What You Need to Know About Differences in Wind-Speed Reporting for Hurricanes

This week’s post was written by Darren Conrad, PE. Engineering Manager, Truss at Simpson Strong-Tie.

With Hurricane Irma wrapping up, the cleanup after Hurricane Harvey’s devastation underway in Houston and more big storms already churning in the Atlantic, it seems like a good time to discuss hurricanes and high wind. There is a great deal of good information out there to help us better understand hurricanes and their impact on people, structures and other property. To improve awareness of wind speeds and their measurement, this article will discuss a commonly misunderstood aspect of hurricane wind-speed reporting.

When a storm is approaching, you will hear meteorologists report wind speeds. They often refer to storm categories. These categories attempt to generalize expected damage to structures based on the wind speed of the storm. The wind speed for a given storm is a measure of the severity of the storm and the danger it poses to life and property. But how do meteorologists determine the wind speed that they are reporting? It seems so concrete and certain, but anyone who has been outside during a storm or windy day knows that wind isn’t constant at any one location over a period of time. It varies continuously in magnitude and direction over time. So how can something so variable be the subject of knowledge that is precise enough to be useful? How do we understand wind-speed measurements and make sure that when comparing them we are doing so in such a way that they are comparable? That is a great question.

The good news is that even though wind is variable, we have a commonly accepted way to measure wind speed and know something about a wind field or event that is occurring at a time and place. This is done by averaging measured wind speeds over specified lengths of time, or picking the highest average wind speed that occurs for a specified averaging interval from a longer period of time. A great resource for understanding how wind speeds are measured and reported can be seen here. From this explanation, it can be seen that a reported wind speed is meaningless without a specified averaging time. The shortest averaging intervals will yield the highest reported wind speeds. The longer averaging times will capture more peaks and lulls and yield lower reported wind speeds. The most common averaging intervals used to report wind speeds are three seconds, one minute and two minutes. Some countries even use a ten-minute averaging interval for reporting wind speeds. So the question arises, which average is correct? And the answer is, none of them and all of them. They are just different ways of looking at measured wind data. That is not very comforting, but one thing we can know is that none of them can be truly interpreted or compared without understanding this idea of averaging time. To make it more confusing, meteorologists and building codes do not use the same averaging interval when reporting or specifying wind speeds. This can lead to misunderstandings.

In general, you will hear meteorologists report sustained wind speeds when covering an approaching hurricane. They might also mix in some peak gusts, but for the most part they focus on sustained wind speeds. Sustained wind speeds for tropical cyclones use a 60-second averaging time. Sustained wind speed is also used by the Saffir-Simpson scale to roughly quantify the likely damage that the wind from a storm might cause typical buildings and other structures. There are criticisms of the accuracy of the Saffir-Simpson scale method, but it is widely used by the public to generalize about the severity of tropical cyclones; therefore, it is likely that the public might and commonly does attempt to compare reported sustained wind speeds to building-code-specified three-second-gust wind speeds to determine if their house or structure will withstand the storm. There is danger in making that comparison.

We need to be careful when comparing the reported sustained wind speed for a storm with the three-second-gust design wind speeds referenced in building codes and design standards. They are not the same and need to be converted before they can be compared for equivalence. After seeing the following example, one could easily see the possibility of the public or a public official comparing the sustained wind speeds reported by the weatherman to the wind speeds used by building codes and design standards and drawing conclusions that may underestimate the force and effect of the storm.

Let’s take a hypothetical situation where a building jurisdiction has adopted a wind speed of 130 mph three-second-gust design wind speed for structures built in that jurisdiction. There are various methods to convert wind speeds between different averaging times, and many factors that may need to be considered when doing that. One method for converting is the Durst method referenced in ASCE 7. Another more recent method recommended by the World Meteorological Organization provides a pretty straightforward conversion between sustained wind speed and three-second-gust wind speeds for near-surface applications. So for the sake of simplicity, we will use it for this example. If we convert a reported sustained wind speed of 130 mph to a three-second-gust average wind speed using this method, it equates to a three-second-gust wind speed for Off-Sea of 160 mph (Off-Sea is appropriate for an approaching hurricane). The adopted130 mph three-second-gust wind speed converts to 105 mph sustained wind speed. This difference could lead individuals in the path of the storm to underestimate its severity if they are not aware of the difference between averaging intervals for wind speeds. They could see the sustained wind speed of 130 mph being reported by the weather service when the storm is over open water and assume that their structure, or structures in their jurisdiction, will stand up fairly well. This would be a serious underestimate, since those structures would need to be designed to resist a 160 mph three-second-gust wind speed using ASCE 7 in order for that to be true. To say that a different way, one might think that their structure was designed for a Category 4 storm (130 mph sustained), when in fact it was actually designed for a Category 2 storm (105 mph sustained) using the Saffir-Simpson scale. Hurricane Irma at its maximum sustained wind speed of 185 mph would equate to a 227 mph three-second-gust wind speed using this conversion method. From a roof anchorage, lateral design and load path design perspective, the difference between 130 mph and 160 mph can be substantial, especially when the building is located on flat open terrain where Exposure C or Exposure D are appropriate assumptions for the design.

