Tag Archives: truss plates

What Fails First in a Fire? Part II

What Fails First in a Fire- the Truss Lumber or the Steel Truss Plates? Part II

If you didn’t read my blog yesterday, you will think me out in left field until you do.  Take a few extra minutes and go back and read Part I. You will be glad you did!

Continuing from yesterday’s blog on hay catching fire, and causing a pole barn to burn:

What do you do if you suspect your hay is heating?

A simple probe inserted into the haystack can accurately monitor temperature. You can make a probe from a 10-foot piece of pipe or electrical tubing. Sharpen the pipe or screw a pointed dowel to one end, then drill several 1/4-inch diameter holes in the tube just above the dowel. Drive the probe into the hay stack and lower a thermometer on a string into the probe. The thermometer should be left for 10 minutes in several areas of the stack to ensure an accurate reading.

Watch for the following temperatures:

150 degrees F is the beginning of the danger zone. After this point, check temperature daily.

160 degrees F is dangerous. Measure temperature every four hours and inspect the stack.

At 175 degrees F, call the fire department. Meanwhile, wet hay down and remove it from the barn or dismantle the stack away from buildings and other dry hay.

At 185 degrees F hot spots and pockets may be expected. Flames will likely develop when heating hay comes in contact with the air.

212 degrees F is critical. Temperature rises rapidly above this point. Hay will almost certainly ignite.

Take precautions. Pockets may have already burned out under the hay surface.

Before entering a barn, place long planks on top of the hay. Do not attempt to walk on the hay mass itself. Always tie a rope around your waist and have a second person on the other end in a safe location to pull you out should the surface of the hay collapse into a fire pocket.

Hay treated with preservatives may produce hydrogen cyanide gas at 240 degrees F, so extreme caution should be taken when fighting a hay fire if hay has been treated with such preservatives. Hay treated with preservatives containing ethoxyquin and BHT (butylated hydroxytoluene) produce hydrogen cyanide gas at around 240 degrees. This gas is deadly. Additives containing primarily propionic acid to not produce hydrogen cyanide during a fire.

In the past, farmers sprinkled salt on wet hay as it was stacked to prevent spoilage, but salt does not prevent spontaneous combustion. Dry ice, liquid nitrogen or carbon dioxide gas pumped into the hay can prevent combustion by eliminating the oxygen from the hay mass.

Hay fires from spontaneous combustion occur infrequently in the arid western U.S., but can be a hazard for new hay or old stacks.

Good storage practices will avoid spontaneous combustion and ensure higher quality hay.

As the pressed steel connector plates do not appear to be the weak link of the truss system, if the investment of time and effort is going to be put into what to do when there is a fire, rather than preventing one to begin with, then the use of fire retardant treated lumber for the trusses might prove to be a better place to spend one’s money.

What Fails First in a Fire?

What Fails First in a Fire- the Truss Lumber or the Steel Truss Plates? Part I

We have a client who also happens to be an engineer. He is planning upon a rather goodly sized (64 feet wide by 96 feet long by 22 foot eave) hay storage building and had some rather interesting concerns.

Here is his first one:

nailing trussesHe is concerned about collapse in a fire, which can happen with hay not properly dried.  He has either read or heard pressed metal truss connector plates are one of the first things to fail, in the event of a fire, causing the trusses to fail and bring the building down.

 

 

I did enjoy the response from Hansen Pole Buildings Managing Partner Eric, The whole fire question just seems odd to me. So he is worried about the building not collapsing in the case of a fire but basically knows in the event of a hay fire the building will be worthless regardless? I mean when you have a building in a hay barn it is toast no matter what it is made out of. Get your marshmallows out and watch the blaze because there isn’t any stopping it.

If the worry is more about fire I would think a sprinkler system would be the only worthwhile investment here.”

I took it a little bit further:

From the testing results I have read, it appears the pressed steel connector plates actually help to protect the wood behind and immediately adjacent to the plate. For the curious, here is the link to the test results: https://www.fire.nist.gov/bfrlpubs/fire07/PDF/f07004.pdf.

The best bet is to prevent hay fires from occurring.

Oddly enough, wet hay is more likely to lead to a spontaneous combustion fire than dry hay. If hay is put into a barn or stack when it has more than about 22 percent moisture, not only does the hay lose forage quality, but it has an increased risk of spontaneous combustion.

High moisture hay stacks can have chemical reactions which build heat. Hay insulates, so the larger the haystack, the less cooling there is to offset the heat.

When the internal temperature of hay rises above 130 degrees F, a chemical reaction begins to produce flammable gas which can ignite if the temperature goes high enough.

