Category: Articles

March 2018 Blog Post

 

What can radar tell us about hail?

 

Have you ever wondered whether National Weather Service (NWS) radars can tell if it hailed on a particular roof?  There is a high probability it can!   Radar information is available online from the National Climatic Data Center at: https://www.ncdc.noaa.gov/data-access/radar-data as well as other sources.  Haag engineers and meteorologists routinely study radar images or analyses to determine the likelihood that hail fell at a specific location. Such information can help our clients better understand hail history.  Radar studies have shown good correlation in determining hail aloft and whether it reaches the ground.  However, there are too many uncertainties with weather radar to accurately tell maximum hail size at a particular location.  While some organizations provide such algorithm output, this type of data must be reviewed with caution and correlated with all other available evidence.  Haag engineers and meteorologists conduct site specific inspections to verify if it hailed and determine the size, hardness, and direction of hail fall.  We look for the presence of scuff marks on wood surfaces and spatter marks on metal and other surfaces that have oxidation, grime, and organic growths.  Such marks are usually visible within a year or two after the hail event. Radar output does not take the place of detailed site specific inspections.

The following is a short treatise on radar to better explain what it can and cannot do.  Radar is an acronym that stands for Radio Detection and Ranging.  It was developed during WWII to track aircraft and missiles.  Advancements in technology have greatly improved radar quality and resolution over the years.  Today, there are more than 150 weather radars operated by the National Weather Service throughout the U.S.  (Figure 1).

These radars emit extremely short bursts of radio waves, called pulses. Each pulse of energy lasts about 0.00000157 seconds (1.57×10-6), with a 0.00099843-second (998.43×10-6) “listening period” in between.

The transmitted radio waves move through the atmosphere at about the speed of light.  By knowing the direction the antenna is pointed, and timing of returned energy, the location of the target can be determined. Generally, the better the target is at reflecting radio waves (i.e., more raindrops, larger hailstones, etc.), the stronger the reflected radio waves, or echo, will be.  This is because the energy reflected is proportional to the target number and target diameter to the sixth power.  The radar antenna is 28 feet in diameter and contained within a fiberglass radome to protect it from the weather.  The antenna is mounted on a tower to limit interference of near ground obstructions (Figure 2).  Every five minutes or so, the radar completes a volume scan, rotating up to 19.5 degrees above the horizon, and providing a “snapshot” of echo intensity and location.  Radar cannot see above 19.5 degrees which might not even see a storm very close to the radar site. The area not sampled above the radar is called the “cone of silence” (Figure 3).  Also, radar does not sample below 0.5 degrees to minimize ground interference.

The radar antenna samples the returned energy at some height above the ground.  Also, radar resolution decreases with increasing distance as the radar beam widens and rises above the ground; the latter occurs due to the curvature of the earth.   Just because radar might detect hail aloft, does not necessarily mean it will fall directly below at the ground.  This is because winds aloft can blow hailstones downstream.  Also, hail melts as it falls into increasingly warmer air.  So, the depth of the warm air is important.

Received radar energy goes through electronic processing where computer algorithms dissect the data so that it can be displayed. Three-dimensional information is placed into color-coded bins on a two-dimensional map of the area.  Figure 4 shows a display of radar reflectivity.  Green colors indicate light precipitation, yellow moderate precipitation, and red intense precipitation.  Darker red and purple colors indicate a high probability of hail at that altitude.  Sometimes, a false echo or spike is found emanating from a radar echo which also indicates the presence of hail.  Figure 5 shows a close-up view of base reflectivity showing the size of a radar bin that was 15 miles from the radar.  The size of the radar bin was much larger than that of a house.  Thus, radar cannot sense hail at a point.

