Superior Steel Components

Truss Notes

• Attachment to Light Gauge Trusses
• Bottom Chord Bracing
• Building Types
• Full Scope Roof System and Blocking
• Fire Treated Plywood
• Reviewing a Light Gauge Steel Truss Quotation

Attachment to Light Gauge Trusses

CFS fabricators are often asked about field drilling holes in trusses for the attachment of mechanical equipment, pipe hangers, etc. There are many factors that affect truss loading, and care must be taken not to exceed design loads. In general, truss bottom chord dead loads are designed to cover all mechanical and ceiling attachment loading.

• Design engineer is responsible for method of attachment to truss
• Maximum loads shown reflect capacity of the flange only. Loads must be incorporated into truss design. Building engineer shall verify adequacy of loads for the actual application.
• Self-drilling screws are acceptable in chords
• Maximum 3/8” diameter hole
• Do not drill within 1 ½ “ from the end of any member
• Do not drill or attach to webs or connectors
• Do not drill or screw into the 1 ¾ “ or ¾ “ flanges of chords

If you require an attachment that does not meet these guidelines, or to order reinforcing material, contact SSC to review your situation.

Bottom chord bracing and UL requirements – Fact and Fiction…

One favorite sales tactic has been to claim that a truss system is less expensive because it does not require bottom chord bracing. This claim can be dispelled by referencing LGSEA Tech Note 551d which states that all trusses must have bottom chord bracing at 10’ oc maximum.

The other claim, and the one that is proving more difficult to dispel, is the idea that the bottom chord purlins that are typically fastened to the bottom of the chord for drywall attachment can also act as the bottom chord bracing of the truss itself and no additional bracing material is required. On a 50,000 sf school in a low wind zone, this could be nearly 500 pieces of hat channel and cause a difference in price of $3,500 in material alone. Understanding, and more importantly, being able to communicate the code and the related UL listings becomes critical. That’s the bad news. The good news is that if you are able to communicate it properly, you will go a long way toward proving your expertise and gaining confidence in you as the go-to person for cold formed components.

A one hour rated ceiling can be used to eliminate the requirement of a fire sprinkler or will allow the fire sprinkler to provide a significant area increase, depending on the type of construction. In other words, for construction of types I-a, I-b, II-a or III-a, a one hour (min) rated ceiling is a must.

It is usually a safe assumption that most projects requiring cold-formed steel trusses due to the non-combustibility are also projects that will require at least a one hour rated ceiling. A simple check to confirm this is by verifying the occupancy category calculations almost always given in the architectural drawings. Once you have verified the need for the rated ceiling assembly, you are able to provide the correct information.

A UL Fire Resistant Steel Truss Assembly will consist of non-combustible roofing, a cold-formed steel truss and a drywall ceiling. For the drywall to provide a layer of protection between the occupied area and the attic, it must be fastened in such a way that it does not break under extreme temperatures. When drywall is subjected to extreme heat, it will shrink. The UL assembly requires that drywall be fastened in such a way that, when it begins to shrink, the members it is attached to can move with the board. This movement reduces the chances of the drywall breaking due to a rigid attachment. If, for instance, drywall was fastened directly to a truss and begins to shrink – the truss and the screw attaching the truss, are not going to move, so the only way for the drywall to move is to separate from the screw and the truss. Once this separation occurs, the drywall is broken and is no longer providing a barrier between the attic and the occupied area.

To allow this movement, a UL listing gives two options for attaching the board assembly to the truss. The first is to screw attach very light resilient channel to the truss and then attach the board to this resilient channel. Under extreme heat, the resilient channel will be weak enough to deflect with the board and maintain the required barrier longer. However, this resilient channel does not have the compression capacity to be considered bracing for the trusses, or can it support the weight of drywall when the truss spacing is greater that 24” oc. In this instance, additional bottom chord bracing is still required.

The other method is to attach the drywall to a heavier gauge hat channel, but the hat channel cannot be rigidly attached to the truss, it must be wire-tied. The use of the stronger hat channel allows the use of steel trusses with increased spacing, but since the heavier hat channel will not move with the board under heat – it cannot be rigidly attached to the truss. Wire-tie is required in this instance and is very labor intensive. Additionally, since the hat channel is not mechanically fastened to the truss it cannot provide axial resistance to the bottom chord, and once again, standard bottom chord bracing is required.

In short, it is not realistic to utilize the bottom chord furring required for a UL rating for bottom chord bracing. If someone states that it is alright, that is misleading.