Hay fires generally occur within six weeks of baling. Heating occurs in all hay above 15 percent moisture, but generally it peaks at 125 to130 degrees F, within three to seven days, with minimal risk of combustion or forage quality losses. Temperature within a stack then declines to safe levels in the next 15 to 60 days, depending on bale and stack density, ambient temperature and humidity, and rainfall absorbed by the hay.

To avoid hay fires, small, rectangular bales should not exceed 18 to 22 percent moisture, and large round or rectangular bales should not exceed 16 to 18 percent moisture for safe storage. If the moisture content exceeds these limits, simply do not stack the bales inside of your new building – allow them to first dry in the field.

In addition, you should check your hay regularly. If you detect a slight caramel odor or a distinct musty smell, chances are your hay is heating. At this point, checking the moisture is too late, and you’ll need to keep monitoring the hay’s temperature.

What do you do if you suspect your hay is heating?

Come back tomorrow and I will impart sage advice!

 

Roof Truss Repair

Roof Truss Repair

A builder writes:

Roof Truss RepairThe previous owner of a pole building I’m working on cut out several internal webs from three consecutive roof trusses to create storage space in the attic. There’s no sign that this has caused any structural problems so far, but I’d like to replace the missing members and fasten them with plywood gusset plates and construction adhesive. Is this feasible, and does this sort of seat-of-the-pants field repair expose me to any legal liability?”

An “arm chair engineered field repair” will expose the builder to liability, should the repair ever fail. Field truss repairs can be made, and they probably would resemble something like what has been suggested. Ideally, an engineer affiliated with the truss manufacturer which produced the trusses would design a suitable repair. But if the truss manufacturer is unknown or no longer in business, the builder should engage an engineer who would both sign off on the design, and inspect and approve the work afterward.

The best place to start is with the manufacturer of the connector plates used on the original trusses. Take pictures of the condition, along with close-up pictures of the connector plates on the trusses and send them to any truss manufacturer. They should be able to point the builder in the right direction.

A typical truss repair might call for a combination of dimensional lumber and plywood gussets “scabbed” beside the remaining web members to make up for the lost members. The repair design should include material dimensions for added members, plus a detailed fastener schedule showing the size, number, and placement of fasteners.

Regardless of the specific design details, under no circumstances should the builder take responsibility for the repair. The fact there have been no structural problems yet doesn’t mean there won’t be some down the road. This may not be enough in the event of high winds, a record snowfall, or a decision by the building owners to move all their old college textbooks into the attic.

Richard Feeley of Feeley Mediation & Business Law, a Marietta, Georgia law firm which provides legal counsel to remodeling companies, confirms a contractor should not attempt a truss repair without engineering support. “If you touch it, you own it,” he warns. A contractor who has not partnered with an engineer to make sure the truss repair meets code and complies with structural requirements is indeed liable, and the risk could be great. Liability could include not just property damage but personal injury if the truss system fails and someone gets hurt.

There can also be licensing implications. If, for example, the job is “permitted” and a building inspector finds after the fact an adjustment which required an engineered design was made.

Feeley points out another legal dimension contractors need to address as well: Be sure you cover the issue in your contract. “You need a change-order policy that covers unforeseen circumstances,” Feeley says. “If you find something that no one expected, you want to be sure you get paid for the work it’ll take to repair it.”

My best advice – don’t field modify any prefabricated roof truss, without adequate engineering.

Truss Connectors: What to Use on Damaged Trusses

As noted yesterday, damaged trusses warrant a repair design by a professional…a licensed engineer.  This is not something for “guesswork” or scabbing on a bunch of lumber.  First the forces of where damage occurred need to be analyzed, and then only an engineer with solid experience in repairing damages should be engaged to elicit the repair design using appropriate truss connectors.

Fastener selection can often prove to be a challenging exercise, due to the wide variety of truss connectors available. The metal plates used in the manufacture of wood trusses have excellent grip due to the numerous teeth embedded into the lumber. When calculating the quantities of other mechanical fasteners required to repair a damaged plate, it is often surprising to see how much larger the connection areas become.

Generally, the nail is the most widely used fastener for wood construction. Nails are commonly referred to by penny-weight. Unfortunately, this designation does not have clearly defined dimensions. There may be four or more different nails commonly referred to by a single penny-weight. For example, a 10 penny (10d) nail could refer to a sinker, common, box, cooler or pneumatic (gun) nail, all of which are slightly different. To eliminate confusion, it is important to note nail length and diameter.

Wood screws are another option. Wood screws can have higher values than nails, but often require pilot holes to prevent splitting. Note many general purpose screws, such as deck screws and drywall screws, may share a common gauge number with a heavier wood screw, but are not considered structural due to the lower grade steel used in their manufacture and should be avoided for truss repairs. Recently, specialty screws have become available, resulting in superior performance in relation to standard screws. These screws are manufactured by United Steel Product, Simpson Strong-tie, and Fasten-Master, to name a few.

