Wood Framing: Roof trusses and non-bearing walls

...The more things change, the more they stay the same....

So i got inspired to inquire about details and framing conditions relating to the use of pre-fab roof trusses that span across or parallel to "non-bearing" walls below.

The state of play, is that our STD details indicate an STC clip that allows for something like  1/4" to 1/2" of deflection.

We also have a note section the typically calls out deflections that are in excess of the code minimum for deflections for the roof trusses.
Taken together, these two elements work fairly well when one considers that the clips have about a 3/4" max delfection capacity, and the live load deflections for typical truss in the 30ft range yield about the same deflection.

The implication of the above is that the non-bearing walls should be constructed low by the deflection amount to allow the truss to deflect under loaded conditions. That is NOT the case in the field. Our experience points to the idea that ALL walls are constructed to the same height at the top-plate.

So how are we to handle this situation - where trusses are routinely being installed with "intermediate bearing points" that are not part of the truss calc, and could potentially damage the bottom chord or "overload" walls below that are not designed for these bearing conditions.

The truth appears to be that we are (almost) powerless to change the construction industry in this area without attracting significant criticism and "push-back" from contractors and owners alike - the gain would appear to be fairly minimal... trusses may deflect more freely - but then there may be other problems cropping up relating to gypboard stress/bending and other corollary cosmetic issues...

So it appears that our approach then moves to one of accommodation, where we appreciate the truss deflection issue; note an adequate solution on our plans, but then "overlook" the issue  and implementation in the field when we observe... as do others in the inspection community...

So, amazingly, if our truss conditions are in the "normal" (say up to about 30ft) range, we use our typical details as noted above, and our plans should pass muster as relates to the theory.

As mentioned, ...the changes... stay the same...

Does anyone have a comeback on this...?

 

Steel Shape Availability

For those who are interested, here a convenient site to find out whether or not it is likely a specified steel shape will be available to the contractor: http://www.aisc.org/steel/searchForm.aspx?id=2044

Modular Construction is Here... and we're missing it...?

See this article in the DJC.

Looks like a 3 story modular project went in on Capitol Hill.

DCI is the engineer... Seems like we should be the ones getting in on this ground floor situation... Do we have any "corporate knowledge" of this facet of MFR...? Would be nice to have this on our resume... Lets talk about this.

 

Concrete Specifications - ACI 318 Ch4 - Durability

The 2008 ACI had the ch4 revised for durability requirements. Anne had mentioned this a while back.

It has a potentially large impact on the specifications for concrete mixes.

We will be updating the structural notes for the concrete section to take this into account.

With some exceptions, the IBC 1904 section points us to ACI ch4, and essentially prescribes some water/cement ratios that may easily overide design considerations for the concrete mix. These prescriptions are related to:

• Freeze/thaw
• Sulphate Exposure
• Permeability requirements
• Corrosion protection of rebar

Has anyone already incorporated these code requirements into their "structural notes" yet... (and not updated the company standards?)...

Please weigh in as you feel inspired...

2012 IBC - Seismic Design - Help?!

I was reminded this morning about upcoming changes to the code for a host of different issues - not least of which is seismic. A geotech engineer was asking me how we are approaching the changes... i was not sure. Hence this note!

Has anyone started to dial into this topic yet. We will need to get up to speed fairly shortly. Also, i predict that we will see a "rash" of work trying to complete their projects under the IBC 2009 (first blush appears that 2012 will more onerous for seismic) prior to the expiration of the "grace period" which normally last 3-4 months after the adoption date. According to this post (see above) - the final date for "switch-over" happens on October 13 - a Sunday.

Nels reminded me that we had a little "introduction" to some of the changes to the 2012 IBC with a SK Ghosh webinar back in January.

In reviewing some notes we took, there are a couple of key words that could appear to some to be "alarming". These could include the words:

• Risk Target (MCEr not equal to MCE)
• Updated source model (Nehrp 2008/9 i think)
• Changes in statisical basis for design (was 2% probability of EXCEEDANCE in 50yr -> now going to a target risk of COLLAPSE of 1% in 50yr)

Can we please start a conversation on this topic.

Headed Hex-Head bolts or J/L bolts - thats the question?