In the past few years, NWS radars have upgraded to dual polarization (Dual-Pol) technology.  Such radars send out horizontal and vertical pulses of energies which provide more information about precipitation type.  Rain typically falls like elliptical plates (not the typical drop or round shape that kids draw) while hail is generally rounded (roughly spherical).  Thus dual-pol radars can better distinguish rain from hail, but the resolution is still not fine enough to distinguish the sizes of individual hailstones .  This improvement still has the same limitations as single-wave radars.  Despite these limitations, weather radar is a valuable tool used in the prediction of hail.  But, a prediction is not a verified result. Radar cannot tell the specific size(s), quantity, direction(s), or hardness, of hailstones at a particular address, or whether the hail actually caused any damage to roofing or other exterior building components. Verification requires ground truth and Haag experts routinely perform such site specific inspections. Since the purpose of site inspections is to determine the extent and severity of damage to building materials, and not simply to determine the whether hail fell or the size of hail, radar data will not replace the need to have well-trained inspectors make evaluations.

  

 

 

 

 

 
Fig. 1
National Doppler Radar Site locations   [Map illustration of the NWS Radar Sites].  National Doppler Radar Sites.  Retrieved from http://radar.weather.gov/.
Fig. 2
NEXRAD Doppler Site Image   [Doppler radar site at sunset.]  Retrieved from http://www.noaanews.noaa.gov/stories2013/images/WSR-88D_Tower.jpg .
Fig. 3
Radar Limitations   [Graphic showing radar beam characteristics.] Cone of Silence. Retrieved from http://www.srh.noaa.gov/jetstream/doppler/radarfaq.htm .
Fig. 4 
Hail Storm, Memphis, TN   [Doppler Radar image showing a storm moving over Memphis]  Available online at  http://www.roc.noaa.gov/WSR88D/About.aspx .
Fig. 5 
NEXRAD Image, Dallas, TX   [Doppler Radar image showing a house point on radar.]  Available online at  http://www.roc.noaa.gov/WSR88D/About.aspx .

By Tim Marshall, P.E., Meteorologist, Haag Principal Engineer

 

 

Tim Marshall is a structural engineer and meteorologist.  He has served as a Haag Engineer since 1983, assessing damage to 1000s of structures (particularly damage caused by wind and other weather phenomena). He has written numerous articles, presented countless lectures, and appeared on dozens of television programs in order to share his extensive knowledge re: storms and the resultant damage.  He is a primary author of many Haag Education materials, including the Haag Certified Inspector-Wind Damage course.  He is also a pioneering storm chaser and was editor of Storm Track magazine.  See his profile here.

October 2015 Blog Post

Haag’s Research & Testing laboratory, which plays a crucial role in the development of Haag courses and publications, has earned its accreditation from the International Accreditation Service (IAS) for product testing! Watch this video blog post for a behind-the-scenes look into the exciting tests performed in the lab (including impact testing with 2x4s and 4″ ice balls!).

video tour: Launching 4″ Ice balls & More!

 

UTILIZING VIRTUAL TOURS IN DAMAGE ASSESSMENT, September 2015 Blog

UTILIZING VIRTUAL TOURS IN DAMAGE ASSESSMENT

By Kevin Kianka, P.E.

If “a picture is worth 1,000 words,” how valuable is the capability to place yourself within an image and pan around?  Or even zoom in?

I call it the “Google Street View effect”:  the desire of tech-savvy users to control their own movements within images and videos that have been taken elsewhere, by someone else.  Though Google’s Street View service is perhaps the most well-known use of this type of technology and has been available for almost eight years, panoramic virtual tours have been utilized in the real estate industry for nearly twice as long.  Now a new wave of users in the damage assessment industry are finding that virtual tours can be extremely useful during inspection and construction.  Utilizing specialized cameras, lenses and software, photographers can create 360° spherical imagery of a specific location, allowing other viewers to become virtual on-site witnesses.

What Type of Equipment is Needed?

Haag has used spherical photography and virtual tours for over three years on a variety of projects, including damage assessments, incident documentation, construction monitoring, and historical documentation and recordation, just to name a few.  The process involves image acquisition, photo stitching/merging, and tour creation.  Hardware includes a standard DSLR camera and photographic tripod, a fisheye lens, and spherical camera mount.