Building Types

Step 1: Building Types

From the 2006 IBC (which is similar to ’03) and widely accepted:

602.2 Types I and II: Type I an II construction are those types of construction in which the building elements listed in Table 601 are of noncombustible materials, except as permitted in Section 603 and elsewhere in this code.

602.3 Type III: Type III construction is that type of construction in which the exterior walls are of non-combustible materials and the interior building elements are of any material permitted by this code. Fire-retardant wood framing complying with Section 2303.2 shall be permitted within exterior wall assemblies of a 2-hour rating or less.

602.4 Type IV: Type IV construction (Heavy Timber, HT) is that type of construction in which the exterior walls are of non-combustible materials and the interior building elements are of solid or laminated wood without concealed spaces. The details of Type IV construction shall comply with the provisions of this section. Fire-retardant treated wood framing complying with Section 2303.2 shall be permitted within exterior wall assemblies with a 2-hour rating or less. Minimum solid sawn nominal dimensions are required for structures built using Type IV construction (HT). For glue laminated members the equivalent net finished width and depths corresponding to the minimum nominal width and depths of solid sawn lumber are required as specified in Table 602.4.

602.5 Type V: Type V construction is that type of construction in which the structural elements, exterior walls and interior walls are of any material permitted by this code.

Referencing to Table 601 – Fire Resistance Rating Requirements for Building Elements – Type I construction requires non-combustible material for a structural frame and for standard roof construction. Type Ia and Ib are very likely to require cold formed trusses when a sloped roof is desired. Type IIa and IIIa also require a 1 hour rating for the structural frame and for roof construction; however a footnote allows an approved automatic sprinkler system to be used in lieu of the 1 hour rating. For these constructions types, it ay be shown that non-combustible components in the roof system may allow the architect to eliminate an attic sprinkler system. That footnote also states that if a fire sprinkler is used to eliminate the fire rating requirement it cannot be used for an area increase (this becomes important when reviewing the limitations of each construction type). Types IIIa and Va do not require non-combustible roof systems.

So what we have determined to this point is that if a building is deemed to be Type Ia or Type Ib, cold formed trusses have a chance to be used. Additionally, Type IIa and IIIa may go steel if the customer is willing to eliminate a sprinkler system or the sprinkler system is being used for the code allowed area increase.

Step 2: What determines the Building Type

If we know CFS product may be used on basically all Type I construction, and some Type II or Type III construction – it becomes critical to know what determines the Type construction a building will be.

Table 503 in the IBC is included with this document – this table gives the limitations for number of stories and allowable building area (ft2).

Many hotels that are built seem to go wood and many seem to go steel. What is it that causes one to be built with wood versus a hotel out of non-combustible steel? Hotels, per the code, fall under the Residential Group (R) as described in section 310. R-1 is the specific designation for a typical hotel. By taking this information and referencing back to Table 503, you can decipher the following limitations for story height and building area:

Footnotes allow the number of stories to be increased by 1 (20ft) and the area doubled with an automatic sprinkler system.

Example:
Section 310 – Residential Group R: For a 4 story hotel (considered R-1) that is 48,000 ft2, the designer could consider this structure Type IIIa construction by using a sprinkler system (area increase) and can use wood trusses since it is construction type IIIa. WRONG – to get the area increase, the sprinkler system is required. That same system cannot be used to eliminate the fire rating for the Roof Construction as well. Above 48,000 ft2 the designer is going to be forced to utilize Type I construction.

Example:

Section 308 – Institutional Group I: Assisted living facilities fall into this category. I-1 is a standard assisted living building; I-2 includes hospitals and skilled nursing homes. These two sections present a significant amount of opportunity for * * , yet they somehow frequently utilize wood trusses. In the case of an assisted living facility with a skilled nursing wing: the number of stories and the total square footage is very limited for group I-2. To maintain Type IIIa construction (IIIb is not even permitted) a building size would be limited 12,000 ft2 and 1 story. The customer could use a sprinkler to increase the area and story height allowed, but then non-combustible trusses would be required. Alternatively, if the building is within the requirements of the code, table 601 still requires a 1 hour fire rating or a sprinkler. Steel trusses could eliminate the need for an attic sprinkler saving the contractor and the owner money and maintenance.

We described just one wing of this structure as skilled nursing, so the additional wings may just be considered I-1 and the allowable areas are greater per Table 503 – however the areas are typically much larger for this type of structure and the wings must be separated from the skilled nursing wing to be considered a separate building group (as described in section 503.1).

Full Scope Roof System and Blocking

What is included in a full scope roof truss system?

All too often, truss “systems” end up being considered simply trusses, bracing and clips. Some fabricators even go so far as to indicate that their trusses do not need bracing due to the shape of the section. In the world of component design, this may be true. Once that component is placed next to other components, it is included as part of a system.

