wood design

[PEin2010]

This is in reference to problem #15 from Hiner Seismic Review. There is a horizontal roof diaphragm made out wood structural panels (WSP) with 15/32" rated sheathing. I have never taken a wood design class, so this may be a very dumb question. But I can't figure out how they said it was CASE 3 for IBC 2305.2.3?????

I don't know how to tell by looking at the diagram whether it is blocked or unblocked? also is the spacing of the nails given? Is it that when there is roof wooden truss supporting the horizontal diphragm like in this case, you don't require blocking? what is blocking in this context anyway?

Thank you in advance

 

[EnvEngineer]

Blocking is placed between 2x4 or other frame members for form a solid frame around the sheeting. Looking down at the top of the frame you would seen the ends of the members (that you nailed into) and the blocks in between. As you may guess blocking will help reinforce the frame and add strength, therefore a blocked frame of the same sheeting, framing member and nailing is rated stronger. Please note the unblocked frames are not allowed in California due to the seismic areas we have.

Here is a link that gives a view of a blocked frame Blocked Frame Link

There are two main differences in the Case number, is the frame blocked and is the direction of the expected force normal or parallel to the lenght of the sheeting.