Tag Archives: gussets

Site Built Roof Trusses

Site Built Roof Trusses

In penning my recent article about sexy prefab roof trusses, recalled from recesses of what little mind I have yet, was a story I will share with you about site built roof trusses.

I was not long into my position as truss plant manager for Lucas Plywood and Lumber when a house framer approached me in regards to trusses for a pole barn he had just sold. Whilst then I was relatively (if not completely) post frame (pole barn) building ignorant – I did know lots about trusses.

Now I am relatively sure this builder wanted me to design these 40 foot clearspan trusses for him. I can only assume he thought he was going to save himself a bunch of money by assembling trusses himself at the jobsite.

Only then 22 years old, I didn’t know much, but did know helping with a design was not a place I was going to get myself deep into. I did end up selling this builder a bunch of 20 and 22 foot long 2×12 #2 Green Douglas-Fir, some 2×4’s and plywood for gussets. I made him provide me with a list as I wanted no part of any liability fiasco.

As far as I know, it all worked.

How this builder expected to come out ahead still baffles me. Truss companies (hopefully) make a profit, otherwise why be in business? When I was a newcomer to truss sales, we used to double lumber costs to come up with a sell price. Toothed steel connector plates ran about 7% of sell price and assembly labor 10%. Left us with a gross profit margin of roughly 1/3rd.

We also had benefits of engineer sealed truss designs, often using higher graded lumber than what a builder would normally be able to buy from his or her local lumber supplier. Our labor costs were reduced by having cutting capabilities of several hundred boards per hour, being trimmed to length and proper angles pretty much as quickly as boards could be fed into saws. Jigs, once set up, provided a means for every truss to be identical in profile.

Have a builder considering constructing trusses for your new post frame building at your site? Unless you are after something quite custom, being built from engineer sealed plans – just say no, as somewhere in this your builder will be cutting corners.

Prefab Wood Trusses are Sexy

Prefab Wood Roof Trusses Are Sexy Though

In 1952, in Pompano Beach, Florida, an inventor named Carroll Sanford had been experimenting with building prefabricated roof trusses using plywood gusset plates and varying concoctions and combinations of glue, staples, nails and screws. Eventually he conceived of light gauge steel plates with punched teeth to connect wooden members.

If this wasn’t a sexy use of technology, then I don’t know what would be.

A burgeoning pole barn (post frame) building industry was largely aided by this new ability to economically clearspan relatively large distances.

What Hansen Buildings does now and since 2002:

FEATURE: Prefabricated face-to-face doubled roof trusses.

BENEFIT: Provides an engineered solution with clearspan widths of 80 feet and (in some instances) more. Endwall trusses make for quick and easy installation, while maintaining roof slopes.

True double trusses provide increased reliability due to their load sharing capabilities: https://www.hansenpolebuildings.com/2018/09/true-double-trusses/.

EXTENDED READING ABOUT THIS SUBJECT:

My all-time most read article: https://www.hansenpolebuildings.com/2011/06/pole-barn-truss-spacing/

Why most people should not order trusses: https://www.hansenpolebuildings.com/2018/10/why-most-people-should-not-order-trusses/

WHAT OTHERS DO: Another feature with a myriad of possible outcomes. I will defer “how” trusses are attached to columns for a later article.

Theories of roof truss spacing become most generally divided up geographically. These geographic nuances do bleed over from one area to next, so are not cast in stone.

Eastern U.S. places single trusses upon two or four foot centers attached to tops of truss “carriers” – headers spanning from sidewall column to sidewall column. Here are a few words about truss carriers: https://www.hansenpolebuildings.com/2018/10/what-size-truss-carriers/.

Midwest most often opts for a single truss aligned with sidewall columns. Spacing might be as little as 7’6” and as great as 10’.

Going West – expect a single truss each side of sidewall columns with paddle blocks to attach roof purlins. Learn about paddle blocks here: https://www.hansenpolebuildings.com/2012/05/paddle-blocks/.

While Eastern and Midwest post frame buildings generally feature trusses at each end. As one heads west, dimensional lumber rafters are often seen – relying upon building erectors to achieve proper alignment with interior trusses.

WHAT WE DID IN 1980: Back in green lumber land – Lucas Plywood & Lumber fabricated trusses out of green lumber. As spans (and dimensions of top and bottom chords) increased they became phenomenally heavy.For building ends, 2×12 #3 rafters were provided.

 

Damaged Truss Repair Part I

Metal plate connected (MPC) wood trusses are found in nearly all types of pole building construction as well as both residential and small commercial projects. While the growth of the structural building component industry has been excellent over the years, it is not without some growing pains. As the use of trusses has increased, so has truss damage.

Trusses are generally considered two-dimensional structural elements. They are typically manufactured from dimensional lumber (2×4, 2×6, etc.) and joined with steel plates, with integral stamped teeth. While quite strong in the plane of the truss (taking vertical loads while standing upright), they are susceptible to damage if allowed to flex out of plane. This so called “out-of-plane” bending most often occurs during unloading or erection, and can result in breaks and splits in the lumber as well as connector plate damage ranging from tooth pull-out to steel failure.

Damaged truss repair should only be done upon an engineer’s recommendations. They may range from the simple addition of dimensional lumber to more involved reinforcements utilizing gussets, beams, prefabricated truss sections, and even steel plates and angles. Unfortunately, there are no code provisions and very limited text book examples to guide engineers at the design stage.

The entire repair process relies heavily on the individual engineer’s ability to apply theory, real world judgment, engineering common sense and knowledge of field conditions to develop a practical, economical, and realistic repair for each given situation. Most engineers who design truss repairs learn the skills from another engineer with past experience. While the theory of connections is documented, the art of damaged truss repair remains an elusive concept which can only be developed over years of practice.

The repair design process varies with the complexity of the damage. Typically, for damaged trusses, the original analysis is a valid starting point for determining the forces in the damaged area of the truss. Almost all MPC wood trusses are designed on proprietary software from the connector plate manufacturer. This software is dedicated exclusively to truss design, and contains all of the lumber and connector plate properties required to design MPC wood trusses. Any new material added should be available locally or provisions should be made through the truss manufacturer to ship less common materials to the job site.

Once the forces have been determined, the focus turns to connections. There are a variety of ways to reconnect members in wood trusses. The two most common are dimensional lumber “scabs”(nailing a piece of 2x lumber alongside one or both sides of the truss) and gussets of plywood or Oriented Strand Board (O.S.B.). In certain instances, plated truss sections are manufactured to solve problems where gussets and lumber will not suffice due to strength or geometric considerations.

The new truss sections are attached to the face of the existing truss and connected in the appropriate aligning members. If available, a portable hydraulic press may be used to install new members within the plane of the existing truss with connector plates. Repaired trusses utilizing a portable press are often indistinguishable from a new truss directly from the manufacturer. This can be very advantageous in instances where appearance is a concern.

Fastener selection can often prove to be a challenging exercise, due to the wide variety of connectors available. As I said earlier, I have no specific “recommendations” for how to repair trusses. This is truly something best left to the experts for safety in structural design – the engineers. Tomorrow I’ll dig into some of the most common connectors used by engineers for damaged truss repair.