Tag Archives: bending moments

Post Frame Knee Bracing in Ohio

Post-Frame Knee Bracing in Ohio

Reader DON in TALLMADGE writes:

“I recently purchased plans for a 32×32 pole building and the trusses are 2×4 and the building supplier did not include knee bracing included in the original plans but the county said they need to be added. Are these really a benefit and do I need them?”

Mike the Pole Barn Guru says: 

A knee brace is an inclined diagonal lumber member connecting to and extending from sidewall columns, usually several feet below truss to column connection, across and attached to truss faces. They are intended to supplement lateral resistance of post frames when loaded by lateral wind forces.

Pole Barn Knee Braces

Knee bracing’s intent is noble – to supplement resistance of post frames (columns along with aligned roof trusses create a post frame) under lateral (wind) loads. They can influence unsupported column length, as (when reduced) column is reduced, it is less prone to buckle.

Pole building frames, prior to installation of roofing and siding, tend to be very flexible. It is steel cladding or sheathing making the building stiff. It would not be unheard of to stand at the top, center of a framed up only building and be able to rock building six to eight inches! Adding knee braces at this point of construction will stiffen the frame and act as a temporary brace.

Knee brace effectiveness is highly dependent on stiffness of connections to post and truss. If brace end connections are flexible or not very stiff due to use of few fasteners, roof diaphragm carries the bulk of load and the brace is ineffective. If brace connections are made very stiff (by installing many nails or bolts) brace could effectively resist wind loading, but overload truss.

Knee braces induce bending moments in truss chords. If used in a post-frame design, load sharing among truss, post, knee brace, connections and roof diaphragm must be included in structural analysis.

Johnston and Curtis, in 1984, performed actual testing on post frame buildings with and without knee braces. They concluded, “As loads were increased, the effect of the knee bracing became insignificant.” This study found knee bracing in post frame buildings provides very little support for horizontal loads. Two years later, as a result of their studies, Gebremedian and Woeste concluded, “Knee braces added little stiffness to the post-frame building analyzed.”

In a presentation to International Conference of Timber Engineering in 1988, Jerry Barbera (then chief engineer for International Conference of Building Officials’ Pacific Northwest office) stated, “When the knee brace is placed on the truss at random the truss will experience considerable stress.”. Further, he said, “Thus the truss designer has to know what the extraneous forces are in order to design for their effects. Both designers have to communicate with each other”.

Walker and Woeste’s 1992 book Post Frame Design states, “Knee braces appear to be a “no-win” solution.”

In all likelihood, pole buildings being proposed as utilizing knee braces are a result of lack of knowledge upon building provider. Knee braces add no benefit to overall structural strength, while potentially adding loads into roof trusses they were not designed to carry. In a right combination of circumstances, this could result in a catastrophic building failure.

Your issue with your local Building Official stems from plans being submitted for permit having included knee braces. 2019’s Ohio Residential Code does require knee bracing to be used for any non-engineered post frame building in Section 328.6. Should you desire to eliminate knee braces, you would need to resubmit plans without knee braces, sealed by a Registered Professional Ohio Engineer.

Installation Guidance on Truss-to-Post Connections

Installation Guidance on Truss-to-Post Connections

Originally Published by Frame Building News May 24. 2022

This article series has been focused on installation best practices as it pertains to long-span metal-plate connected wood trusses in post-frame buildings. We’ve explored the reasoning behind why truss handling on the jobsite should be minimized, and how proper jobsite storage and use of the correct hoisting equipment can be effective in achieving that goal. We’ve also explored how long-span trusses need to be adequately braced to the ground during installation, then properly restrained and braced to each other before sheathing is applied. Most recently, we looked at effective ways to apply permanent bracing to a truss system to ensure it performs as expected over the life of the building.

All of those elements are extremely important to mitigate the chance of something going wrong during truss installation and you end up at best having to hold up the project to make a repair, or at worst cleaning up a large and expensive pile of spaghetti. Yet, following all those best practices are for naught if the connection of the truss to the post or column is not made correctly.  Unlike truss bracing, there are several ways to do this properly, and different regions of the country approach columns and truss connections differently. Instead of going into depth on the myriad of options, this article will talk about the big-picture issues that must be addressed.