Arbitrarily a discussion came about on this issue.

I was surprised by it, and did not particularly think that there were any significant and "strong" thoughts out there regarding this issue.

I was wrong.

The brief poll i took in the office showed that some folks are using the hex head anchor bolts in place/lieu of J or L bolts at all anchor bolt locations (including shearwalls). The thought offered was related to a reduction of J/L bolts per Appendix D.

App D DOES mention J/L bolts, but only in the context of PULLOUT (as far as i can tell).

The basic thought there is that the pullout of the J/L bolt is related only to the projected area of the "hook" portion, and this "area" is multiplied (arbitrarily) by a 0.9 factor. In contrast the hex head bolt pullout is based on a net "brg" area multiplied by a 8.0 factor. This is apparently based on testing results.

Fair enough.

The main issue here is that the J/L bolts are being spec'd in non-pullout uses such as in sole plate applications to shearwalls - NOT for holdowns - which are themselves again subject to the "nuanced" intricacies of the "infamous" Appendix D.

Contractors appear to not "give a hoot" about which type of bolt is used for the "non-PT" cases. Of course they would still like to "wet-stick" things - but that aside - there appears to be no premium one way or another on the anchor bolt issue. They indicate that post install is almost always prefereable using drop in or titen or shot type applications where applicable.

Based on this, we should probably decide how to handle this in our office so we can try to be consistent. It may well be "reasonble thing" to go to the hex-head anchor bolt wholesale. Can we have a discussion on this? Take the poll at least...

Additional Reinforcement at Holdown Embedments (eg PT slab etc.)

I talked with some folks about this yesterday. This is a quick Blog to summarize what i found. I'm not sure that we have "uniformity" on the issue - but we'd like to get there!

So we are stuck with dealing with Appendix "D" concerning embed items including holdowns. It used to be a few years ago, that we would install a HD, size a plate for "idealized" shear cone, and be done with things... Now its a little different...

In App "D". the are various failure modes - one of which is described in D5.2: Concrete breakout of anchor in tension. At the end of this section, section D5.2.9 indicates that "... where anchor reinforcement is developed in accordance with Chapter 12..." and goes on to say that you can design "break-out cone" to be "restrained" with rebar per chapter 12. Chapter 12 deals with development and lap splices of rebar.

You'll recall that Rick Fine was with us a while back and talked about a studrail solution and a "rebar chair" solution to the HD question. Studrails can be expensive, have lead times, and can be unwieldy to consider - especially at edge conditions. (incidentally - the stud rail option would be making use of the provisions of Ch11 for shear - not exactly what App "D" had in mind)





Rick also looked at the "rebar chair" option. The problem with this is that the chair legs need to be developed per Ch12. This can lead to some complications - one of which was Rick suggesting to use 14" thk decks (apparently mostly on account of chair dimensions... This does not  seem like the right approach to me).





We have recently used closed stirrups with longitudinal rebar as shown here. It seems to me that this approach will work, but there are some difficulties in placing the "large" configuration where tendons are concentrated, and where there may be obstacles that "interfere" with the spacing of stirrups etc. - thereby compromising the whole idea... One idea to improve on this concept may be to have the stirrups be two part elements, with a closure or "hat" bar on top to facilitate installation and other entities that may encroach on the "arrangement" . I like this config at edge conditions though, and so may end up with various embedment "reinforcement" details depending on location and size of holdown force.



Steve has used the "bent bar" approach shown here which i really like: it is simple, easy to install and should work well for our purposes to restrain the "concrete cone". It uses the provisions on Ch11 for shear friction reinforcement, which i think is an appropriate approach when applied to HD conditions we see all the time. This solution is well suited to typical slab conditions "in the field" og the slab... at slab edges, we would still need to scratch our heads.  For slab edges where there are walls below, we can use the "lap length" approach as noted below.






 








At Cantilevered edge conditions, we could use a detail similar to this one. We should probably try to limit the holdown sizes on cantilevers, and try to terminate them in walls below if necessary.



 






I feel this is something we should try to standardize on, so that we have a uniform approach to holdown reinforcement across projects and across clients. If you have thoughts on this issue please weigh in! Also weigh in the Ch11 vs Ch12 thing too...