Purpose of both the uplift hanger and strap at ridge connection

[Said the Sky]

In the attached picture - is the strap at the ridge meant for unbalanced loading? what is the reason to spec both hangers (that has uplift capacity) and a strap? I always thought it was either have a "collar tie" within 1/3 distance from the ridge or have a strap or special connectors that have uplift resistance. I think the hangers and the sheathing nailed on top of the rafters does what the strap is doing. looking forward to your guy's thoughts on the matter.

 

[RontheRedneck]

Conspiracy theorists might say that its just a product of Simpson's marketing.

The hangers don't have a lot of "bite" since the OSB web of an I-joist is so thin. There might be some rationale for the straps due to wind uplift.

I don't see too many I-joist roofs due to all the hardware required.

 

[LuK13]

Disclaimer: I'm not too familiar with the hanger and have never done an I-joist roof.

I was taught that the purpose of the collar tie or strap was to keep the rafters from separating at the ridge. To me separating has an upward component and an outward component. For those hangers the outward component of that separating force would put the hanger nails that are in the ridge into withdrawal, and you don't want to count on a nail in withdrawal. The strap is in tension and the strap nails are in shear if the rafters try to separate.

That's my thoughts, I'll hang around to see if someone knows better as I'm sure I'll encounter this construction at some point.

 

[DaveAtkins]

The reason for the strap is to resist tension when the roof system is experiencing uplift due to wind.

For downward (gravity) loads such as dead and snow, the two rafters go into compression, and the ceiling tie or collar tie goes into tension (like a truss).

For upward (wind) loads, the opposite occurs. The ceiling tie or collar tie goes into compression, and the rafters go into tension. A tension tie is needed at the ridge to prevent the two rafters pulling away from the ridge beam.

DaveAtkins

 

[XR250]

@DaveAtkins

I-joist roofs don't typically use collar ties - they are mainly vaulted ceilings.
I agree with Ron Redneck on this one.

 

[JStructsteel]

Since its 4:1 to 12:1, I think its for the lateral tension load on the joist. Perhaps the hanger does not have that capacity.

 

[Said the Sky]

thanks JSstruct, I think this is the case possibly. I will resolve my wind load into its components and check the nails in withdrawal capacity I think nails have about 4 newtons per mm of embedment for withdrawal capacity for SPF wood, which gives me about 200lbs withdrawal capacity of three - 3" long nails, I only need 80lbs per rafter since I'm in a low wind zone.

I've found something on a google search regarding collar ties that might be of interest.