Truss Profile 3

[JStructsteel]

Architect has asked me to show a truss profile for the truss manufacturer. I am not sure why I would do this, unless they want a specific look? Isnt the truss manufacturer best at picking the profile to optimize the economy of a truss?

What do you guys show for a profile? Span, slope, and overhang length, and some diagonals would be called out as 'for show only' truss manufacturer to provide layout?

 

[Settingsun]

"Refer Architect's drawings"

 

[JStructsteel]

They dont show anything, as this is just a hip roof attic space. I could care less how the truss looks as long as right span, pitch and over hang. I have that on my plan and sections.

I will see what exactly they are wanting me to show. I would think truss guys would ignore what I show unless its a specific dimesion wanted between diagonals.

 

[Settingsun]

I was being tongue in cheek. I don't know your relationship to the architect, but do consider whether what they want should be on their drawings instead of yours before agreeing to change yours.

 

[JStructsteel]

I guess my issue is why are we dictating anything about the truss layout if a specific 'look' isnt needed.

 

[phamENG]

Sometimes it is good to show truss profiles. This doesn't mean you should show all of the web members, but there are certain key pieces of information the truss manufacturer needs:

1) Out to out of the bearing walls/beams, location of intermediate bearing points (if any), width of bearing points
2) Heel heights
3) Roof slope
4) Locations for attic rooms, duct chases, mechanical platforms, etc.
5) Overhangs

I've drawn truss profiles that just show a triangle with these 4 pieces of information.

For common trusses, it's become more common practice to leave this information sprinkled around the drawing set for the truss manufacturer to hunt for. A lot of older drawings I've seen have dedicated truss profile sheets. I think it's a good practice since it minimizes the risk of problems in shop drawings (assuming you have a well coordinated set).

That said, I usually like to leave this piece to the architect since most of the items are architectural in nature.

 

[gte447f]

Do not do this. It's unnecessary, unless you have something unique going on with the design/layout. A competent truss designer only needs the span, pitch, and overhang, as you say (and of course the design loads). If the truss designer is asking for more, that is reason to be concerned about their competence.

 

[JStructsteel]

PhamENG, I agree if I need something specific with the trusses. Otherwise yes, I show span and bearing, overhang length and slope in my plans and sections.

Perhaps its an architect thing to just show stuff on their drawings to think they have control of everything, lol

 

[gte447f]

I do agree with phamENG that it is good to show a "generic" (i.e., does not dictate the web member profiles) truss profile for special situations like an attic room or loft within the truss. Otherwise, all the basic stuff like roof slope and bearing wall locations can be shown on the plans.

 

[KootK]

1) Speaking as a former truss designer, I favor phamENG's approach.

2) Yeah, the truss designer will eventually figure all of this stuff out for herself if she has to but she'll likely waste a lot of time doing it. And perhaps a lot of the design team's time too. Within the truss industry the delineation of responsibility is thought to be:

a) The design team defines the roof and ceiling planes into which the trusses fit. This nicely mirrors the fancy layout software that the truss folks use that has them model the "trussed volume" in 3D before the individual trusses go in.

b) The truss designer defines the profiles of the individual trusses based on their final locations and -- obviously -- the webbing configuration if the design team has to no weird preferences in that regard.

This makes sense based on who controls what in the process.

3) In my experience, most architects are not capable of fully specifying all of the geometric information that a truss designer needs. They're good for OH, pitch, and heel height if they're on the ball but that's about it. They usually aren't capable of thinking through the framing arrangement in the detailed manner that a good structural engineer can/will.

4) Generating the kind of profiles that phamENG describes takes very little time. And, in my opinion, it should probably be a nearly automatic outgrowth of the engineer thinking through the details of the roof system in the first place. In my experience, too many engineers treat pitched roofs as no different than flat roof systems when they lay out their framing. This can lead to all manner of poor layout choices.

5) In more complex, pitched roofs, there can be a number of possible layouts. If the engineer doesn't care what the outcome is then, so be it, the truss designer can do whatever suits them. If the engineer doesn't care what the layout is, however, then it strikes me as only fair that the engineer should provide enough direction to the truss designer such that the truss designer isn't burdened with a bunch of rework trying to figure out what the EOR wants.

6) To an extent, doing the truss profiles is simply a way to encourage coordination and decision making between the architect and the EOR. In many instances, overhang length and heel heights have structural implications for things like uplift and the best scheme for transferring diaphragm shear down through the heel height and into the shear walls.

