truss Archives - Page 3 of 3

What’s on a Truss Design Drawing?

by Seth Duncan | Nov 7, 2023 | Educational, General

Whether you’re starting a new project that includes trusses or retrofitting an existing one, you’re going to need Truss Design Drawings (TDDs) that are certified by a Professional Engineer. A TDD includes the essential characteristics of the truss itself, the inputs that went into loading and designing it, and the key outputs of the analysis. A TDD must include certain information in order to be valid; let’s walk through what information that is together.

Required Information

According to ANSI/TPI 1-2014 section 2.3.5.5, the following information must appear on a valid TDD:

Building Code used for design: This is the building code used to develop the loading conditions and other constraints for the truss design, e.g. IBC 2018. Which code to use is determined by the jurisdiction in which the truss will be erected.
Slope or depth, span and spacing: This includes how much space is occupied by the truss profile as well as the distance between this truss and the next one
Location of all joints and support locations: This means dimensional information about where members of the truss intersect and where the bearings on which the truss rests are located relative to the truss
Number of plies: A multi-ply truss is one where the same design is duplicated a certain number of times and then all the duplicates are fastened together into a single, much stronger component. The TDD must specify the number of these duplicates if one or more exist
Required bearing widths: The bearing area required to support the truss is a function of the material of the member that rests on each bearing and the forces at that location (see TPI 7.4.1)
Design loads as applicable: this includes all of the loading conditions the truss has been designed for, including the live and dead loads that span the entire top and bottom chords, environmental loads due to rain, snow, seismic, and wind, additional loads and where they’ll be applied, and lateral loads like drag strut loads. Any factors used in loading calculations, such as duration factors, need to be listed as well

Truss Pal TDDs include a list of every load case that was considered. Each load case is a combination of load types along with their duration factors. See the Notes section for any additional loads that were applied

Adjustments to Wood Member and Metal Connector Plate design values for conditions of use: These are factors that adjust the typical material strength properties of the members or plates. Examples of this include the Wet service factor, Quality control factor, Temperature factor, and Incising factor
Maximum reaction force and direction: This refers to the reactions at each bearing in the down, up, and horizontal directions
Metal Connector Plate types and joint offsets: The types of metal plates must be specified as well as the offset distances for any plates that are not centered on a joint
Size, species, and grade for each Wood Member: The basic lumber properties of each wood member must be specified
Truss-to-Truss connection and field assembly requirements: For trusses that carry other trusses, the TDD must describe how they should be connected. Any other detail that directly impacts the truss’s performance, like material that will be attached to it in the field, should be specified as well
Deflection ratio and/or max vertical and horizontal deflections: This refers to the “L/d” ratio and total truss deflection under particular load combinations (see TPI 7.6)
Max axial tension and compression forces in Truss members: The largest forces experienced by each wood member and whether they are tensile or compressive must be listed

Truss Pal TDDs include all of the forces that occur between each truss joint that are greater than or equal to 250 lbs

Fabrication tolerance: This is captured in the Quality Control Factor (Cq) and is used to account for imprecision in the placement of connector plates on the truss (see TPI 6.4.10)
Required permanent member restraints: This could be sheathing, a rigid ceiling, or evenly spaced purlins over the chords or lateral bracing for webs. The locations of the restraints must be specified as well
Truss Designer: The individual or organization who produced the truss design

Additional Information

Besides the additional information about forces and load cases already mentioned, Truss Pal TDDs also provide the following:

Maximum CSI: A summary of key member check results that indicates how stressed the members are
Maximum JSI: A summary of key plate check results that indicates how stressed the connector plates are
Exposure Criteria: This table reflects information provided about whether or not portions of the truss, such as an end vertical or cantilever, are exposed or covered up, which primarily affects whether wind loads are considered in those areas
Camber: Camber is the curvature of the bottom chord that would be required to compensate for the deflection of the bottom chord due to dead load
Weight: This is the self-weight of the truss including all lumber and plates

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Truss Spacing: How Far is Too Far?

by Seth Duncan | Oct 30, 2023 | Educational, General

Roof trusses are typically spaced 2′ on center for residential and commercial projects. That spacing can be reduced if needed to better support additional truss loads, longer truss spans, or other factors. Also note that structures designed using the IRC are prescriptively required to have trusses spaced two feet or less (R802.10-11).

