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!

 

The Challenge of Specifying Truss Girders

The Challenge of Specifying Truss Girders

Truss Girders

Structural engineers often encounter the challenge of specifying truss girders when developing a structure’s load path during the creation of the structural framing plan. A deeper understanding of their functions, placement, and design considerations is crucial for avoiding costly waste, rework, and miscommunication with downstream manufacturers and installers. Let’s dive into the world of truss girders and explore their critical aspects.

The Role and Structure of Truss Girders

Defining a Girder Truss: A girder truss is a truss that supports other trusses. It ranges from 1-5 plies in thickness, with the additional plies lending extra strength and additional material for connecting fasteners to penetrate.

Nailing and Ply Requirements: For a girder supporting trusses with hangers, a minimum of 2 plies is often necessary. This requirement stems from the nailing needs for hangers, typically rated for nails penetrating 3 inches of material.

Optimal Placement of Girders

Hip Ends and Wall Junctions: You’ll commonly find girders at hip ends or where a wall changes direction. In corner scenarios, the girder usually resides on the “outside” of the adjoining bearing. This placement ensures the trusses it supports have a consistent span and design as the adjacent run of trusses, rather than requiring shorter carried trusses.

Residential Home Considerations: In residential settings, a hip girder setback of 6 feet or less from the end wall is ideal. This allows for toenailing the jack trusses onto the hip end, saving time and resources compared to more expensive (and more expensive to install) connecting hardware. More complex houses may require varied hip girder setbacks to accommodate consistent truss to truss spacings.

Vaulted Ceilings and Roof Openings: Girders are also crucial around changes in ceiling profiles, such as vaulted ceilings running perpendicular to the main ridge line. Additionally, they play a vital role around roof openings like chimneys or in supporting the second floor of a building.

Decision Factors for Girder Placement

Shortest Girder Span Preference: Some structures have ambiguity in the optimal location to place a girder. Given a choice, the shortest girder span is often preferable for ease of installation and cost-effectiveness. However, sometimes the building design dictates girder placement, especially to avoid large point loads on structural elements like garage door headers.

Floor Girders vs. Roof Girders: While similar in placement, floor girders have their unique constraints. They can’t carry as much weight as roof girders and are limited to a maximum of 2 plies (due to limitations of fasteners to connect these wider trusses). In certain deep floor scenarios, a rectangular roof-style girder can be used instead due to its higher weight capacity. (Roof-style trusses have the lumber oriented on edge, whereas Floor-style trusses have the lumber oriented flat-wise).

Practical Approaches to Girder Design

Stair Opening Considerations: Also, don’t forget about floor girders around stair openings. For stair openings, a practical approach to developing the loading involves taking half the length of the stair run, multiplying it by the floor load, and then applying that as a distributed load to the supporting girder.

Conclusion: The Art of Truss Girder Design

Specifying truss girders is a blend of technical knowledge, practical experience, and adaptability to the unique demands of each building. Understanding the nuances of their placement and structural requirements is key to efficient and effective structural engineering.  Ultimately, the location and size of your girder trusses depend on a wide variety of parameters depending on the building. Let Truss Pal help you develop your structural framing plans for your project. Truss Pal can provide full truss placement diagrams as well as material take offs and IFC models for a more complete picture of your structural framing and coordination needs.

What goes into a Truss Placement Diagram?

What goes into a Truss Placement Diagram?

Cropped Truss Placement Diagra

The Basics

All modern building codes reference a Truss Placement Diagram, which “identifies the proposed location for each individually designated truss and references the corresponding truss design drawing” (IBC 2021, 2303.4.2). This is typically accomplished via a top-down view of the structure, a truss ID labeling the shape that represents each truss, and a dimension that locates each truss relative to the building’s edge. The purpose is to guide the installation of the trusses.

The diagram is typically not the primary source for engineering information like how the trusses are designed or how they are to be anchored. Truss Placement Diagrams are merely a supplement to or summary of information found in the Truss Design Drawings. If they aren’t referencing information like that from a sealed document, they should be sealed themselves.

Additional Information

Connecting Hardware

The Truss ID label may contain the hardware that should be used to connect each end of the truss to its bearings, i.e. hangers and/or nails. In this example, an index is used that references a table nearby.

Truss identifiers

Index Name Quantity
1 HUS26 1
12 LUS26 30
23 H2.5A 270

 

Graphics like this indicate hanger connections:

Hanger Connections

Roof Details

Key features of the roof will often appear so they can be used for reference when orienting the diagram and locating the trusses. This includes, but is not limited to, openings in the frame for a skylight or chimney, dormers, a projection of the roof showing slope changes, and portions of the framing that will be built on-site.

 

Roof Details Snippet                      

Truss Details

Some of the most pertinent information for installing trusses may be called out in a Truss Placement Diagram. This includes things like heel details, special truss-truss connections, and the locations of valley sets and piggyback trusses. Girder trusses typically receive their own dimension lines indicating their “setback” from a parallel edge of the building.

Truss Details

Truss Pal

Do you have questions about a project but don’t have a structural framing layout yet? Save time by letting us design a truss placement diagram for you based on the construction documents for the project. Don’t have construction documents? We can design layouts from scratch too.