There is a lot more background and detail to this very complicated discussion, but the general point is to know your averaging times when comparing reported wind speeds, so as not to underestimate a storm’s force. If a storm is headed your way, hopefully you have already selected the proper hurricane tie for your structure; you have a well-defined and properly designed continuous load path; and you are protecting your exterior openings from windborne debris. Remember, the objective is not to protect the window or door product itself. Unless you are in the insurance business, you are preventing the breach of the opening to keep wind from pressurizing the structure, increasing loads on the structure and potentially causing catastrophic failure.

Know how to secure your structure against high winds, and be safe.

Designing Overhangs on Gable Ends

It seems that each major hurricane tends to teach those of us in the construction industry some lesson. With Hurricane Andrew, the lessons were the importance of protection from windborne debris, and the importance of proper construction of gable ends.

There are two main areas where gable ends can fail. One is a failure of the hinge at the connection between the top plate of the wall and the gable end framing, if the gable end is not balloon-framed with continuous studs. This is now addressed in the International Residential Code. Since 2009, Section R602.3 has required that “Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.”

For existing construction, the International Existing Building Code specifies a method for retrofitting gable ends in Appendix C.  For new construction, Simpson Strong-Tie shows a couple of solutions for bracing top plates of gable ends in our High Wind–Resistant Construction Application Guide on Page 48.

Figure 1, Gable Wall Bracing Methods

Figure 1, Gable Wall Bracing Methods

The other common wind-related failure at gable ends is uplift of the roof decking at the overhang. This can be from two causes: inadequate nailing of the sheathing to supporting framing, or inadequate connections of the framing at the rake edge that supports the roof. As far as this author can tell, this area of light construction is not covered in the International Residential Code for wood framing, but it is covered for cold-formed steel framing, where Section R804.3.2.1.2 contains requirements for “Rake overhangs.” The two methods shown are the cantilever outlooker (Option 1) and the ladder outlooker (Option 2).

Figure 2, IRC Gable Overhand Details

Figure 2, IRC Gable Overhand Details

Figure 3, Gable End Wind Damage

Figure 3, Gable End Wind Damage

In the photo above, it appears that the cantilevered outlooker method was used, and that there was a failure of the outlooker connections at the gable end and the first full truss. If you look closely, the end nails from the full-height truss that were in the end of the outlookers can be seen in a couple of places.

If a truss roof is used with this method, the gable truss is manufactured 3½” shorter than the others. Then a 2×4 outlooker is placed over the dropped gable, and butted into the side of the adjacent full-height truss. Then the barge or fly rafter is attached to the end of the cantilevered outlooker. At the overhang, wind can cause uplift on both the bottom and top surface. The uplift at the end of the outlooker imparts an uplift force at the gable truss, which must be resisted by a tension connection such as a hurricane tie, and a downward force at the connection to the full-height truss.

Figure 4, Cantilevered Outlooker Method

Figure 4, Cantilevered Outlooker Method

The other method commonly used to support the sheathing and the barge rafter is the ladder method. With this technique, lookout blocks are used to connect the barge or fly rafter back to the gable framing. One way this can be constructed is as a full ladder, with parallel fly rafter and ledger with block framing in between. Either this assembly can be constructed on the ground and then raised and fastened in place, or it can be built in place at the overhang. Or there are also examples where a ledger is not used, and the block framing is just connected directly to the top chord of the gable truss or gable rafter. This method is less wind-resistant, and in literature is limited to a 12″ overhang.

