The seismic retrofit guideline FEMA P-1100 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings contains a detail (Detail 1, sheet D3) that recommends joist blocking not found in the 8 other seismic retrofit guidelines. One might speculate that research teams who analyzed damage after past earthquakes never saw the damage that this detail would prevent. This webpage looks at the reasons this detail is included in this guideline and why using it might be a good idea just to be sure. Previous earthquakes are no predictor of what kind of damage will occur in future earthquakes and it is better safe than sorry.
These blocks cannot be found in the following 8 retrofit guidelines: The California Existing Building Code, Standard Plan A, the Los Angeles Department of Building and Safety Plan Set 1, the Simpson Strong-Tie Plan Set, the City of Seattle Guidelines, the City of Portland Guidelines, or the Simpson Strong-Tie Homeowner’s Guide. Nor is it found in the building code for new construction.
Now we will be looking at each one of these guidelines to see how they solve the engineering challenge this detail tries to address.
The speculation behind this detail is that the floor joists may roll over as shown in this detail. Some people would claim that these joists can roll only to the extent that the cripple walls on the perpendicular walls collapse, and this blocking may not be necessary.
Detail 1, Sheet D2 as shown in the retrofit guideline Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings. The focus of this detail is the block described as (N) 2x blocking which is supposed to prevent rolling over of the joists.
In the California Existing Building Code, which has had several revisions in the past 30+ years, this joist blocking is not part of the standard.
Standard Plan A has a similar detail without joist blocking.
Los Angeles Department of Building and Safety Standard Plan Number 1 Has a Similar Detail.
The City of Portland Retrofit Guidelines Has a Similar Detail. As In Every Other Guideline, The Joist Blocking Is Missing.
The Simpson Strong-Tie Plan Set Does Not Include This Blocking.
Simpson Strong-Tie’s Seismic Retrofit Guide for Raised Foundations Does Not Include Joist Blocking.
City of Seattle Retrofit Guidelines Do Not Include This Joist Blocking.
As far as I can tell, such blocking is not required in new construction- though correct me if I am wrong.
This is all the information I have on this connection, and I hope you have found it helpful.