Forces to Be Reckoned With

David Bonhoff, Ph.D., P.E., is a professor emeritus at the University of Wisconsin-Madison.  He has written several technical articles providing thorough analysis of various aspects of post-frame buildings, including truss-to-post connections. In a recent discussion on connection best practices his primary focus was on providing resistance to all of the forces that are applied to these connections. “Engineers typically address these forces as a shear force that acts perpendicular to the column, and an axial force that acts parallel to the column,” says David. “Then there are bending moment forces where the members within the connection want to rotate.”  

Of those three forces, the bending moment is generally the biggest concern for the building designer and often dictates the size of truss and column members.  That said, all of these forces influence the amount and direction of load being resisted by a truss-to-post connection, yet the connection has to successfully resist them all.

An example of a truss-to-post connection with a solid-sawn column (Hansen Pole Buildings, LLC photo)

Steve Kennedy, P.E., an engineer for Lumber Specialties who has been designing long-span trusses for post-frame buildings for many decades, says, “When enough load acts on a truss, whether it’s a snow or wind load, and the truss wants to move, that truss-to-post connection needs to be stiff enough to resist it.” If it doesn’t provide sufficient resistance the truss will move and the end result will be failure at that connection.

A Pantheon of Columns

Post-frame columns are typically either solid-sawn or laminated. A laminated post is any column assembly that consists of two or more layers of dimensional lumber joined together by either glue, nails, a combination of nails, screws and/or bolts.  Glue-laminated columns are typically horizontally laminated, while mechanically laminated columns are most often vertically laminated.  

Some laminated column assemblies are unspliced, meaning each layer is comprised of a single piece of dimensional lumber. For several reasons, it has become more common for laminated assemblies to come spliced, where at least one of the layers is comprised of multiple pieces of dimensional lumber.

While all of these approaches are acceptable, NFBA’s Post-Frame Construction Guide points out that spliced, mechanically laminated columns offer a significant advantage when it comes to enabling “saddled” truss-to-post connections (page 5):

“These columns can provide efficient truss connection details because the length of the different laminations can be varied, creating a slot of the truss to slide into.”

On the Level

The first step in ensuring a good truss-to-post connection is making sure the bottom chord of the truss rests completely on the top of the column. “You want the bending moment (rotation) forces in that connection transferred to every ply in the post,” says David.  This is done through the fasteners to the outer plies and through physical connection to the inner plies.

David also stresses that if the truss (or trusses, if the column is supporting multiple plies) is sandwiched between plies of the laminated column, the truss should be in full contact with the outer plies. “When they’re snug, that will produce considerable friction between the truss and post members, adding stiffness to the connection.”

Consistent column height and every truss-to-post connection is also important. “Whether the column is solid-sawn or laminated, it’s critical all the posts provide a consistent height to ensure the truss bottom chords are level once they’re installed on top of the columns,” says David. 

He points out it can be difficult (though not impossible) to resolve a situation where the inner ply of the column is too high. “In that scenario, it’s beneficial to have one of the outer column plies spliced. You can then mechanically laminate it in the field once you’ve cut down the inner ply to the height you need,” says David. “Another approach is to purposely leave the inner ply short and you can insert shims in the field to get that full connection in the inner ply.”

On occasion, it may make sense to shim the column at the foundation. “If you are setting columns on a slab and your slab isn’t perfectly level, you can shim the bottom of the post with one or more thin layers of polyethylene,” says David. “It’s a great material because it adds a moisture barrier to the bottom of the post where it comes into contact with the concrete, and the plastic has stronger resistance to crushing than the wood.”

A “Fasten-ating” Approach

“The key to the truss-to-post connection is making sure multiple fasteners are sufficiently spaced apart so they don’t share the same wood grain and aren’t too close to the end of a member that they cause it to split,” says David.

Truss-to-post connections can be accomplished with nails, screws, and/or bolts. David says, “The availability of proprietary fasteners has really increased over the last decade, so post-frame builders have a lot of options today. You just have to be careful that whatever fastener you are using you understand how many you need and how far apart they need to be spaced.” 

