split in timber truss 1

[charliealphabravo]

I have encountered a split in one of the trusses I posted about previously.

http://www.eng-tips.com/viewthread.cfm?qid=322881

I'd like to get some feedback from the forum on the significance of the condition and thoughts about a repair. I have attached a photograph of the split.

On the negative side the split occurs at a split ring connection, is rather wider and deeper that I would expect for a check, and it occurs where the tension load is highest: about 6 Kips total in the (2)3x6 diagonal webs coming up to the connection from the right. I am also concerned that there are no secondary drains on the roof and that the loads could be potentially higher.

On the positive side, the split looks rather old (subjective opinion), only one of the two 3x12 top chords is split, and the bolt passes through the 3x8 compression web which is also not split. According to my preliminary calcs the split rings are good for about 5K each and per the plan there are 6 split rings total at the connection.

I am considering several solutions for a potential repair if I determine that the connection is compromised. The simplest and most direct involves adding a thru-bolt or SDS screws in the tension web above the check. My concern here is that I would be adding some further rotational restraint to the connection.

Thanks all.

 

[SlideRuleEra]

I agree with you to add a thur-bolt. Also, you could consider scabbing 1/2" or 3/4" plywood plates (cut to fit neatly) to both sides of the split member. Use appropriate glue and nails to connect the plates to the member. Use small nails (say 6d or 8d common) just to keep the plates in place until the glue sets. From your photo, this truss is in very good condition - don't want to do anything drastic.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[cap4000]

I inspected an 80 year old 70 foot span bowstring truss with the bottom chord split at the rings. What I designed was thick steel U-clamps with 3/4 inch bolts and thick plates directly over the splits. It was a temporary fix as the insurance company was going to cancel the building owners policy.

 

[BAretired]

It appears that a mistake was made in fabricating the truss. There are two connections in the top chord, one for each diagonal. The tension diagonal is connected near the bottom of the chord. The compression diagonal is connected near the top. It should have been the other way around. You are lucky that you don't have a split at the other connector.

BA

 

[BAretired]

On second thought, the lower connection is probably connecting both diagonals, but the tension diagonal really needs another connection near the top of the top chord.

BA

 

[msquared48]

Looka to me like there are two sets of diagonals, one a single member with two bolts in double shear, and the other two members with one bolt in single shear to the doagonals.

To me this means that the single member diagonal connection is twice the capacity of the two member diagonal connection. Kinda of an uneven shear distribution through the web members and uneven forces on the bolts causing splitting in the top chord...?... possibly?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[charliealphabravo]

You can see the original plan and my preliminary T/C pattern in the prior thread that I referenced

...and yes, the compression load at the single web is about 12 kips, more than twice the load at the double tension webs.

You kinda hafta make an educated guess where the split rings are. The plan is a little ambiguous.

 

[woodman88]

The attached links, for the NAVFAC MO 111.1, Inspection of Wood Beams and Trusses (free download) at

Http://www.everyspec.com/USN/NAVY-General/NAVFAC_MO-111x1_SEP1985_16804/

talks about splits in wood trusses (see section 3.1.2).
Also the Wood Handbook (another free download) from

Http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

(see page 8-24, Cross Bolts)
Both articles seems to be concerned with end splits only, not with your condition of mid-member check/split. But they may be of interest to you.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BAretired]

The single member is in compression and the double members are in tension.

Seems like another bolt through both tension members near the top of the chord would help a bit, but not as much as split ring connectors.

BA

 

[hokie66]

Don't think I would be adding any more bolts, but agree with cap4000. Clamp the member vertically, one to the left, one to the right of the connection.

 

[charliealphabravo]

I have discovered a missing split ring and am concerned that there may be others, particularly where the splitting is occurring. I am seeing the same splitting on several other trusses.
Does anyone see any issues with using a flat probe or long fine wood drill bit to feel up or down between the plies for the split rings? The trusses shouldn't see significant load again until the winter.

Thanks again all.

 

[msquared48]

I don't.

There are concrete rebar detectors there that give a visual representation of the rebar and might work here to see if the ring is present without any of the drilling or probing though.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

See this string:

Portable Concrete and Rebar Test Equipment

thread507-325906

Called the Hilti Ferroscan... Haven't used it yet, but might consider it.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[charliealphabravo]

Sorry to bump this again guys but I have a related question. The chord timbers are specified on the plans as 3x12. My measurements of depth were consistently 11" rather than the 11-1/4" I would expect for dry dressed nominal dimensional lumber. Anyone know offhand what the reason might be and if I should consider it to be significant in terms of the original intended design capacity? Again these trusses where constructed in Alberta around 1950.

Thanks again all.

 

[woodman88]

It depends on what the moisture content of the lumber was at the time you measured it. Lets say it is DF-L(N) surfaced green (20%mc or higher) at 11.5". As it is 11" now, you have a 100(1-(11/11.5))= 4.4% change in tangential dimension. Per the Wood Handbook DF-L(N) can have a 6.9% shrinkage (see table 4-3 on page 4-6) from green to ovendry mc tangential. You can get the Wood Handbook at the attached link.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[hokie66]

Yes, I think that is just cross grain shrinkage.

Back to the missing split ring(s). I would want to take one of the side webs off to examine the installation. Pick one near centre span where the web force is small if all joints are meant to have split rings.

How did you discover the missing split ring? Was there that much of a gap? If so, cross grain shrinkage would be a reason for that as well. You discovered one missing, but did you find others that are there?

 

[charliealphabravo]

As BAr noted, the lower bolt in the connection is very close to the bottom of the chord. In fact in one place it is so close that you can see the ring extending from below the chord...only one ring! So one must be missing if there are 6 total at the connection...two for the tension webs and 4 for the compression web. I'll attach a pic. Again as BAr noted, the tension web also ties into the compression web so it is not as bad as it looks.

The shrinkage of 3X (2-1/2") members is about 1/8" so in some places the ring between the plies can be seen.