 

[BAretired]

It has been more than fifteen years since I dealt with truss fabricators, but there was one issue which came up over and over again. On a complicated roof shape, long web members required bracing, sometimes one brace but occasionally two. The brace was shown as a small rectangle abutting the web on the truss elevation. Each truss was different and often, the web members did not align from one truss to the next. Each truss design was treated like a separate project.

The truss fabricator made it clear by notes on every truss elevation, that bracing members were not included in their package; it was the responsibility of others. Site inspection revealed that bracing was not installed in all locations. Sometimes, the only way it could be installed was with a "tee" brace, which cost the general contractor more than he had included in his bid.

In many cases, aligning web members could easily have been done, but it seemed to me that no thought was given to the issue at the time. I believe that each truss was designed to reduce cost to the fabricator, even if it cost others more.

BA

 

[phamENG]

I believe that each truss was designed to reduce cost to the fabricator, even if it cost others more.

Ladies and gentlemen, the modus operandi of the modern construction industry.

 

[XR250]

I never show truss profiles unless there is something really special going on. The Architect usually handles it or it is simply left to the truss designer to figger it out. That is why they make the big bucks!

 

[KootK]

In all but the simplest of cases, I believe it quite impractical for the truss designer to align web locations between dissimilar trusses. It's not really even feasible with the stepping hip trusses of a common hip roof.

Bracing is definitely a problem though. I'd very much like to just say:

TRUSS WEBS REQUIRING BRACING

1) At common runs of four trusses or greater, the truss supplier shall design and supply web lateral bracing stabilized by X-bracing terminating at the roof diaphragm."

2) At all other conditions, the truss supplier shall design and supply web bracing in the form of strong back reinforcement.

3) Strongback reinforcing may be used to brace the webs of any truss at the supplier's discretion.

This is usually what needs to happen anyhow and I think that it's a bit nuts that the EOR should be expected to comb through the truss shop drawings and spec the bracing system on the fly at that late stage in construction. The result of that system is just lots of braces getting missed.

EOR's tend to delegate design and coordination wherever they can in order to produce "minimum credible drawings" and maximize their own profit margins. This is little different from what the truss suppliers are doing when they punt on the bracing. When it comes to business, we all reduce to rational actor's in the Adam Smith sense of the term. If change is desired, then change will have to be incentivized.

 

[RontheRedneck]

To BAretired -

Keep in mind that truss companies are like so many other businesses - Lowest bid gets the job.

Or as I like to say: "People are willing to pay for quality - As long as it doesn't cost any more".

I like sending trusses out that don't require a lot of bracing. But if the customers aren't willing to pay for it, then I price myself right out of a job.

So you're trying to place blame at the wrong place in the chain.

 

[XR250]

I agree with KootK about the web bracing. However, I can't remember the last time I have witnessed a problem with a truss due to deficient bracing - even considering how shitty most bracing is installed.
I usually only see straight runs with no diagonals running to the either diaphragm.

 

[RontheRedneck]

XR250 said: " I can't remember the last time I have witnessed a problem with a truss due to deficient bracing..."

I have seen whole buildings come down.

Every instance I know of it was long webs that required 2 rows of CLBs (Continuous lateral braces) on one or more webs. In some cases the CLBs were not installed. In others there were 2 rows of CLBs, but they were not anchored to anything. So the webs all bowed the same direction.

It it thankfully not common. But it does happen.

 

[phamENG]

I saw a large church sanctuary almost come down. About 200 feet of trusses, all braced with a pair of 2x4's that were anchored into the end wall with two 8d nails. Those nails pulled out and 60% of the trusses were bowed a good 8" out of plane. Best part was that I was 40feet off the ground and Tarzan-ing my way through the trusses when I figured out what was going on.

 

[XR250]

Wow! Makes sense due to shear size of the building and long clear spans. My work is typically residential so things are smaller and there are less runs of similar trusses due to things being cut-up. Also, I believe having "non-bearing" interior walls helps considerably in masking bracing deficiencies as they will pick up the load before the webs can buckle too far.

 

[jayrod12]

I'll chime in on deficient truss bracing examples. There's been at least two I've seen personally in the last few years. One was a giant garage for farm equipment, 60 foot truss spans, 200 foot long building. Put all the trusses up one day with minimal bracing, came back the next day to a pile of matchsticks on the ground.

Second one was a 4 unit townhouse, this time instead of being on the ground, they were all laying over on top of the walls.

To be fair, I would say I haven't definitively seen a failure due to lack of truss bracing that wasn't during construction.

Our general notes however indicate that all truss members shall be sized to eliminate individual member bracing to avoid those types of issues. We still show the permanent bracing required for the trusses as a whole unit as per the TPIC document. Temporary construction bracing is the contractor's responsibility where I practice.