The camera tripod is set and a series of photographs are taken (typically 6) utilizing the camera mount.  The spherical camera mount has settings that ensures that there is a 360° horizontal coverage, creating the spherical image.  (Some specially-designed cameras can even take a 360° image without any manual effort.)  The camera system is then moved to the next desired location and the process repeated until all desired locations are documented.  Typical setup locations include any key Points of View (or POV, see Fig. 1), witness locations, areas where damage is present, and areas that show inconsistencies in construction or manufacture (or anything out of the ordinary for a scene).

Later, back in the office, users stitch the six individual images together, creating a final spherical image for every noted setup location.  Tours can then be  created utilizing specialized software which combines the multiple spherical images into a single file.  This file can be linked to a map with a “radar” showing the POV of the camera, along with links to all the photographs, videos and PDFs used within each spherical image.  (The map is typically created to allow the tour users to have a point of reference in the context of the whole scene.)

The end product can be delivered as a self-executable file (EXE), or it may be loaded onto a website or mobile device, among other formats.  Key information can be placed within the virtual tour, including still photographs, field notes, videos, links to OSHA standards or websites, or other applicable documentation.  The resulting product allows persons never on a site to “be there” with the photographer.  They can see “first-hand” notable conditions and, crucially, view a photograph within the surrounding context while controlling their movements in the virtual space.

See a 3D Tour in Action

Check out an example of a virtual tour here.  In this short video of a 3D Tour created by Haag personnel, you’ll be able to see how easily a viewer can maneuver through a Virtual Tour of the Battleship USS New Jersey.

 

Kevin Kianka, P.E., Director of Haag BIM/Modeling Program, 07/2015

Kevin Kianka, P.E., serves as the Director of BIM/Modeling Program for Haag Technical Services, overseeing the office production of deliverables for clients, in addition to serving as a Project Manager on multiple projects. He recently spearheaded the effort to petition for and obtain Haag’s FAA Section 333 Exemption. See his profile and contact information here.

 

 

Ready to Utilize Drones on the Job? Factors to Consider in Commercial Use, July 2015 Blog

Ready to Utilize Drones on the Job? Factors to Consider in Commercial Use

By Kevin Kianka, Operations Manager, Technical Services

Drones are a hot topic in the media these days, whether they are being used to help report the news (such as to show recent flooding in Houston) or whether they are the actual topic of the news (such as when drones impacted the airspace of a jet on its final approach to LaGuardia Airport and led to flight diversions).  While the term “drone” is used most often in the media, the official Federal Aviation Administration (FAA) designation is Unmanned Aircraft System (or UAS).  Other designations include quadcoptor, unmanned aerial vehicle, and unmanned aerial system.

Regardless of what you call it, the UAS is fast becoming a valuable asset to the architecture, engineering, inspection, real estate and insurance industries.  Previously inaccessible areas and views of properties are now obtainable via the use of a UAS. And the control is literally in the user’s hands.  A UAS can be utilized to provide documentation of damage and collapses, aerial views, and other images that were previously too costly on small to moderate projects.  Now the UAS provides cost effective options that were once unobtainable for the average homeowner or inspector.

However, before you go out and buy a system or hire someone to perform a service using a UAS, there are several factors which must be considered.

First, before you fly, it is imperative that you determine whether you have permission and authorization to fly.  If you are going to fly a UAS inside of a building or enclosed space, permission from your client and the property owner/manager will get you started.  Note that the industry you are working in may have additional restrictions or requirements.  Make sure you check all available resources to determine if there are any restrictions to your indoor use of a UAS.

If you move outside, additional regulations impact you:  specifically and most importantly, those regulations which are established by state and federal agencies.  In addition to obtaining permission from your client and the property owner, you must determine if the US state you are working in allows the use of a UAS for commercial purposes, as individual states are legislating UAS usage.  The commercial use of a UAS in open airspace is further regulated by the FAA.  As of the date of this document, such use requires you to obtain FAA Section 333 Exemption for any commercial application; governmental operations have different requirements.  (Note: Haag Engineering Co. recently received approval to utilize UAS systems under the FAA Section 333 Exemption.  For more information on all we can offer, read the press release here.)  The Section 333 exemption comes with additional requirements.  To name a few, not only must a FAA-licensed pilot operate the UAS, but s/he must do so from an area that is at least 500’ away from all nonparticipating persons or structures.  The pilot may work only over a private or controlled-access property, and the pilot must obtain express permission from the owner.

