Summary
Wind and rising water forces cause different types of damage to a building. In general, wind usually affects a building from the top down, whereas moving water affects a building from the bottom up. The most important evidence is found at the building site and its surroundings even if the building is destroyed. In essence, the building site tells the story. Having the proper knowledge of how wind and rising water forces affected the building is crucial to figuring out what happened.

Tim Marshall is a principal engineer and meteorologist with Haag Engineering Co. He has assessed damage to thousands of buildings from dozens of hurricanes during the past 30 years. Tim also has served as a damage expert in several wind vs. rising water cases in Hurricanes Iwa, Opal, Ivan, Dennis and Katrina.



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