Figure 5, Ladder Outlooker Block Method

Figure 5, Ladder Outlooker Block Method

If the gable overhang is to resist wind loads properly, it must either be designed, or constructed in accordance with some pre-engineered prescriptive detail. Figure 4 shown above was originally published in a Simpson Strong-Tie Technical Bulletin, the High Wind Framing Connection Guide. But this Guide is no longer published. As shown earlier in Figure 2, there are some prescriptive details in the IRC for cold-formed steel construction. These are limited to an overhang length of 12″ and apply for up to 139 miles-per-hour ultimate wind speed. For wood-framed construction, comparable details are contained in the American Wood Council Wood Frame Construction Manual. For the cantilevered outlooker method, connection design loads are published for various wind speeds. Cantilevered outlookers are permitted to extend out up to 24 inches, while the ladder outlookers are only permitted to extend out 12 inches. See below for excerpted figures and tables from the Wood Frame Construction Manual, courtesy of the American Wood Council.

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

In addition to the framing design, the connection of the roof decking at this location is critical. If you’re building to traditional construction methods, with 6″ nail spacing at panel edges and 12″ nail spacing at interior supports, the close nail spacing ends up at the nonstructural outer member, while the nailing at the actual roof edge over the gable is only 12″ on center. As shown in the details above, newer documents do indicate the importance of spacing the nails over the gable end at the closest spacing, both because these are subject to the highest withdrawal loads and because this is the edge of the diaphragm for transfer of lateral loads.

The Journal of Light Construction has a discussion of the unbraced gable end overhang on one of their Forums.

The Florida Division of Emergency Management provides some information on wind resistance of gable overhangs and some possible means of retrofitting them here.

Have you seen or designed with different methods for framing gable overhangs?

Reminders from Hurricane Katrina

This week is the 10th anniversary of Hurricane Katrina, and we have all seen articles on the lessons learned from the storm. Engineers learn something new from every storm. However, I think that Hurricane Katrina just gave us some very strong reminders of things we already knew.

Hurricane Katrina reminded us that hurricanes are flood events as well as high-wind events. And I don’t mean the flooding in New Orleans. No, I mean the flooding along the Gulf Coast from Louisiana to Florida.

I witnessed the complete devastation of the Mississippi Gulf Coast from Waveland to Biloxi. Structures within the first few (and often many) blocks from the beach were simply flattened by water. Fortunately, these areas are coming back, but the structures being built there now bear little resemblance to the homes that graced the beach 10 years ago.

I remember my father-in-law having his new house built on the coast in Waveland more than 20 years ago. As a young engineer, I gave it the once over and noted that the builder had connected the roof framing to the top plate, but little else. I made some recommendations, such as continuing the connections down throughout the rest of the house to the foundation. The builder followed my suggestions and then presented my father-in-law with the bill “for your son-in-law the inspector.” He was happy to pay it. Nevertheless, although the house was wind resistant, it could not stand up to the rushing waters from Hurricane Katrina.

Katrina reminds us that the only way to get away from floods, other than not building near the water, is to elevate structures above them. Due to flood regulations, new houses along the Gulf Coast are now elevated high in the air, in the hope of avoiding flooding from future storms. Simpson Strong-Tie is proud to have developed some products during the last few years that make it easier to build structures elevated on pilings.

One such product is our CCQM column cap that strengthens the connection of support beams to masonry piers. Another is the Strong-Drive® SDWH Timber-Hex HDG structural screw, which is meant to replace through-bolts to make the connection of a beam to a wood piling easier and more reliable.

CCTQM Installation

CCTQM Installation

Elevated house built with CCQM Column Caps

Elevated house built with CCQM Column Caps

 

SDWH TIMBER-HEX HDG Screw

SDWH TIMBER-HEX HDG Screw

Hurricane Katrina reminds us of the value of building codes. After the storm, the LSU Hurricane Center conducted a number of simulation studies on the effect of a direct, Katrina-like storm on the states of Louisiana, Mississippi and Alabama. The simulations were run on the existing stock of buildings, and then run again on the same stock of buildings, assuming that certain features that result from modern building codes were present. These features included shutters or impact-resistant windows, enhanced nailing of the roof deck to the roof framing, framing connected together with hurricane clips and straps to achieve a continuous load path. In addition, in the Louisiana study, a secondary water barrier over the joints in the roof sheathing was added.

