Blocking at end of wood trusses 3

[NFExp]

I'm looking for comments/information on blocking required at end of wood trusses. We generally design our diaphragms as unblocked however we specify 2x blocking at the ends of trusses with a heel height greater than 4". For trusses with a heel height less than 4" we do not require blocking. We justify using the blocking to prevent truss rotation and to help distribute shear.

If diaphragm shear is transfered by truss connectors, is it possible to have a truss heel height of 6" without blocking. If so how do I justify this?

Thx

 

[ronstruc]

IMO you can't. A diaphragm boundary needs to exist to transfer diaphragm shear into a lateral resisting element even with an unblocked diaphragm. You need to have some way of getting the shear out of the diaphragm at the boundary. I don't believe that trusses will transfer the diaphragm shear forces. How does the diaphragm shear get into the trusses at the boundary? I don't believe you have a complete load path without blocking between the trusses and connected to the roof sheathing.

 

[msquared48]

If you but the ends of the trusses against a rim joist and the diaphragm is nailed to the rim, then you can laterally brace the truss ends with connectors nailed to the rim.

Other than that specific circumstance, I would have to agree with ronstruc.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

While I agree with all the reasons above - I have often seen it not installed - esp with overhangs.

 

[woodman88]

If you are using the IBC or IRC a minimum 2x boundary member is required per the code for horizontal diaphragms. See the IBC Table 2306.2.1(1) or IRC Table R602.3(1). Metal Plate Connected Wood Trusses will not transfer any loads perpendicular to the trusses.

Garth Dreger PE
AZ Phoenix area

 

[DaveAtkins]

I don't require blocking for trusses with small heels (but keep in mind I am in an area where seismic loads are not an issue). The truss heels can transfer a small amount of lateral load by acting like a portal frame--the truss heels are the columns, and the roof sheathing is the continuous beam.

DaveAtkins

 

[mmillerpe]

According to the 2006 Edition of the BCSI, the only recommend transferring up to 50 plf lateral load between the diaphragn and shear wall through the truss heel. The following limitations apply to the recommendation:

Max spacing of 24" oc.
Max heel height of 6".

 

[NFExp]

Thanks mmillerpe. I just found it in the BCSI and it looks like it will provide the justification I was looking for.

 

[woodman88]

You should note that the 2009 IBC Section 2303.4.4 states that the "registered design professional" is responible for the design for transfer of loads to the supporting structure. Also that the BCSI is not a referenced standard to the code. The TPI 1-2007 (National Design Standards for Metal-plate-connected Wood Truss Construction) is a referenxced standard. I do not have a copy of it and do not know if or how the BCSI applies in it. I would in any case require the Truss Engineer to add a note about the truss(es) being able to transfer the 50 plf (100# per truss) shear load perpendicular to the truss(es) at the heels on the truss designs or in a letter for the trusses.

Garth Dreger PE
AZ Phoenix area

 

[FFENGINEER]

Do you need to leave space for air flow, i.e. not block to the bottom of the sheathing?

 

[NFExp]

FFengineer

A particular contractor (our client) has been complaining about installing the blocking. Before I tell him that we absolutely needed the blocking, I wanted to investigate further.

 

[woodman88]

NFExp
If you can't get the truss design(s) to state that a 50 plf shear load at the heel can be transfer to the wall, you can also state that the blocking is only needed in every other, or every third, or etc. bay as need for the shear force.

Garth Dreger PE
AZ Phoenix area

 

[archeng59]

I always show 2x blocking between trusses at the bearing wall or beam regardless of truss heel height. I do not like to rely on the truss not rolling over. Depending on the diaphragm shear load being transferred into the shear wall, the blocking typically does not have to be installed between each truss. For most cases, blocking between every other truss is appropriate to transfer the diaphragm shear into the shear wall or LFRS (lateral force resisting system). Not installing between each truss allows for the airflow mentioned earlier. When contractor's complain (which is rare), it's typically because they have never installed the blocking on previous projects even when it was specified. I tell them if they want to omit it and be responsible for future failures, they can omit the blocking. Always gets installed after that but with grumbling.

 

[msquared48]

"they have never installed the blocking on previous projects even when it was specified."

Sorry, but I don't put up with that crap. They can pay me now or pay me later their choice, but the blocking WILL be installed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[NFExp]

I have to say that we don't put up with contractors complaints but we do try to provide value engineering. I personally try to listen to the contractors and if I can justify making the change I will do so. If I can't justify making the change I let them know.

This was a perfect case. Our standard was to always specify blocking at the ends of trusses. This particular project was a small, 1-story, box shaped structure with little lateral load. If I can justify reducing the blocking, the contractor will be pleased and maybe we will get some repeat business. If I can't reduce the blocking I'll explain why and the worst case is that the contractor doesn't believe me and goes somewhere else next time.

 

[msquared48]

When they don't do it and not tell me, leaving me to discover it later when the Building Inspector questions the lack of blocking, talk to the hand. I have no sympathy in that case and they CAN pay the piper.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

In providing "value engineering" to contractors you can always add a requirement that they get the owners permission before using it.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

You lost me Woodman...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

msquared48, Contractors want to "value engineer" to save themselves money. But, they bid the job per the contract documents which have the details they want to change. So, any money that is saved by "value engineering" should go back to the owner as this is a change from what was bid and not to the contractor.

Garth Dreger PE
AZ Phoenix area

 

[hokie66]

I think Garth's comment was in response to NFExp's post above. I don't see how a bit of blocking relates to value engineering, but as Garth say, if there is a benefit, it should go to the owner. Apparently NFExp's client is the contractor.