Wood Connections for Rehab Project

[PittEng88]

Hi Everyone,

I am in the process of rehabbing a historical stone masonry building with timber framing on the inside. The walls will be repointed and the existing timber framing is to be removed and replaced. During the design process it was noted that some of the older connections for the framing would not work under the current code. So, they had to be redesigned. The roof system for this building consists of 2x8 rafters spanning approximately 25' and are supported by a 8x8 purlin at their mid-span. At the ridge, the rafters are nailed together and strapping is provided. At the eave, 2x10 Joists are used to restrain the thrust forces from the rafters. Angled 4x8 Struts every 10.5' provide support for the purlins. The vertical reaction from the strut is then transferred down into a column and to the foundation (no problem there). However, the thrust force from the strut is a different story. I ended up using double 2x10 joists (to match up with the 4x8 strut) sandwiched between two 1/4" steel plates. I welded another 1/4" plate to the back of them so that the forces can be transferred from the strut through end bearing and not through the bolts. I then called out for one 1" dia. thru-bolt to be used, to allow for rotation.

At the eave I need to provide a 1' overhang, so a welded plate at the end of the rafter will not work. Instead, I used two 1/4" steel plates bolted to the members with one 1" dia. thru-bolt in each member, again to allow for rotation.

I have provided a few sketches. The first is a section cut of the building, incase my description above was a little confusing. The second sketch is of the strut to joist connection at Detail A, and the third sketch is of the joist to rafter connection at Detail B.

I was just wondering, if you guys would be willing to give these details a quick look? Just to ensure that I did not miss/overlook anything. If you guys would like the numbers, I can provide them as well. It has been a little bit since I have designed in wood, so any comments or concerns will be greatly appreciated.

Thanks for your time,
Mike

 

[SlideRuleEra]

What catches my attention in the detail is one bolt per member. With wood, pay special attention to the connections, the limiting factor will probably be the strength of the wood, not the strength of the bolt.

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[SlideRuleEra]

...additional comments:

Detail A-A shows double 2 x 10s neatly matching the width of a 4 x 8.
Double 2 x 10s have a combined thickness of 3"
A 4 x 8 is 3 1/2" thick. Spacers will be needed to match difference in thickness.

The connector plates with one bolt in each member will be unstable when uplift occurs on the roof. The bolted members will tend to rotate around the single bolt.

You mentioned "allowing for rotation". Is that the reason for the 1/2" clearance between members at the connections? You may want to revisit allowing a gap, I don't believe it is a good idea. Even the tightest connections when combined with the relative flexibility of wood take care most movement.

Also, I question the need (and desirability) of slotted holes in the tie-down plates.

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[KootK]

My initial thoughts, all in relation to the heel joint:

1) I believe that, with one bolt per member, you're actually pretty close to unstable under normal, gravity loading. You would require a moment connection between the bracket and the wall which would be dodgy in my opinion.

2) I don't like the single bolt arrangement from a redundancy perspective either, particularly at an important heel connection. Suppose you get some mid-depth splitting in your tension chord. That one bolt might slide right through it.

3) I'm very surprised that the single bolt works in your compression chord. I would have thought that perpendicular to grain issues in the connection would kill that.

4) Because your truss reaction will be to the right of the top/bottom chord node, the saddle may tend to rotate CCW until the 1/2" gap is closed. After that has occurred, you'll get substantial local bending moments in the chords.



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.

 

[Brad805]

While I am sure the numbers justify a problem, how is the roof performing? You did not mention that you noticed significant sagging of the various wood members and your details do not seem to suggest there is much of an issue with that. Based on the section I would surmise the bldg could be in the 100year old category and it likely has timbers and thick plaster ceiling. Since it never fell over, I assume the existing members are all nailed together in some fashion and the gaps you are showing in your details are something you wish the contractor to implement. I too am suspicious of your connections and I fear that you might be doing more harm than good with the proposed details. From a construct ability standpoint, I think both details are difficult.

I am not sure where you are, but in Canada one is not always required to upgrade to the current code. Most codes allow quite a bit of flexibility with regard to upgrades when it comes to historical structures. If the structure does not have a sprinkler system I would do that before implementing the proposed repair details unless you have more reasons to share.

