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Who is Responsible for Design of Permanent Truss Bracing?

This article was triggered by an email questioning truss bracing from Hansen Pole Buildings’ client JASON in WELLINGTON who writes:
“My inspector is telling me that the truss documents take precedence over the building plans. I told them the building plan has the x bracing and the t-bracing. He didn’t care. He wants all the shown bracing from the truss documents. I am not sure what to do. I think the inspector is being ridiculous.”

In my humble opinion this inspector has an absolutely incorrect opinion. Included in Hansen Pole Buildings’ Construction Manual are engineer sealed letters from two significantly large truss plate manufacturing firms, clarifying who has responsibility for design of Permanent Truss Bracing. These companies typically supply engineer sealed drawings for metal plate connected wood trusses (MPCWT) manufactured by purchasers of their truss plates. Copies of these letters should be provided to this inspector.
Inspector can also be given this link (to ANSI/TPI 1-2014): https://static1.squarespace.com/static/53442b51e4b072e71999c8c5/t/56d9d1038259b560ad3a0821/1457115397817/ANSI_TPI+1-2014StdONLY-WEB_WP.pdf

Included in ANSI/TPI 1-2014 (incorporated by title in Building Codes) are:

In Section 2.2 DEFINITIONS

Building Designer: Owner of the Building or the Person that contacts with the Owner for the design of the Building Structural System and/or who is responsible for the preparation of the Construction Documents. When mandated by the Legal Requirements, the Building Designer shall be a Registered Design Professional.”

Permanent Building Stability Bracing: Lateral force resisting system for the Building that resists forces from gravity, wind, seismic and/or other loads.

Permanent Individual Truss Member Restraint: Restraint that is used to prevent local buckling of an individual Truss chord or Web member due to the axial forces in the individual Truss member.

Registered Design Professional: Architect or engineer, who is licensed to practice their respective design profession as defined by the Legal Requirements of the Jurisdiction in which the Building is to be constructed.”

And be pointed to Section 2.3.2 Requirements of the Building Designer

Section 2.3.2.3 Review Submittal Packages:The Building Designer shall review the Truss Submittal Package for compatibility with the Building design. All such submittals shall include a notation indicating that they have been reviewed and whether or not they have been found to be in general conformance with the design of the Building.

(Author’s note – General Note 9, Sheet S-0 of Registered Design Professional sealed plans provided to client, specifically addresses Section 2.3.2.3 above.)

Section 2.3.2.4(c) All anchorage design and connections to the Structural Elements and the Permanent Building Stability Bracing required to resist uplift, gravity, and lateral loads.

Section 2.3.3 Requirements for the Permanent Member Restraint/Bracing of Truss Systems.

Section 2.3.3.1 Method of Restraint. The method of Permanent Individual Truss Member Restraint/Bracing and the method of anchoring or restraining to prevent lateral movement of all Truss members acting together as a system shall be accomplished by:

Section 2.3.3.1.3 Project Specific Design. A project specific Truss member permanent Lateral Restraint/bracing design for the roof or floor Framing Structural System shall be permitted to be specified by the Building Designer or any Registered Design Professional.

Building Officials and inspectors have a veritable mountain of materials referenced by Building Codes. Tremendous volume of these references becomes more than any one person (or small group of persons) can possibly know completely contents of all. It would be unrealistic to expect otherwise.

This entry was posted in Building Department, Pole Barn Structure, Trusses, Professional Engineer and tagged T-Bracing, truss bracing, engineered plans, X bracing, permanent bracing on November 23, 2018 by admin.

Can I Remove Bottom Chord Bracing?

Truss Bracing In The Way?

Thirty three years ago, when I first dipped a toe into the post frame industry, roof truss bracing was pretty much an afterthought. Even though I came to pole buildings from the prefabricated metal plated truss business, bracing (especially of bottom chords) was pretty much a subject left up to the person doing the building.

Back then, it was not unusual to see 40 foot and wider spans, with absolutely no bracing at all between pairs of trusses spaced every 10 to 14 feet.

Apply a bit of a load to those trusses, from either wind or snow, and the bottom chords begin to form an “S” curve, as they are buckling in the weak direction.

Solution to the S? Add bracing.

Modern engineered prefabricated roof trusses do take into account and recommend bracing. Oddly, the drawings will specify what appears to be a single 2x lateral brace, on truss spacings greater than 10 feet. Why I say “odd” is because a single 2x brace, of any size, which spans over ten feet, will buckle in the weak direction just like the trusses did.

Roof truss bottom chord bracing, properly designed and installed will not only keep the trusses in plane (standing upright), but will also brace the endwall columns. By the use of strategically located X bracing, loads from the endwalls and truss bottom chords can be transferred into the roof diaphragm.

A “load path” must be created from one end of the building, to the opposite end.