Once you have determined that you actually have permission to fly, you can analyze and identify the hardware that you’ll need to do the job.

A UAS can vary in cost from several hundred dollars to tens (if not hundreds) of thousands of dollars.  Selecting the right UAS is an involved process, and the actual UAS itself is just one piece of the puzzle. Users also need to consider what data they are trying to collect and what they are going to do with that data.  Whether you are using a digital camera, FLIR, LIDAR, or other type of apparatus, data acquisition is required to document the view from the UAS.  The next and most important piece of the puzzle to define what you are going to do with your data.  Are you simply taking pictures?  Do you want to take measurements from the data?  Do you want to create panoramic images?  These are many questions that need to be answered before someone implements the use of a UAS.

In addition, training is crucial.  Do not expect to purchase a UAS on Monday and use it on Tuesday.  It is best to plan on several days and weeks (if not months) of practice using the hardware and software before utilizing it on a paying project.

Importantly, a UAS will not replace a required workflow; it will only assist you in completing that workflow.  Likely, an inspector will still need to access a roof or complete a hands-on inspection of a bridge or apparatus to complete her/his analysis.  For example, an aerial image might indicate circular areas of granule loss on an asphalt roof; a crack in an elevated concrete structure; cracking in the pavement of a parking lot; or damage to elevated piping in a facility.  However, a hands-on inspection is still required to confirm the severity and extent of any damage captured visually by UAS data.  The UAS is merely a tool to assist in that process.

Below are some basic questions that you should ask before you consider using a UAS.

  • Does the client allow the use of a UAS?
  • Will the property owner/manager/controller allow the use of UAS?
  • What data are you trying to collect and what are you going to do with it?
    • Can you perform the data processing internally or will a consultant be required?
    • If a consultant is required, is s/he qualified to do this work and what will the cost be?
    • Will the data meet the accuracy and precision needs for the project?
    • Do you (or your consultant) have the data-processing expertise needed to produce reliable and defendable results?
    • Will the UAS be operated outdoors?
      • Does the city, county and/or state where the services are being performed allow the use of a UAS?
      • Do you or your employer have a FAA Section 333 Exemption?
      • Does the UAS Operator (Pilot in Charge) hold either an airline transport, commercial, private, recreation or sport pilot certificate from the FAA?
      • Will the UAS operate at least 500’ from all nonparticipating persons, vessels, vehicles or structures, unless protected by a barrier?
      • Will the UAS operate over private or controlled-access property with permission from the property owner/controller or authorized representative?

Once these are satisfactorily answered, you may be well on your way to using a drone for the first time on the job!

Kevin Kianka, P.E., serves the Director of Operations, based in Haag’s Sugar Land (Houston), TX office and leading Haag Technical Services efforts nationwide, including all services related to 3D Laser Scanning, 3D Modeling, Drones (sUAV’s), GIS, and other advanced technologies. A licensed Professional Engineer in Texas, New Mexico, Colorado, New Jersey, New York, Pennsylvania and Florida, Mr. Kianka obtained a Bachelor of Science in Civil Engineering from Drexel University (Philadelphia, PA) and has over 15 years of experience in the field of Engineering.

Earthquake Risk Assessment:  Can We Predict the Next ‘Big One’? May 2015 Blog

EARTHQUAKE RISK ASSESSMENT:  CAN WE PREDICT THE NEXT ‘BIG ONE’?

By David L. Teasdale P.E., Haag Principal Engineer

An earthquake is little more than a release of energy when two sides of a fault line slip relative to each other.  A fault is a break in the rock crust, generally miles below the surface, and faults were created long ago as tectonic plates drifted to their current positions.