Machine bolts are generally used where high forces are involved. Truss connectors in wood trusses rarely benefit from high strength bolts, making them unnecessary. Carriage bolts are not recommended; the lack of a washer and solid bearing on the head of the bolt results in poor performance in relation to machine bolts. Lag screws may be used, but are of limited capacity due to the relatively thin (1-1/2-inch) thickness of the truss members. The lack of penetration limits their lateral strength significantly.

The use of adhesives for truss repairs raises considerable debate among engineers. The quality of some adhesives today allow for bonding wood with greater strength than the wood itself, resulting in much smaller connections. However, the conditions under which many truss repairs are performed can compromise the glue bond. For example, freezing temperatures, surface dirt, and unsupervised labor make the capacity of the glued joints difficult, if not impossible, to judge.

Mechanical fasteners can be counted to verify conformance with the repair specification. Unfortunately, once the glue is applied and covered, it cannot be seen without destroying the repair. Therefore, adhesive use is best left for controlled environments, such as factories and jobs where the responsible engineer can observe the application of the adhesives. If an adhesive is to be used, the curing time must be considered. In many instances, a truss must be repaired in place. Once a repair is completed, temporary supports are often removed and construction materials are placed on the truss immediately. This practice does not allow proper time for the adhesive to cure. One solution to this problem is to provide mechanical fasteners, designed to support the construction loads, allowing the adhesive the required time to gain sufficient strength before the full design load is applied to the truss.

One other school of thought is to specify the adhesive, but not to consider it in the calculations. This is most often done when repairing floor trusses. The mechanical fasteners transfer the forces and the adhesive provides insurance against squeaks. If the adhesive is omitted, the repair is still valid. If it is used, it only adds strength and stiffness to the repair.

A common practice used by many engineers to reduce the connection size is to “clinch” the nails used with gussets. In order to properly clinch nails, a gusset is placed on each face of a single ply truss. The gusset must be clamped or held in place securely during the nailing procedure. Nails are driven through the front gusset, through the truss, and through the gusset on the back face. The nails must then be bent over flat against the surface of the back gusset.  Please bear in mind clinching must be performed properly. Unfortunately, many repairs specifying clinched nails are performed incorrectly at the jobsite.

Due to the lack of code provisions governing truss repairs, engineering judgment becomes critical in the determination of what is acceptable. While the design capacities of various materials are available to aid an engineer’s calculations, there are times when repairs tend to bring the art of engineering to a level almost equal to the science.  Need a truss repair?  Call an engineer experienced in designing repairs for damaged trusses.  I always have, and always will.  Like concrete finishing, there are just some things best left to the experts.

Metal Connector Plates Have Teeth

Metal connector plates are used to connect wood truss members and are considered the most important invention in the light frame wood construction industry. This is due to their versatility, cost effectiveness and ease of installation. Typically composed of galvanized steel they are mainly manufactured using one of three metal thicknesses: 20, 18 and 16 gauges. The galvanization means the steel is coated with a zinc alloy for corrosion resistance. Stainless steel is also used to manufacture metal connector plates since plates are used in highly corrosive environments.

The interior pieces of a truss are webs and the continuous outside ones are the chords. Top and bottom chords typically directly carry the loads and transfer those loads to adjacent joints (the confluence of webs with the chords). The truss members can be of different sizes, grades and/or species of lumber. When constructing trusses, the members have to be in the same plane for the metal plates to connect them.

As shown in the picture metal truss plates have integral tooth slots. Each tooth has a particular shape, size and twist design depending on the plate manufacturer. These characteristics affect the load carrying capacity of the plate. Plates with teeth on one face are placed on both faces of a truss joint and are pressed with hydraulic or roller presses to embed the teeth into different web and chord members. Once the plate connects the web and chord members, it resists the forces coming from them. All plates on a truss joint are designed to withstand forces due to tension, compression, shear, and moment.

To ensure the plates can resist the forces coming from the truss members, each plate manufacturer tests them in tension and shear at different angles. A plate’s load carrying capabilities will differ at different angles. Tension and shear values of plates are also related to the metal strength used for plates. Plates are also tested for lateral resistance or gripping, which is directly related to the lumber strength. The stronger the lumber is, the higher the lateral resistance value will be. Each manufacturer publishes the design values of their truss plates.

Just as metal connector plates to join wood truss members created a burgeoning prefabricated roof truss industry, the affordability of wide span wood trusses has revolutionized the pole building industry. This advent has allowed for clearspans of 80 and even 100 feet to become practical using wooden pole barn design.