For example: a 60 foot long building, with five equal sidewall column spacings (or bays) of 12 feet each. Bays one and five are braced with X’s from the end truss bottom chord to the top chord of the second pair of trusses; and from the end truss top chord to bottom chord of second pair of trusses. Common sense says this is a fairly rigid bracing system as both directions take the loads into the plane of the roof.

In bays two, three and four, lateral bottom chord bracing is applied in the plane of the bottom chords. This bracing is anchored to the relatively “stiff” first and fifth bays.

Let’s complicate matters….

Imagine bay #3 has a large door, through which building owner desires to drive a vehicle in, place on a car lift and raise it so it is between the pairs of trusses. Wonderful, until the vehicle hits the truss bracing.

Oops….

Solution: remove the lateral bottom chord bracing in bay #3 and change the bracing in Bays #2 and 4 to X bracing. Every truss is still braced in both directions, and the load between the second and third pairs of trusses is transferred by the roof diaphragm.

If the building is designed by a RDP (registered design professional – architect or engineer), it is essential this modification be ideally made in the initial bracing design, so as not to incur additional engineering fees for a change.

Obviously this solution will not work to remove lateral bottom chord bracing in adjacent bays, but it does afford some added flexibility in design solutions.

This entry was posted in Trusses and tagged T-Bracing, bottom chord bracing design, lateral bracing, X bracing on October 3, 2013 by admin.

Engineered Buildings Part II: Continuous Lateral Restraint Systems

As I said yesterday, a properly engineered building is a fully engineered building. Either it is engineered, or it is not. I have been appalled to hear what clients feel are reputable companies tell me they sell buildings at a much lesser price if the client does not require sealed plans. My question to them was pointed. “You design them the same, though, right?” Their answer was the one which left me with a sick feeling in my gut, “well….” they hesitated, clearly not liking being put on the spot, “they are engineered to be stout.” Pushing the issue I innocently mimicked, “stout?” Again their hesitancy said “guilty as charged” all over their answer, “Yes….well…they are built to what our engineers feel are robust.” Excuse me but….”stout”? “Robust?” This is where all I can think of to say to clients is, “Buyer Beware”!

To continue from yesterday…

Many wide width buildings have seriously under-designed interior columns, especially those using columns which are nail-laminated (several 2x plies nailed together). Other major deficiencies include no accounting for additional loads induced by drifting snow and improper truss web bracing. With respect to the latter, roof trusses utilizing continuous lateral restraint systems to brace longer compressive web members, may be improperly installed and often fail to include diagonal bracing to prevent bracing shifting. This can result in web buckling and subsequent truss failure.

From my view, the more major concern is not improperly installed continuous lateral restraint systems in these buildings, but using them to begin with on the web portion of the truss. In my opinion buildings with trusses over 2 foot on-center should have T- or L-bracing to all long compression webs. Use by builders of continuous lateral restraint systems (rather than L- or T-bracing) results from truss designs produced with software developed for residential buildings. Using L- and/or T-bracing saves lumber and provides greater stability, braces are easier to install (they can be attached on the ground) and do not cause progressive collapses.

With a continuous lateral restraint system, when one truss fails, the lateral restraint attached to that truss pulls on similarly buckled truss webs located on each side of the failed truss. The truss on one side of the failed truss is helped by this action and does not fail (as its bowed compressive web is somewhat straightened out).

Meanwhile the truss on the other side of the failed truss becomes more compromised as its buckled web is pulled further out of alignment. This almost always snaps the web of this truss, resulting in its collapse. The second truss collapse brings down the next truss in a similar fashion. Like dominoes, trusses continue to fail until there are no more trusses to pull down. This entire failure process explains why this failure type is characterized by a partial roof collapse ending at a wall.

It is quite apparent, to me, a vast majority of building purchasers are under the impression they have purchased a properly engineered building, when in fact they have not. In some cases, these clients are intentionally misled which is highly unethical if not criminal.

Frequently trusses are quoted with a “balanced design snow load” which was used as an input to a truss design program by a local lumber yard employee. Given this number, a builder or building owner assumes they are getting a fully engineered building. This could not be further from the truth. Trusses so designed seldom account for all loads to which trusses are subjected, nor do they account for the exact manner in which trusses will be connected to other components, receive loads from other components, and/or be braced by other components. Furthermore, a truss is only one element in an extensive building system and each of these elements must be properly engineered with special attention given to unique interactions between elements.

Back in the day, when I ran my own truss plant besides having a pole building construction business, we quoted trusses without consideration for unbalanced snow load – meaning snow drifting. The Building Codes in use at the time did not address it as an issue and the available computer design programs just didn’t have the capabilities.
So if you as a building designer or truss supplier are trying to “cheap out” when folks don’t know the difference, failure to follow building codes designed to save lives (human and otherwise) is at the least shoddy design work and could be criminally negligent. Wider span trusses in snow country will be more costly, however cutting corners at the risk of property, animals or human lives, is just not worth the risk.