Truss Tuesday: Long Truss Span

Long Truss Span

This week’s truss: Long Truss Span

Question:

What is the longest truss you’ve seen in the wild? Do you think this 65’ long agricultural truss would work in a pole barn?

Top and Bottom Chords are 2×6 Southern Pine #2
Webs are 2×4 Southern Pine #3

Building Code: IBC 2021

Loads(psf unless otherwise noted):
TC Live Load 20
TC Dead Load 10
BC Live Load 0
BC Dead Load 5

Application: Agricultural

Wind: 110 MPH

Terrain Exposure Category: C (Scattered Obstructions)

The answer will be posted on Thursday! This is not a production truss! This is simply an example for Truss Tuesday conversation. Join the conversation on LinkedIn.

Updated Thursday, February 29th 2024

Answer: Yes! This truss will work in this situation; however, most component manufacturers would split this into two separate trusses to make shipping easier.

Truss Tuesday: Moving Load

moving load

This week’s truss: Moving Load

Question:

This truss has two plate pairs at 101% capacity with a 2000lb concentrated moving load applied to each panel and mid-panel point (i.e. at each vertical and halfway between each pair of verticals or the end of the truss). Without changing the load, how would you modify this truss to bring it under capacity?

Top Chords: Douglas Fir Larch Select Structural
Bottom Chords: Douglas Fir Larch Number 2
End Verticals: Douglas Fir Larch Number 3
Webs: Douglas Fir Larch Number 2

Building Code: IBC 2021

Loads(psf unless otherwise noted):
TC Live Load 20
TC Dead Load 10
BC Live Load 0
BC Dead Load 10
Application: Residential

Wind: 110MPH

The answer will be posted on Thursday! This is not a production truss! This is simply an example for Truss Tuesday conversation.

Passing Moving Load

Updated Thursday, February 22nd, 2024

Answer:

There are many ways you could fix the issues with this truss, but stacking the webs at the failing plate pairs gives the teeth more area to bite and brings the capacities within acceptable parameters.

Truss Tuesday: The 1000lb Chandelier

Welcome to Truss Tuesday! Every Tuesday, we will present and interesting truss and/or loading situation to see if you can figure out if it works or not. None of these trusses will be actual productions trusses, we just want to have a little fun. We will follow up with the answers on Thursdays.

Vaulted Ceiling

This week’s truss: The 1000lb Chandelier

Question:

You just bought a beautiful 1000lb crystal chandelier for your valentine and want to hang it from your vaulted ceiling. Is this a good idea…or a bad idea? Did you keep the receipt? 

Top and Bottom Chords are 2×6 Southern Pine #2
Webs are a mixture of SP #2 and #3

(psf unless otherwise noted)

Building Code: BCNYS 2020

Loads:
TC Live Load 20
TC Dead Load 10
BC Live Load 0
BC Dead Load 10
Application: Residential

Concentrated dead load applied at the center of the vault

Wind:
155MPH

Updated Thursday, February 15th, 2024

Answer:

It would indeed hold the chandelier, but you would have to decide whether it’s a good idea. B1 is at 100% capacity with a 1000lb concentrated load at the vault’s center. Make sure you purchase quality hangers for that fancy chandelier!

 

What’s on a Truss Design Drawing?

What’s on a Truss Design Drawing?

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.

    Marked Up Truss Design Drawing

    Required Information

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

    1. 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.
    2. 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
    3. 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
    4. 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
    5. 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)
    6. 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
      1. 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
    7. 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
    8. Maximum reaction force and direction: This refers to the reactions at each bearing in the down, up, and horizontal directions
    9. 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
    10. Size, species, and grade for each Wood Member: The basic lumber properties of each wood member must be specified
    11. 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
    12. 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)
    13. 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
      1. Truss Pal TDDs include all of the forces that occur between each truss joint that are greater than or equal to 250 lbs
    14. 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)
    15. 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
    16. 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:

    1. Maximum CSI: A summary of key member check results that indicates how stressed the members are
    2. Maximum JSI: A summary of key plate check results that indicates how stressed the connector plates are
    3. 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
    4. 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
    5. Weight: This is the self-weight of the truss including all lumber and plates
    Truss Spacing: How Far is Too Far?

    Truss Spacing: How Far is Too Far?

    Truss Spacing

    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!