One way to look at the truss is this; it is a steel component that is 40’ long, 10’ tall and 1.75” wide. A roof systems must have capacity in all directions, although a roof truss (component) may only be designed for capacity in the vertical (and in the case of shear trusses – parallel) directions. In the 1.75” direction, the component has no real strength – no matter who roll forms the steel. The bracing applied perpendicular to the truss is what gives the component its strength in the weak direction and allows it to act as part of a system.

A full scope roof truss system becomes essentially anything that the project engineer draws as part of the structural support system above the top plate. Instances arise where nothing additional to the trusses is drawn – in these cases SSC still wants to supply what we know is required for the system and include that. For instance, a building might have cantilevered trusses with a standard heel. The structural drawings probably indicate bent metal at the heel for the edge attachment of deck. The 2006 IBC requires the transfer of load from the diaphragm (roofing material) to the resisting member (bearing wall). Since the truss cannot provide this load transfer perpendicular to the truss (remember – only 1.75” thick), and the bent metal is attached at the end of the cantilever – well past the resisting wall – something needs to be added perpendicular to the truss to transfer the load directly from the diaphragm. This is why we use shear blocking – it is a method to transfer this load as required by IBC. Above 12” tall, this is typically a short truss built to go between trusses. Less than 12” tall, SSC will typically use a piece of break metal. Note that these items are attached to the sheathing (plywood or metal deck) and to the bearing wall, they are not attached to the trusses since they are solely a method of transferring load between trusses.

Additional requirements for a full scope system include:

• Bent metal which is required for the edge attachment of the sheathing material
• Outriggers – where gable ends or dormers exist
• Dormers over the roof – including the face of the dormer that must be designed to resist wind load
• Gable end walls. Providing gable end trusses is almost never adequate. Again, think 1.75” think, the gable truss does not have any significant capacity in the perpendicular direction and truly a pre-fabricated rake wall is necessary.
• Kickers – often the engineer counts on the truss system to brace his/her beam from rotation. This is not something that is quoted unless it is specifically shown on the plans. When it is shown, it must be addressed.
• Soffit framing – similar to kickers. When it is shown attached to the truss it must be addressed. Typically, this is an item that is best field framed due to the fact it is an aesthetic item.

Additional opportunities that exist within a full scope: BIM – if significant mechanical units exist in the attic, there is an opportunity to offer a 3D CAD rendering of the roof truss system, including web locations. The mechanical contractor can take this 3D rendering and know, before construction, that the mechanical units and ductwork will work within the roof system.

While this list is not all inclusive – it highlights items to look for and be familiar with prior to discussing a project.

FRT Plywood

Cold-formed steel trusses are used on projects for a very specific reason – to accomplish a residential look with a non-combustible system when required by code for that particular building type. Basically, when an architect wants an aesthetically pleasing commercial structure, but is not allowed to use combustible construction – cold-formed trusses become an option. The ultimate residential look on a building can be accomplished by providing a shingled roof. Many owners and architects prefer the look of shingles over the more industrial look of a standing seam roof. If they are going to go to the expense of a sloped roof, then the additional cost of shingling the roof often makes sense.

To apply shingles, a roof substrate that is nail-able is required. Unfortunately, the most economical, non-combustible and nail-able substrate is fire treated plywood – and as a result, it is frequently used.

We are not a building architect or engineer, and there is no reason for us to pretend to be. However, we are construction people and we should give our customer the benefit of our construction experience, especially as it relates to FRT plywood.

When offering FRT plywood on a project as specified by an architect or engineer, the architect is specifying the least expensive product that can be more used to provide a surface for applying shingles. Unfortunately, if SSC bid that project with a more reliable alternative, the pricing would not be in line. All plywood projects MUST use screws for the fastening of plywood to the trusses using a 3/6 screw pattern (screws 3” oc at the edges and 6” oc in the field). The pattern most commonly used is 6/12. In addition, stabilizers or coped track (blocking) are required to be used at the plywood seams perpendicular to the trusses. This provides another surface for plywood attachment and should restrict the ability of the plywood to curl at the edges.

Some issues with FRT plywood are related to moisture. However, these can be avoided. Plywood is an organic material – FRT plywood is an organic material treated with chemicals that make it more susceptible to the effects of moisture. When there is moisture around plywood, it will absorb the moisture and have a tendency to expand. When the air is very dry, the plywood loses its moisture and shrinks. The IBC code gives minimum requirements for ventilation in an attic in an effort to ensure that wide swings in temperature and humidity do not adversely affect the building materials.