Agricultural/post frame trusses are spaced much wider, typically 4’-12′, which again can vary. These truss spaces are typically spanned by purlins.

The three most common spacings for “Open-web” or floor trusses are 16″, 19.2″, and 24″ on center. Sometimes the smaller spacing is chosen in an attempt to reduce the floor deflection and vibrations in the floor system. However, some research indicates that this approach is limited in its effectiveness and is better-solved by increasing the floor depth, shortening the span, or modifying how the sheathing is attached to the trusses.

Truss spacing may be constrained by the materials being used to span between trusses. For example, most sheathing comes in 4’x8’ sheets and is best installed with a two-foot staggered layout. This means that having trusses centered every two feet will ensure that there is a convenient truss chord to which the sheathing can be nailed at each end. This is something to keep in mind when considering using a smaller truss spacing in an attempt to achieve lower material costs for the trusses themselves. It could require more work or material during framing and sheathing.

The only way to know for certain what truss spacing can be used is to have your trusses analyzed using your desired spacing. Let Truss Pal run your designs for you and provide you with truss design drawings that will show you what options you have!

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What is The most difficult part of any truss repair?

by Seth Duncan | Oct 9, 2023 | Repairs

The most difficult part of a truss repair is getting the necessary information! Here are the five things an engineer needs in order to efficiently design a metal plate connected wood truss repair:

1. The details of the original truss

Ideally, the original truss design drawings, which describe the assumptions and considerations (such as loading, material properties, etc) that went into the original truss design, would be supplied along with the request for repair. If those cannot be found, a detailed description of the truss in question along with photographs and descriptions of the truss materials will be needed to create a new truss design drawing. From there, an engineer can look at the various forces and design a sufficient repair.

2. Locations and descriptions of the damaged or overloaded portions of the truss

Not all types of damage on a truss can be repaired the same way. A connector plate that has popped off will probably require a plywood gusset repair. A cracked member could probably be handled with the addition of dimensional lumber scabs attached to the front and/or back. Also, there may be repair considerations in one location that are not present elsewhere (see Obstructions or constraints below). Knowing both the location and extent of the damage is critical for designing an appropriate repair.

3. Obstructions or constraints

Some repair designs are impractical if the truss to be repaired is confined to an area where the repair material cannot be realistically installed. For example, a gable truss with a broken web will likely require a beefier scab member attached with more connectors to a single face of the broken piece instead of scabs on both front and back that would be sufficient for other trusses.

4. Available materials/resources to perform the repair with

Knowing what materials are available at the job site will help an engineer design the repair that is most practical to apply on location. Knowing what sheathing material is on hand or if a local truss manufacturer is available can help determine if a replacement or scab truss may be the best option for an efficient repair.

5. Loading and building code requirements

Finally, if the truss being repaired is required to comply with newer loading standards or building codes than the original truss was designed for, an updated truss analysis and design drawing could be needed to understand what reinforcement (if any) is needed to bring it up to code.

Being sure to communicate all of the above information when requesting a truss repair will ensure a better outcome and turnaround time for your repair. If you need to repair a truss and don’t have a truss design drawing or if you need a truss evaluated under additional loading conditions, let Truss Pal create one for you!

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Introducing Truss Pal!

by Seth Duncan | Oct 2, 2023 | General

Have you ever had a project that required the analysis of an existing structure with prefabricated metal plate-connected wood truss framing? Having trouble tracking down the original Truss Design Drawings? Have the Truss Designs but need to know how they would handle additional loading or modifications? Truss Pal can help!

Instead of relying on manual methods or begging a truss manufacturer to run a design for you, let us provide you with a Truss Design Drawing that matches the trusses you are working with.

Whether you are evaluating an existing structure for retrofitting additional loading like solar panels or a new HVAC unit, or you are attempting to get a sense of whether prefabricated wood trusses are a good fit for your construction project, let us help you by quickly providing detailed and accurate Truss Design Drawings.

We are familiar with the urgency that comes with Truss Repairs. We understand the concern of specifying a floor truss system that could end up being overly flexible or squeaky (but technically within code!). We’ve been involved in conversations about how to make trusses work on a job where the site-built dimensions don’t match the plans. Let us use our experience to help you with your project today.

If you have any truss issues but don’t see what you are looking for on our website, let us know by sending a question to salessupport@trusspal.com and we will try to connect you to someone in the industry who can help.

Stay tuned as we expand our offerings in the coming months!