Howe Truss Connections

[PSUengineer1]

I observed a Howe truss configuration on a manufactured home (wood-framed trusses with metal press plates). Truss span=26 feet, 2x4 members. House built in 1979. The web members are connected with metal press plates; however the plates have consistently disconnected just above the first node along the top chord. There is a gap of approximately 1 3/4 inches between the vertical edges of top chord members near this node (see attached pics). Was this a standard way of connecting the truss top chord members in 1979 (with gap between the members)? I opine that this roof has been caving in since it was built due to this construction. Recent heavy snowfall was the last straw that caused the roof to cave in considerably.
Thanks for any comments you can provide on the attached photos.

 

[mike20793]

It looks like it's the top chord splice plates that are the issue and the plates connecting the web members to the top chord are intact and in good shape. Some of those look to be disconnected due to out of plane forces, but there aren't any obvious signs of distress around the plates, so I don't think it has happened after the fact. Could those be hinge connectors at splice locations? I've never seen them before, but surely someone else here has.

 

[KootK]

Hinge plates: Link. Their use is pretty rare in my experience. Usually to deal will some manner of shipping dimension limitation. The modern versions of these connectors have published capacities. The design was obviously insufficient the structure that you're reviewing. Most often, hinge plates are installed as part of the "overbuild". You know, piggy backs and odd ball features. Structural elements but barely. It seems pretty sketchy to have one as part of your hard working, primary truss compression chord.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

On the plus side, you should be able to:

1) Easily justify some healthy fee to the owner as their lives are probably in danger and;
2) Come up with an oddly easy truss fix. Just some scabs either side of the joint.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

What are the disadvantages to using those comnectors? I end up getting piggy backed trusses all the time but that looks like a nice alternative for cap trusses.

 

[dhengr]

PSUengineer1:
That splice plate is an intentional hinge plate. For height reasons, during transit, the manuf’er. was able to hinge the top half of the top chord down on the web members. Then, the top chord was hinged up when the roof was being completed. The 1.75" gap looks like a larger gap than would normally be needed, I would want a larger end distance on each of the nailing plate parts at the first row of bent nail points. And of course, the entire hinged joint does have to be designed for whatever loads might exist in the top chord near that first node. Finally, it does not look like there has been much movement axially in the top chord (down its length). But, with the sheathing joint right at the hinge point, there has been a shearing movement in the plane of the roof sheathing right at the joint in the sheathing. I don’t recall exactly what the roof sheathing joint location was, as I’ve seen that hinge detail in the past, but I would not want the sheathing joint right at that hinge gap. I would want solid sheathing over that hinge joint gap to support the top chord laterally. That lateral movement seems to be the primary failure mode of the top chord and the hinge plates. Maybe the top chords could be moved back into line (means moving the sheathing w.r.t. its joint at the hinge), the hinge plates straightened as best as can be, and then that sheathing joint be blocked so that it can transmit the shearing forces. Maybe a 2x8 or 2x6 right under the sheathing, and btwn. the trusses, for shear nailing to the sheathing; and then a second 2x8 or 2x6 under the first block to add lateral stability to the hinged gap and top chord members.

 

[KootK]

What are the disadvantages to using those comnectors?

Speaking as a former wood truss guy, my issue is the interplay between joint stability and quality control. As the designer, you'll get spiffy values to work with as shown below (Mitek Website). I'm sure that tension and shear are fine. Compression capacity, however, will have everything to do with the size of the gap and the degree of initial imperfection present at the time of fabrication. I've no doubt that the published compression capacity comes with a particular limit on the size of the gap. However:

1) Does the designer, often not an engineer, know the limits on the gap and why that's important? Or is it in fine print some place?
2) Do the non-english speaking folks putting the truss together know the limits of the gap?
3) Does the truss installer whole may be doing some of his own truss assembly know or care about the limits of the gap?
4) As a former shop laborer, I can also tell you that the hinge plate is a convenient place to bury any tolerance issues that you may be having.
5) If you don't have your truss roller/press machine calibrated just right, you'll end up shipping these things out all bent up to start.

Like I said, for overbuild situations where your not really dealing with true top chord compression forces, I think the hinge plates are a spiffy idea. For compression members that are part of real "truss", I'm pretty skeptical in the absence of uncommon MPCWT quality control.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.