Norby_acn
Structural
- Jun 26, 2019
- 13
This is a question born out of curiosity more than anything and peripherally related to some research I was doing. I would also like to preface this by saying I am ignorant of the full scope of European design codes so forgive me if I am missing something obvious.
In the design of glulam beams as covered by the NDS, there is a shear reduction factor (Cvr) of 0.72. It applies only to glulam members (not to sawn lumber) and applies specifically to the shear design of members which are non-prismatic, notched, or at connections. This makes sense to me because it is a way to account for stress concentrations at the edges of notches. However, in section 3.4 of the NDS where it discusses the shear design of members at notches, it gives an additional reduction in capacity based on a ratio of the notch depth versus the un-notched (some variation of [dn/d]^2).
In effect, we doubly punish glulam members in shear design if they are being notched. I can rationalize this maybe by the thought of perhaps the extra reduction being to account for failure between laminations or something... However, I have recently been doing some research on the Eurocode and noticed that they provide a more detailed and less conservative shear reduction than our NDS code (kv in section 6.5.2 in EN 1995-1-1: Eurocode 5 for those interested). I also do not seem to find another similar shear reduction value listed which would correspond to the NDS Cvr factor.
I guess my ultimate questions are... Why do we apply a Cvr value to glulam (and not to other wood products) and where does that value come from? Is the NDS/American code overly conservative when compared to the European code here or am I missing something? Why does the NDS not provide a more detailed shear reduction calculation?
I am aware of my lack of knowledge when it comes to the European code, but after going through some of the research papers (particularly by Thomas Tannert) and code material, it seems like the US is lagging behind in some of these wood design areas and I'm trying to understand the gap between the 2.
In the design of glulam beams as covered by the NDS, there is a shear reduction factor (Cvr) of 0.72. It applies only to glulam members (not to sawn lumber) and applies specifically to the shear design of members which are non-prismatic, notched, or at connections. This makes sense to me because it is a way to account for stress concentrations at the edges of notches. However, in section 3.4 of the NDS where it discusses the shear design of members at notches, it gives an additional reduction in capacity based on a ratio of the notch depth versus the un-notched (some variation of [dn/d]^2).
In effect, we doubly punish glulam members in shear design if they are being notched. I can rationalize this maybe by the thought of perhaps the extra reduction being to account for failure between laminations or something... However, I have recently been doing some research on the Eurocode and noticed that they provide a more detailed and less conservative shear reduction than our NDS code (kv in section 6.5.2 in EN 1995-1-1: Eurocode 5 for those interested). I also do not seem to find another similar shear reduction value listed which would correspond to the NDS Cvr factor.
I guess my ultimate questions are... Why do we apply a Cvr value to glulam (and not to other wood products) and where does that value come from? Is the NDS/American code overly conservative when compared to the European code here or am I missing something? Why does the NDS not provide a more detailed shear reduction calculation?
I am aware of my lack of knowledge when it comes to the European code, but after going through some of the research papers (particularly by Thomas Tannert) and code material, it seems like the US is lagging behind in some of these wood design areas and I'm trying to understand the gap between the 2.