The studies found that the decrease in wind damage from the simulated storms was astounding. In Louisiana, the study showed a 79% reduction in economic losses due to wind. In Alabama, the study revealed a 72% reduction in economic losses due to wind. The Gulf states seem to have received the message loud and clear. In the years following Hurricane Katrina, Louisiana adopted a statewide building code and Mississippi adopted a uniform building code for the four counties along the coast. Recently, Alabama has also adopted a statewide residential and energy code. But in general, building codes are still quite varied in coastal states. This report from the Insurance Institute for Business and Home Safety evaluates the effectiveness of building codes in coastal states.

Finally, Hurricane Katrina reminds those of us who do damage surveys that you need to know what you are getting into before you go. As soon as the storm hit and we saw the scope of the damage, four members of the Simpson Strong-Tie Engineering Department in our McKinney, Texas, office decided we needed to go see the damage first-hand before any repairs were made. So two days after the storm struck, off we went to Jackson, Mississippi. There, we rented two vans stocked up with food, water and fuel. Unfortunately, the fuel and the food/water ended up in separate vans. Before long, we were separated in traffic and could not communicate due to loss of cell signal.

Our team spent two days viewing the damage first-hand along the Louisiana and Mississippi coast, but spent a lot of time our last day trying to find some fuel so we could make it back to Jackson. I remember spending the night in a hotel without power full of storm victims, and then months later receiving the bill and being charged for a movie!

SE Blog 4SE Blog 5

SE Blog 6

What do you remember from Hurricane Katrina? Let us know in the comments below.

Florida Product Approvals Made Simple

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This year, the new 5th Edition of the Florida Building Code was released and is now in effect statewide. First printed in 2002, the Florida Building Code was developed as part of Florida’s response to the destruction caused by Hurricane Andrew and other hurricanes in the state.

Another component, which I would like to take a closer look at in today’s post, is a separate Florida Product Approval system designed to be a single source for approval of construction products for manufacturers, Designers and code enforcers. This single system streamlines the previous approach of different procedures for product approval in different jurisdictions. While statewide approval is not required, many jurisdictions, manufacturers and specifiers prefer using the statewide system to the alternative, which is called local product approval. To ensure uniformity of the state system, Florida law compels local jurisdictions to accept state-approved products without requiring further testing and evaluation of other evidence, as long as the product is being used consistent with the conditions of its approval.

The rules of the Florida Product Approval system are in Florida Rule 61G20-3. Here is some basic information about Florida Product Approval.

The Florida Product Approval system is only available for “approval of products and systems, which comprise the building envelope and structural frame, for compliance with the structural requirements of the Florida Building Code.” So users will only find certain types of products approved there. However, if you work in areas where design for wind resistance is required, the Florida system can be a gold mine of information for tested, rated and evaluated products. Not only will you find products like Simpson Strong-Tie connectors with our ICC-ES and IAPMO UES evaluation reports, but thousands of other tested and rated windows, doors, shutters, roof covering materials and other products that don’t typically get evaluation reports from national entities. The specific categories of products covered under the Florida system are exterior doors, impact protective systems, panel walls, roofing, shutters, skylights, structural components and windows.

To protect consumers, a recent law passed in Florida states that a product may not be advertised, sold or marketed as offering protection from hurricanes, windstorms or wind-borne debris unless it has either State Product Approval or local product approval. Selling unapproved products in this way is considered a violation of the Florida Deceptive and Unfair Trade Practices Act.

Once a manufacturer understands the process for achieving a statewide approval, it is not difficult to achieve, but it can be expensive. The manufacturer must apply on the State of Florida Building Code Information System (BCIS) website at www.floridabuilding.org. To prove compliance with the code, the manufacturer must upload either a test report, a product certification from an approved certification entity, an evaluation report from a Florida Professional Engineer or Architect, or an evaluation report from an approved evaluation entity (ICC-ES, IAPMU UES, or Miami-Dade County Product Control). Then, the manufacturer must hire an independent validator to review the application to ensure it complies with the Product Approval Rule and that there are no clerical errors. Finally, once the validation is complete, staff from the Department of Business and Professional Regulation reviews the application. Depending on the method used to indicate code compliance, the application may be approved at that time or it may have to go through additional review by the Florida Building Commission.

Here are several ways to find out if a product is approved.