Therefore, all parts of the United States have fault lines, and any part of the United States has the potential for an earthquake as the tectonic plates continue moving.  As might be expected with any moving, flexible plate, parts of that plate move differently, and the plate surface deforms.  (Recall from high school geology that the rise of mountains and other land features is attributed to this deformation.)  Continued plate movement and localized surface deformations cause different sides of a fault to bind against each other, and binding builds up internal strains in the rock.  Built-up energy may eventually be released through slipping along a fault line, and we feel that slip as an earthquake.

While every part of the United States has fault lines, not every fault line is actively moving and binding, and those that are active are not storing energy at the same rate or in the same kind of rock geology.  Therefore, the risk of an earthquake varies by region for many different reasons, and risk assessment is hampered by the sheer size of the moving parts, the variability of materials, and our understanding of the process.  Presently, risk assessment is based on the size and frequency of past earthquakes.  California is on the leading edge of North American plate, and faults are more active, as it collides with the Pacific Plate to the west and rides up over the “subduction zone”. (Refer to  http://www.sanandreasfault.org/Tectonics.html for further explanation.)

It is presently believed that periodic release of energy through smaller earthquakes helps prevent a larger earthquake, but discovery of new faults and awakening of dormant faults is ongoing with time.  No scientist knows precisely what might happen next, and consideration of the unknown always fuels discussion of the inevitable “big one”.

Study of California geology and past earthquakes leads seismologists to consider the maximum credible event around a magnitude 8.0.  Magnitudes are logarithmic, and each magnitude level is about 32 times greater than the previous one (M5 is32 times greater than M4).

Therefore, the maximum credible event for California is more than 1,000 times greater than the Northridge earthquake in 1994 (M6.7).  Buildings of different age, construction materials, and design behave differently when shaken, but good detailing of connections goes a long way toward safely resisting earthquake shaking.  In the United States, one can generally assume that most buildings will perform well with little damage at magnitudes below 5.0, even when they are not constructed using earthquake standards, but damage is influenced by magnitude, duration of shaking, distance from the involved fault, depth of the earthquake focus, frequency content, and other factors.

Earthquakes in the Midwest and other regions of the United States are much less frequent, but areas like the New Madrid fault, the Wasatch Fault, and the Middleton Place – Summerville Seismic Zone (Missouri, Utah, and South Carolina, respectively) have all experienced extraordinary shaking events in their histories similar to a maximum credible event in California.

The frequency of these events is around once every 500 years, on average, so residents of these areas may not feel the urgency about earthquakes that Californians might feel.  Residents of central Oklahoma routinely experience short shaking events up to about M3.5 without significant alarm.  Curiously, building codes handle the risk of infrequent strong events by including seismic requirements in design but reducing the force levels.  These measures will save lives in a surprise shake of moderate proportions, but they will not likely make much difference if these areas experience a large event.  Therefore, much of the United States is effectively “playing the averages” when it comes to earthquakes, but averages over geologic time are just that, averages.  The next one could be in 1,000 years or 100 years.

Much effort has been made to predict earthquakes, and these efforts usually involve measuring fault movements with the hopes of understanding when they are apt to slip.  Where the strain energy theory of earthquakes is prevalent, it is often thought that small tremors can presage a larger event, and seismologists have seen small excitations in their studies that might someday offer a reliable advance warning of several minutes or more.

Predicting how large that event might be, however, is still well beyond current knowledge.  Given the difficulties in evaluating earthquake initiation where faults can be studied over time, one can see the difficulty of evaluating fault activity in new areas like those where hydraulic fracking is underway, for example. Seismologists can postulate theories as to how water injection might lubricate faults and cause movement, but moving the earth is not as easy as all that. Further study of a region along with consideration of other factors like depth, procedures, and time lapse since the last injection often lead geologists to discount the effects of human activity. In the meantime, the rest of us can sleep better believing that a historically inactive region likely will not have a lot of strain energy stored in its faults, and small tremors will reduce that energy even if they result from human activity.  It’s as good a theory as any at this time.