 

[PittEng88]

Thanks for the replies everyone, all very good points. I forgot to mention that all the timber in this project is to be rough cut nominal lumber. So the dimensions do add up. The original timber framing for this building is completely shot. The timbers are severely dried out and contain heavy splitting in numerous locations. Awhile back, this building was flooded and the interior footings were washed out causing them to fail and pretty much pull the entire timber frame with it. Now the floors and the roof are sagging significantly. The town wants to save the exterior of the building (stone walls) and convert it to a community center. So due to the change in occupancy, I need to bring it up to current code.

SlideRuleEra: I was trying to follow some of the detailing examples from the AITC Timber Construction Standards and some of the examples shown in "Design of Wood Structures ASD/LRDFD" by D. Breyer. So between the two references the 1/2" gap did come from allowing for rotation. However, the more I think about it, the more I agree with you that the flexibility of the wood will take care of this. Also, from a constructability stand point, I can see how the 1/2" gap would be a nightmare to obtain.

Kootk:

1) I forgot to callout the lags on this detail. So, instability really shouldn't be a problem here; unless I'm missing something else.

2) I too am not a fan of single bolts for a connection as well, but as I mentioned above, I was trying to follow some of the examples in Breyer's book and a lot of them show single bolt connections. The justification for this was that fixed connections in wood are not ideal due to the potential for splitting, and the single bolts allow the members to rotate. Also, I figured that as long as I provided the minimum end distances to the bolt, splitting would not be an issue.

3) I was a little surprised too, but the numbers do workout. However I know that doesn't automatically mean that the connection will work.

4) This is the part that really didn't sit right with me. I had feeling that is what will happen, so I completely agree with you here.

For the strut to bottom chord member (Detail A): if I were to remove the 1/2" gap and the slotted holes in the tie down plate, and substitute the 1" thru-bolt in the bottom chord for multiple smaller diameter bolts, would this detail be ok?

For the heel connection (Detail B): Again if I remove the gap and substitute the 1" thru-bolt in the bottom chord for multiple smaller diameter bolts, and then shift the bolt in the rafter so that it is over the wall (possibly add multiple bolts here as well), would this detail be ok?

Again, thanks for everyone's help!

 

[Brad805]

These are great projects. Any chance you could share a pic or two of the existing connections? I do not quite follow the existing connections. I figured out we were dealing with rough lumber.

I understand the need to upgrade.

 

[PittEng88]

The first two pics show how severely bowed the roof is. The third one shows how the struts and purlin frame together. I really don't have a clear picture of the connections in question, because they just have these struts sitting right on top of the columns w/o anything to take up that thrust force. The only thing I can think of is that they assumed the floor beams sitting on top of the columns, which are 10x12, and the columns themselves, which are 10x10, to be stiff enough to resist this force. However, with forces I am coming up with I do not feel comfortable transferring that thrust force to the beam and column.

 

[KootK]

1) I forgot to callout the lags on this detail. So, instability really shouldn't be a problem here; unless I'm missing something else.

Lags doesn't really change anything for me here. The hardware can still rotate more or less freely about the bolts.

Also, I figured that as long as I provided the minimum end distances to the bolt, splitting would not be an issue.

I was less concerned with splitting due to the bolt shearing through the lumber and more thinking just of the typical splitting that you tend to get at the ends of timber members in general as a result of longitudinal shear.

Your proposed improvements for both details sound good to me.

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.

 

[PittEng88]

Lags doesn't really change anything for me here. The hardware can still rotate more or less freely about the bolts.

I understand what you meant now, and I agree with you.

I was less concerned with splitting due to the bolt shearing...Your proposed improvements for both details sound good to me.

Thanks for the help, once I get these details updated I will upload them to show you what I got.

Also, in Breyer's book he talks about avoiding rigid truss connections. So, I was wondering for the heel connection if adding multiple bolts to both members would turn this into a rigid connection? Or is this ok due to the flexibility of the wood?

 

[Sawbux]

The major problem in connections in timber is splitting, either from the cross-grain prying forces in the connector or shrinkage in the wood. For this reason it is preferable to have many smaller connectors rather than one large one. Connections with many nails or screws would be more dependable.

One thought is to add collar ties, if that is not objectionable due to historic reasons. Another solution might be to add tie rods between the wall plates, hiding them in the floor joist spaces.

 

[Brad805]

That is pretty poor framing. I had much higher hopes given the effort in the stonework. I am not convinced the roof looked much better before the flood, and the value of the remaining roof has little historic value since it appears to have been re-clad. I would be inclined to replace the roof.