GRK analyzed as lag screw vs. wood screw

[skeletron]

In CSA O86, there is a distinction between Wood Screws and Lag Screws. The yield modes are more or less the same, however the safety factor is 0.6 for lag screws vs. wood screws. More importantly, Wood Screw provisions account for a higher embedment strength in the point side member (rope effect), while the Lag Screw provisions make a distinction based on the direction of the load vs. grain.

Wood screws larger than #12 are to be analyzed as lag screws because of the shank/thread dimension limits. When reviewing the literature for GRK screws, it seems that they give capacities that are all over the map.

1. The comparison table (PG. G26) shows a value of 366lbs or 466lbs for all diameters
2. The tables on PG G28-G30 show various values for different embedment, but note that Wood Screw provisions have been used
3. Tables on PG G48 seem to suggest an NDS approach, and offer different capacities for parallel and perpendicular loading.
4. When reviewing a similar type of self-drilling screw (ASSY), their documentation makes it clear that Lag Screw provisions are used. And this is reflected in their tables. However, from the naked eye the ASSY screw is quite similar to the GRK screw. Both of which appear to be more refined than a lag screw or wood screw alternative.

How are other designers handling these types of fasteners? Is it sufficient to go off the manufacturer's data, even though the provisions may not be as specified by code? Is it sufficient to use ESR reports and apply a judgement/FOS to the results to get capacities within the threshold between NDS, CSA Wood Screws, and CSA Lag Screws (or sim)? What is the capacity of a GRK?

 

[Craig_H]

I personally use the wood screw provisions, as outlined in their catalog. I went through your exact process of humming and hawing how to handle them, and ended up defaulting to the manufacturer's guidance. My justification is that these do not fall squarely into either category (wood screw or lag), and are therefore similar to other proprietary fasteners (such as hangers, truss plates, etc). While they do not have a CCMC report (wish they did), they reference their catalog values on testing to an ESR Report, which seems like the next best thing.[/sup]

 

[skeletron]

As a follow-up, I emailed ITW to acquire lag screw values and to get their feedback. Here is the synopsis of their response:
1. The unique design of the GRK-RSS with the T-star drive is not covered by ASME B18
2. It is interpreted that the IA Cross Recessed Oval Countersunk Head wood screw in the standard is the closest representation of the RSS.
3. Certain features of the screw are not considered as they are not comparable.
4. GRK has been tested, approved, and published ICC-ES report
5. ASME B18 shows only values on shank diameter, not on the strength of unique design of screws

The rep provided another report sheet that has the lag screw values. Probably a good resource. I agree that using the catalogue table values is adequate as it looks like they have been aligned with some test data.