David Teasdale, P.E., Haag Engineer

David Teasdale specializes in Structural Evaluations, Earthborne and Airborne Vibrations, Geotechnical Evaluations, General Civil Engineering, & Wind Engineering and Related Storm Effects.  He is the primary author and presenter of a Haag seminar course on earthquake damage assessment titled “California Earthquake Adjuster Accreditation“.  See his profile here.

Hailstorm Data Sources and Hail Characteristics, April 2015 Blog

Hailstorm Data Sources and Hail Characteristics

With the advent of Spring, we are entering the climatological “hail season” for the central and southern United States, and you should expect to start getting calls to make roof inspections related to hailstorms and potential hail-caused damage. We want to assist you in preparation by discussing the various sources for hailstorm data, and how to compare that with the information you can obtain during your site inspections. Some hail data is from free governmental sources, and some is from private sources including maps or lists from third-party meteorological consulting firms that typically are fee-based.

Governmental Data Sources

The climatological data from the federal government is under the umbrella of NOAA (National Oceanic and Atmospheric Administration). Severe weather reports are obtained by local offices of the National Weather Service (NWS) through eyewitness reports by individuals, trained storm spotters, or emergency management officials; media and social media reports; official NOAA recording stations, and occasionally the reports of observation teams dispatched by the NWS (NWS teams are mainly used to document tornado tracks). The severe weather reports can be found at three types of NOAA websites:

NWS offices: http://www.srh.noaa.gov/

Local Storm Reports issued by individual NWS offices. This data is considered preliminary and generally only remains available for a limited amount of time, often less than one week.

Storm Prediction Center (SPC): http://www.spc.noaa.gov/

Nationwide reports added on a near real-time basis and listed as “daily” reports that start/stop at 6:00 a.m. Central Time. The archive of daily reports is retained permanently, but this data (regardless of age) is considered preliminary is it has not undergone quality control and may have errors and omission.

National Climatic Data Center (NCDC): http://www.ncdc.noaa.gov/stormevents

Nationwide reports searchable over a specified date range after selecting the state and county of interest. The NCDC database has undergone quality control and is considered “final” data. The data is usually several months behind the current date and the site will not have information on the most recent storms. The “Event Details” listed may provide additional information about the storm or if any property damage was reported.

The local NWS offices forward their “Local Storm Reports” to the SPC for posting during the storm events, and then the NWS offices prepare monthly severe weather summaries to the NCDC for inclusion into the Storm Event Database. The current NWS “severe” criteria for entry in the database for hailstones is 1.0 inch diameter or larger (although sometimes hail sizes as small as 0.75 inch diameter are listed). The hail reports are generally listed as “point locations”, although often the geo-codes (latitude and longitude) provided with the reports are not exact because a database with listing of the latitude/longitude at the center of cities is used to create the geo-codes.

Non-Governmental Data Sources

There are numerous private or educational institution websites that will provide links to hailstorm or severe weather reports, although for the most part, these sites will be routing you to the above-listed NOAA information or re-packaging it in some way. One organization that provides a different non-governmental source of eyewitness hail reports is COCORAHS (Community Collaborative Rain, Hail, and & Snow Network), http://www.cocorahs.org/. The volunteer spotters of this nationwide network can report hail of any size, and searches can be made by state or county for user-defined date ranges.

The final data sources for hail information we will discuss are third-party maps or lists based on radar imagery. There are numerous firms that offer maps of individual storms or provide “site-specific” estimates of maximum hail size of a single storm or a date range. It is important to note that actual hailstone sizes can be larger or smaller than those listed, and even occurrence of hail at the site is not guaranteed. It should be understood that several factors can influence the accuracy of the estimated hail sizes and the proximity of the estimated hail to the location of interest when analyzing radar signatures.  All service providers of this kind of report use the data obtained from the same radar signatures; however, the output from different providers of the same storm can be quite different. The service providers attempt to process the data through proprietary algorithms to determine whether hail fell and the size of that hail.  Meteorological researchers and NOAA personnel actively study this methodology, and the NWS uses similar algorithms for predicting the occurrence of severe hail (at a county level), although published studies have revealed widely varying success in determining maximum hailstone size, and no peer-reviewed study has indicated accuracy of determining hailstone size at a specific address. Simply put, the radar data is not fine enough to be directly measuring individual hailstone sizes. As such, these reports are not a substitute for site-specific observations.