We have seen several projects where this ventilation was not provided, or was covered up somehow. The result was plywood pulling from the steel it was attached to creating major waves in a shingled roof system. This has occurred as quickly as 6 months and as long as 4 years after the plywood was installed. Incidentally, mold is also a result of excessive moisture with organic materials.

If FRT plywood is to be used, SSC ensures that our customer is aware that proper ventilation must be provided AND that the roofer puts the roofing material down immediately after the plywood is installed. FRT plywood that is exposed to the elements for an extended period of time (2+ days) after installation is more susceptible to movement than plywood that is covered up immediately. The heating of plywood during daily exposure to the sun and then cooling overnight seems to cause FRT plywood to move no matter what the ventilation is. This seems to be the result of top layers being exposed to excessive wetting and drying while the underside is subject to standard ventilation. FRT plywood, however, should never be covered when it is excessively wet.

Assuming all this criteria is met during installation, and the proper ventilation is provided – FRT plywood should be just fine. Alternatively, there are other products available using non-combustible material that provide a nail-able base for shingles.

Other solutions include:

• Loadmaster's Roof deck system
• Martin Fireproofing’s loadmaster equivalent
• Firestall roof deck by Homasote

Reviewing a Light Gauge Steel Truss Quotations

A lot of work goes into the design and bidding of a light gauge metal truss system, however, in order for the end user of this quotation to be completely covered some review work is still required.

In a standard metal truss quotation you will have 6 items:

1. Light Gauge Metal Trusses
2. Permanent Lateral Web Bracing (Web Compression Bracing)
3. Truss-To-Bearing Clips
4. Sealed Truss Drawing Packages
5. Bracing Layouts
6. Erection Bracing (Stabilizers)
7. Sales Tax

1. Included within the scope of the light gauge metal truss system are the metal trusses, truss-to-truss connections, and delivery F.O.B. jobsite. Remember that unless installation is quoted and purchased unloading of the metal trusses at the jobsite is the responsibility of the General Contractor.
2. A truss system gains maximum stability and strength only when it is properly braced. The bracing of the truss system is required for it to function safely and properly. The bracing material used is a common hat channel and can be purchased from a variety of sources, including SSC. SSC realizes that some contractors have this material in-stock, so we have decided to break this item out as a line item.
3. Truss-To Bearing clips are required to hold the truss to a bearing support. Typically you will see that a 3x3x3 16 gauge clip will handle the reactions created by the truss systems, however, in certain situations a clip capable of handling larger reactions is required. In all situations SSC will break out the quantity and type of each clip required and provide this as a separate price as some contractors have these clips in stock.
4. When a truss system is ordered SSC will create a package of information and submit it to our customer, who in turn submits the information to the architect & structural engineers. This package is called a submittal package and is required to be verified and approved by all parties before the truss system is fabricated. Submittal package requirements typically involve SSC sending out 6 sets of roof truss drawings, connection and bracing details, and sealed roof truss engineering drawings. When trusses are purchased, submittal drawings are sent out at no additional cost to our customer.
5. In addition to our submittal drawings a customer may choose to order a bracing layout. Bracing layouts contain the same information that the submittal package contains except that the bracing layout is a graphic representation of how the trusses are to be braced and is also sealed by a licensed engineer. Typically a bracing layout is requested by either the first time truss installer or is requested by the architect / structural engineer for approval. If SSC, Inc. performs the installation a bracing layout will not be required but may still be purchased if requested by the customer.
6. Stabilizers are a product that solves many problems related to the installation of light gauge metal trusses. Stabilizers are a small piece of metal that fit between each truss (they come in 2 lengths – dependant upon the spacing of the trusses) and attach to the truss top chord. The stabilizers act as a spacing device ensuring that you maintain the exact spacing required by the design of the truss. By using a stabilizer you eliminate the need for temporary bracing on the top chord. Stabilizers provide the contractor with timesavings, as they don’t need to be removed from the truss system once the trusses are permanently braced unlike regular temporary bracing does.
7. SSC makes every attempt to calculate the sales tax on the projects it quotes. In some instances the sales tax calculated may be incorrect and upon ordering of the metal truss system the sales tax will be correctly calculated. Some projects may be tax exempt. In the case a project is tax exempt the customer will be required to provide SSC with an exemption certificate and then disregard this line item.

In addition to the standard quotation SSC may choose to quote the following items:

1. Installation
2. Closure Metal
3. Hat Channel / Structural Purlins
4. Metal Deck / Roof Sheathing
5. Eave / Soffit Framing