Zee purlin fixity

[JStructsteel]

Anyone have advice on Zee roof purlin fixity. I see usually they are designed as a fixed connection. I have read 10-20% lap is adequate. 20% would be 5'-0" which seems reasonable but also not.
I was thinking 2'-0" each side. 2 bolts each end, then clip at the beam w/ 2 or 4 more bolts.

Thoughts?

 

[human909]

It is my understanding that the lap is there to achieve fixity but it is also to provide double the steel in the section of highest moment. Aka the hogging moment around the support.

So yes you might be able to achieve a good degree of fixity but overlapping by 8% but you'll not get the benefits of the extra steel in all areas.

Cold formed steel engineering is highly complex. If the manufacturers have guidance about capacities etc but require 15% overlap then I suggest you follow that.

If you don't follow the manufacturer's recommendations then you are out on your own. Make your own astute judgements, but do recognise that this aspect of steel engineering is not an easy path to walk.

 

[Tomfh]

Follow the span tables and lap length tables and details. They do it all for you.

 

[bones206]

Check out this design guide and look through the examples at the end.

https://mbma.com/sites/default/files/documents/MBMA_Roof_Framing_Design_Guide_for_Metal_Building_Systems_2024%20Edition%20V2_1.pdf

 

[JStructsteel]

It is my understanding that the lap is there to achieve fixity but it is also to provide double the steel in the section of highest moment. Aka the hogging moment around the support.

So yes you might be able to achieve a good degree of fixity but overlapping by 8% but you'll not get the benefits of the extra steel in all areas.

Cold formed steel engineering is highly complex. If the manufacturers have guidance about capacities etc but require 15% overlap then I suggest you follow that.

If you don't follow the manufacturer's recommendations then you are out on your own. Make your own astute judgements, but do recognise that this aspect of steel engineering is not an easy path to walk.

Thanks. I am on my own. This is not a metal building, but using Zee purlins. Im not worried about the double section, just getting good fixity.

 

[bones206]

I think the lap location will be iterative. Check the combined stresses at the critical section at the end of the lap. If the checks don’t pass, extend the lap length until the stresses at that critical section pass.

 

[JStructsteel]

I think the lap location will be iterative. Check the combined stresses at the critical section at the end of the lap. If the checks don’t pass, extend the lap length until the stresses at that critical section pass.

I havent reduced the size to account for this. The section alone (if it was just spanning over the support) is adequate. I just wanted the fixity to get that section to work. Deflection is controlling beyond that.

 

[bones206]

According to Metal Buildings System Design and Specifications by Alex Newman, the minimum lap length required to assume purlin continuity over the support is 2'-0", with minimum 1'-0" on each side of the support. '

I think your proposed 4'-0" lap would be adequate.

 

[Tomfh]

From the lysaght book:

 

[sdz]

Anyone have advice on Zee roof purlin fixity. I see usually they are designed as a fixed connection. I have read 10-20% lap is adequate. 20% would be 5'-0" which seems reasonable but also not.
I was thinking 2'-0" each side. 2 bolts each end, then clip at the beam w/ 2 or 4 more bolts.

Thoughts?

Lapping gives continuity, not fixity, at least in my understanding of the words.
You get continuity and additional capacity over the support which is treated as a pin support for design.

 

[JStructsteel]

Lapping gives continuity, not fixity, at least in my understanding of the words.
You get continuity and additional capacity over the support which is treated as a pin support for design.

I think I get that, not looking for fixed support, but a continuous beam via overlap.