Hail Data From Site Inspection

The data that can be observed during a site inspection regarding hailstone quantity, direction of hail fall, hailstone size range, and estimated maximum hailstone size at a particular location is greater and more complete than can be obtained from the data sources listed above. Also, it is prudent to ask the building owner or site contact if there are any photographs or video of the actual hailstones or hailstorm event. Depending on the quality of the images and video, this can provide useful information on the hail sizes, hailstorm duration, and hail fall direction.

At the inspection site, various surfaces and materials including utility boxes, air-conditioning units, fences, windows, siding, gutters/downspouts, fascia, plastic and metal vents and roof appurtenances, and the roofing materials can be inspected for spatter marks, dents, and other forms of damage related to hail impact. Painted surfaces and exposed materials often form a layer of oxidation or become covered with dirt, grime, algae, or other organic materials that can be cleaned away when impacted by hail, resulting in spatter marks. Spatter marks are temporary markings left by removal of surface oxides, grime, organic growths, etc. caused by hail impacts that can provide an approximate hailstone size and direction of hail fall.

Since spatter marks tend to fade from oxidized surfaces after a year or two, they provide a helpful temporary record of recent hailstorms.  Although the visibility and longevity of spatter marks can vary based on the material and amount of oxides and organic materials removed, harder hailstones can remove a greater amount of surface material and tend to produce a “crisper” edge to the spatter marks with greater contrast, while spatter marks from softer hailstones show a greater scattering of material from the hailstone breaking apart upon impact. Harder (or larger) hailstones also would produce deeper dents in metal than smaller and softer hailstones. Dents produced in light-gauge metals when impacted by hail leave a permanent record of hailstones that have struck exposed surfaces over the years.  Spatter marks and dents can be evaluated to help determine the approximate size of hail at a location and provide insight regarding the time passed since passage of a hailstorm. Determining the age of a dent in metal can sometimes be challenging, but dents that contain spatter marks or have a shiny surface from removal of grime and oxides would be indications of recent denting. Accumulation of hardened grime and organic growths in a dent generally takes a considerable length of time. Examining vertical surfaces such as siding, sides of mechanical units, and fences for hail-caused dents and spatter marks can provide information about the direction of hail fall.

Hail Impact Forces and Threshold Sizes

Impacts from larger hailstones result in higher forces (impact energies) than impacts by smaller hailstones because larger hail is more massive and falls at higher velocities than does smaller hail.  Also, harder (frozen solid) hailstones transmit their energy over smaller areas than do softer hailstones of the same size, because softer hailstones tend to break apart upon impact. Consequently, impact forces from harder hailstones result in higher material stresses than impacts from softer hail.  For these reasons, harder and larger hailstones are more damaging to roofing materials than smaller and softer hail.  Of these two parameters, the size of hail has much more influence on the maximum possible force at impact and accordingly, the ability to damage roof coverings. Hail damage thresholds listed in the HCI courses, Haag publications, and published research papers by Haag personnel are for hailstones that are at the upper range of density and hardness striking perpendicularly to roofing material of average quality and thickness.  Therefore, not all hailstones that are of threshold size or larger will result in damage due to variations in density, hardness, angle of impact, and material quality or thickness.

Note that the hail data sources as discussed above do not provide any specific information about quantity, hardness, direction, or size range of the hailstones for a particular location. At most, the direction of hailstorm movement can inferred by mapping the hail reports with the time or by looking at the shape of the hail swaths (with radar-based maps). However, these characteristics of hail fall can be determined and documented during a thorough inspection of your inspection site, and will provide support for your roof inspection findings whether you find hail-caused damage to the roof covering or not.