  1. For Simpson Strong-Tie products, we maintain a page on www.strongtie.com that lists our Florida Product Approvals.
  2. The Florida Department of Business and Professional Regulation maintains a page where users can search Product Approvals by categories such as manufacturer, category of product, product name, or other attributes such as impact resistance or design pressure.
  3. A third-party group we work with has created a website called www.ApprovalZoom.com that lists various product evaluations and product approvals. In addition to listing Florida Product Approvals, they also list ICC-ES evaluation reports, Miami-Dade County Notices of Acceptance, Texas Department of Insurance Approvals, Los Angeles Department of Building Safety Approvals, AAMA certifications and Keystone certifications among others.
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Florida Department of Business and Professional Regulation Product Approvals search

The process for searching for approved products on the Florida BCIS is fairly simple.

  1. Go to www.floridabuilding.org
  2. On the menu on the left side of the page, click on Product Approval. Or, click this link to go directly to the search page.
  3. On the Product Approval Menu, click on Find a Product or Application. Note that at this location you can also search for approved organizations such as certification agencies, evaluation entities, quality assurance entities, testing laboratories and validation entities.
  4. Ensure the proper Code Version is shown. The current 2014 Florida Code is based on the 2012 International Codes.
  5. At this point, several options can be searched. You can search for all approvals by a specific product manufacturer or a certain type of building component by searching Category and Subcategory, or if searching for a specific product, by entering the manufacturer’s name and the product name.
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Select the option highlighted in red

I hope you find the information contained in the Florida Product Approval system useful. Do you have other needs to find approved products?

What Did Sandy Teach Us?

Shane Vilasineekul

Guest blogger Shane Vilasineekul, engineering manager

This week’s blog post is authored by Shane Vilasineekul, who is the Simpson Strong-Tie Engineering Manager for our Columbus, Ohio branch covering the Northeastern United States. Shane is our latest blogger for the Structural Engineering Blog. He will be posting occasionally on topics that are relevant to our work. We will continue to post on a regular weekly schedule to the blog. In the future, we hope to expand the voices on the blog to include more Simpson Strong-Tie engineers, along with other industry colleagues and associates. For more information on Shane, see his bio here. Here is Shane’s post:

In the weeks following Hurricane Sandy, I had an opportunity to visit some of the hardest hit communities in the region. At the time, many of New Jersey’s barrier islands were still completely closed off to civilian traffic and all accessible bridges were blocked by military guards. Our local territory manager has great relationships with building departments, so we were able to walk portions of Long Beach Island, NJ with an inspector. The storm surge washed out several sections of the protective sand dunes on the south end of the island in the neighborhood of Holgate and this is where we spent much of the day.

Holgate, NJ

Holgate, NJ

Scoured foundation temporarily shored. Holgate, NJ.

Scoured foundation temporarily shored. Holgate, NJ.

For a structural engineer, there was a lot to observe and many things I could write about here (maybe a future post), but what strikes me the most when looking back is the long- term impact this event will have on the region. The cost of Sandy goes beyond the loss of life and property (72 lives, $50 billion and growing). It would be difficult to estimate a dollar amount that accounts for the displacement of people and disruption to their lives, the hit to local economies that depend heavily on tourism, and the effect on the national economy and taxpayers; but I imagine it would be a staggering sum. So what, if anything, can structural engineers do about it? Continue reading

Top 3 Roof Deck Design Considerations for High Wind Events

Was it JFK who said, “The time to repair the roof is when the sun is shining?” He was likely using the roof as an analogy for the economy, but I take things literally and wanted to talk about roofs.  The time to think about the design of your roof and its function in a high wind event like a hurricane or tornado is right now.

Wood screw vs. common nail

During a high wind event, a roof deck is expected to perform many functions. It should prevent water intrusion from rain, withstand impacts and protect those inside from hail. It also needs to act as a diaphragm – transferring lateral loads to shear walls and resisting the vacuum effects of wind uplift forces. Continue reading

Building a Storm Shelter to ICC-500 Design Requirements

According to the National Weather Service, 2011 ranked right up there as one of the worst years on record for tornadoes, having set records for the earliest date of the first tornado, the most states reporting tornadoes, the greatest monthly total, the greatest daily total, and the highest estimated property and crop losses. (Take a look.)

You may wonder: What can I do to protect building occupants (perhaps even my family) in a tornado? It is possible to build your home to higher wind resistance than normally required so that it can resist weak to moderate tornadoes? See my previous blog post, “Designing Light-Frame Wood Structures for Resisting Tornadoes. It Can Be Done!” and also our tornado technical bulletin for more information. But to resist the strongest of tornadoes, the most economical solution is a storm shelter located nearby or in your home.    Continue reading