“Ideally, you want a 12-16 inch truss heel at the connection so you have sufficient area to space out your fasteners,” says Steve. “Again, the whole goal of the connection is to resist the rotation of the truss members as the truss is resisting loads like gravity, wind, snow, etc.”

From his experience, David prefers bolts. “Bolts have the advantage of going all the way through the truss-to-post connection, so they provide increased resistance to shear and also suck the plies together to create greater friction. The larger diameter of the bolts also increases their resistance to corrosion. In addition, with bolts you need a lower total number of fasteners for the connection, so you can space them farther apart, providing greater bending moment resistance.”

The Bottom Line

Proper long-span truss installation in a post-frame building presents several challenging elements. While minimizing lateral bending during handling and adequately bracing trusses during installation are very important to ensure trusses aren’t damaged, compromising their ability to perform over the life of the building, proper truss-to-post connections are the most vital aspect of truss installation. While there are many options when it comes to column configurations and fasteners, the key is ensuring the truss bottom chords are level, are in full contact with the column material, and are fastened using an approach that maximizes the stiffness of the connection to resist truss member rotation. 

Calculating Stairs Rise and Run

What is Wrong With this Picture?
Stairs, they seem to confound and befuddle just about everyone. In my early years as Sales Manager at Coeur d’Alene Truss, I used to volunteer to go measure houses up to confirm plan dimensions would match up with what was actually being built. Usually yes, but on occasion – not.

One of this area’s best framing crews used to call me to measure trusses just so I could tell them how to cut stairs. Even though they had framed hundreds of houses, math skills to determine stairs were outside of their toolbox. For this reason every third-party engineer sealed set of Hansen Pole Buildings’ structural building plans including stairs has tread rise and run spelled out.

Back on task to this photo.
2018 IBC (International Building Code):
“2304.12.1 Locations requiring waterborne preservatives or naturally durable wood.
Wood used above ground in the locations specified in Sections 2304.12.1.1 through 2304.12.1.5, 2304.12.3 and 2304.12.5 shall be naturally durable wood or preservative-treated wood using waterborne preservatives, in accordance with AWPA U1 for above-ground use.”

This means those portions of stairs in contact with this concrete slab-on-grade must be pressure preservative treated.

Stair stringers per 2018 IBC Table 1607.1 must be designed to support a minimum 40 psf (pounds per square foot) uniformly distributed live load for stairs and exits of one- and two-family dwellings and 100 psf for all other uses.
Let us assume a minimal rise from top of slab to top of next floor of nine feet. With a maximum rise of 7-3/4 inches per tread and minimum run of 10 inches, this would require 13 treads with a horizontal distance of 130 inches (10.83 feet).

Here is how to calculate what it takes to carry residential stair loads:

Moment = (40 psf live load + 10 psf dead load) X 36 inches wide X 10.83 feet^2 / 8 = 26,390 inch-pounds

Learn about Bending Moments here: https://www.hansenpolebuildings.com/2012/09/bending-moment/
With this given rise and run, remaining portion of 2×12 after cutouts is 5-1/8 inch. Using Fb (Fiberstress in bending) for SYP 2×12 #2 of 750 we will solve to determine if what is present is adequate structurally:
26,390 inch-pounds / (750 X 2 X 6.566) = 2.68 where it must be 1.00 or less to adequately carry this load.

But, you might ask, where did these other two variables appear from? Two (2) is because we have two stair stringers to carry loads. 6.566 would be Sm (Section Modulus) of remaining portion of 2x12s after cuts are made to accept treads.
So our photo has stringers 268% overstressed – not good.
This can be resolved by adding another 2×12 stringer at center of stairs and nailing a 2×6 alongside each 2×12 stair stringer.

IBC 1015.2: Guards shall be located along open sided walking surfaces that are located more than 30” measured vertically to the floor or grade below at any point.
This means a guardrail must be on each side of these stairs.

IBC 1015.4: Required guards shall not have openings that allow passage of a sphere 4” in diameter.

Easiest solution here would have been to have vertical supports for hand railing spaced with a maximum distance between of four inches.

A “toe plate” should be incorporated into these stairs at the rear of each tread to fill space between treads and meet with four inch maximum space requirement.

Granted, this requires some math but given the variables, just plug them in and away you go!