Richard F. Herzog P.E., Meteorologist, RRC, and Haag Principal Engineer (04/2015)

Richard Herzog’s primary areas of consulting are Roofing Systems, Building Envelope Systems, Evaluation of Wind Damage to Structures, Construction Defect Evaluations, Meteorological Investigations, Development of Hail Analysis Software, and Alternative Dispute Resolution.  He serves as a primary advisor in the creation of many Haag Education seminars and products.  See his profile here.

Dents in Metal R-Panel Roofing – What can they tell us about a hailstorm? March 2015 Blog

Dents in Metal R-Panel Roofing – What can they tell us about a hailstorm?

by David Teasdale, P.E., Haag Principal Engineer

Hail-caused dents

Most metal R-panel (raised-rib) roofing is structural, spanning across purlins or other structural members without substrate support.  Even when backed by a roof deck, sheet metal comprising raised ribs is not supported. One statement that can be made about most metal roofing is that it is fairly easily dented.  Since the dents are permanent, metal roofing offers an accumulated history of hail events at a given site.  In general, if a significant area of open, exposed metal roofing is undented, most inspectors would reasonably conclude that large hail had not fallen at the site since the metal roofing was installed.  When the metal is observed to be dented, inspectors often can derive or infer various characteristics about past hailfall, such as hailstone size and direction of fall.

Hailstone size

A variety of factors affect the impact energy of a hailstone (e.g., size, hardness, impact angle), and they will also affect dent size and shape.  With respect to impact of sheet metal, studies by Haag and others have shown there is an inner dent caused by the direct deformation of the hailstone, and there is also a surrounding area of change that results from buckling of the nearby metal.  Because the buckled area varies with hailstone size and impact energy, the inner dent offers the best information about the hail size.  Discerning and measuring the inner dent in the field can be difficult, and most inspectors document the total dent width.  (Lightly rubbing chalk across a dent captures the total buckle.)  There is nothing wrong with measuring the total dent size; however, the inspector should understand what he or she is actually measuring. The total dent is usually not the shape obtained by metal deforming to the surface of the hailstone, and the relationship between the total dent diameter and the hailstone size varies with hailstone size, metal type, and metal thickness.

Testing

Haag Engineering performs ongoing impact tests on roofing materials that help correlate controlled laboratory tests to observations that can be made in the field. One recent test involved impacting 26-gauge, galvanized metal panels to compare dent sizes.  A test panel was impacted by ¾”, 1”, 1¼”, 1½”, 1¾”, and 2”  molded ice spheres.  These were propelled by the Haag IBL-7 to develop no less than free-fall energies of normally occurring hailstones of the same size. Targets for impacts included the flat pans and flat area of the trapezoidal lap seam.  In general, testing showed that the panels could be dented by any sized hailstone, and the ratio of dent size to hail size varied from about 40% to 70% when considering the inner dent across this range of hail size.  When considering the total dent diameter, the ratio was greater, and dents around 1” across required large hailstones around 1½” to 1¾” in size.  Dents formed in the top of a rib were elongated, and they were hardest to interpret. No fracturing or spalling of the galvanized coating was visible at any of the hail-dented regions.

Field Observations

Field measurements of dents can vary depending on lighting, angle of view, and other variables, including the inspector’s judgment.  This is  particularly true when  small hail is involved.  Certainly, large hail-caused dents in steel panels correlate with very large hail, and they may be useful in searching weather records for a storm date.  Of particular note, Haag research has found that impact on the top of a rib near its edge could create a dimple in the side of the rib that implied a false sideward direction to the hailfall.

David Teasdale, P.E., Principal Engineer

David Teasdale, Principal Engineer and VP of Engineering, specializes in Structural Evaluations, Earthborne and Airborne Vibrations, Geotechnical Evaluations, General Civil Engineering, & Wind Engineering and Related Storm Effects.  He is the primary author and presenter of a Haag classroom and online seminar course, titled “California Earthquake Adjuster Accreditation”.  See his profile here.