Sloped Wide Flange Stiffener

[skylite]

2 pcs of 20 feet Wide Flange w8x21 will be connected at a slight sloped at middle without any central beam ridge or support. They will make up rafters supporting light roofing shed . The connections at the middle is shown in the above image. Can anyone recommend what kind of horizontal stiffener to make it stable? It will be located in a seismic zone so the connections need to be competent. Or for the span and sizes.. won't you recommend it at all?

 

[Guastavino]

Is mitering it with full-pen welds an option?

And W8x21 spanning 40 feet? Any interior supports? Red flag in my mind.

 

[skylite]

Is mitering it with full-pen welds an option?

And W8x21 spanning 40 feet? Any interior supports? Red flag in my mind.

It is sloped.. so it's not like connecting them straight which can give bad deflection. The original plan was to have middle support, but we are studying now if it is possible with no support. The load is only light roofing cover and not steel deck or floor or loading is not a problem. But I'd like to know if spanning the W8x21 40 feet at a slope would be stable if the middle connection would be competent.

 

[KootK]

It is sloped.. so it's not like connecting them straight which can give bad deflection.

Unless there's a tension tie connecting the low ends of the rafters, sloping the beams will not improve deflection.

But I'd like to know if spanning the W8x21 40 feet at a slope would be stable if the middle connection would be competent.

As shown, what your connection detail lacks is a positive connection between the rafter top flanges. If the loads can be accommodated, my preference would be for a bolted end plate ridge connection.

For the rafters to be stable, they will need to be laterally restrained by some manner of roof diaphragm, either shear rated decking or discrete, in-plane bracing.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[skylite]

As shown, what your connection detail lacks is a positive connection between the rafter top flanges. If the loads can be accommodated, my preference would be for a bolted end plate ridge connection.

For the rafters to be stable, they will need to be laterally restrained by some manner of roof diaphragm, either shear rated decking or discrete, in-plane bracing.

There is no roof diaphragm or shear rated decking or discrete, in-plane bracing. But we can easily accommodate middle column support beneath it. If we do that.. what becomes of the requirements for each rafter to be laterally restrained?

 

[KootK]

With the central post, the ridge connection becomes inconsequential but the rafters still require lateral restraint. In the absence of a diaphragm of some sort, the rafters would need to laterally restrain themselves via weak axis bending. If you post a framing plan and section, we'll be in a better position to make recommendations.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

You can splice two pieces of steel to make one beam. I would do it with either bolted end plates as KootK suggested, or welding both sections with butt welds to a vertical plate which will properly distribute the forces around the sloped connection.

But an 8" beam on a 40' span? Too skinny. You insist that the loads are light, but what about wind? Maintenance? I wouldn't want to walk on this roof, too much like a trampoline.

 

[wilberz]

With the central post, the ridge connection becomes inconsequential but the rafters still require lateral restraint. In the absence of a diaphragm of some sort, the rafters would need to laterally restrain themselves via weak axis bending. If you post a framing plan and section, we'll be in a better position to make recommendations.

the x is the column.. the vertical are purlins on top of the rafters with roof sheet connected to the purlins. May I know what is your meaning of "laterally restrain themselves via weak axis bending"? The column themselves support the rafter so the rafter is stable.

x---------x---------x
| | | |
| | | |
| | | |
x---------x---------x

 

[KootK]

May I know what is your meaning of "laterally restrain themselves via weak axis bending"?

I'll do my best. Imagine that an earthquake occurs and each of your rafters needs to be prevented from shifting laterally (left to right in your sketch). That movement needs to be limited. Some options for doing that include:

1) Connect rafters to a shear rated deck diaphragm.
2) Connect rafters to a discrete, in-plane bracing "diaphragm".
3) Utilize the weak axis flexural stiffness of the rafters themselves.

All methods assume that the column tops themselves are restrained from lateral movement somehow, perhaps by bracing them or making them into moment frames. Options one and two, which are the most common, are off of the table per your previous comments. That leaves option three. Wind analysis would be similar.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[skylite]

I'll do my best. Imagine that an earthquake occurs and each of your rafters needs to be prevented from shifting laterally (left to right in your sketch). That movement needs to be limited. Some options for doing that include:

1) Connect rafters to a shear rated deck diaphragm.
2) Connect rafters to a discrete, in-plane bracing "diaphragm".
3) Utilize the weak axis flexural stiffness of the rafters themselves.

All methods assume that the column tops themselves are restrained from lateral movement somehow, perhaps by bracing them or making them into moment frames. Options one and two, which are the most common, are off of the table per your previous comments. That leaves option three. Wind analysis would be similar.

How do you utilize the "weak axis flexural stiffness of the rafters themselves"? By selecting stronger beams with larger section modulus in the Wy axis? Are you also stating that there is weak axis bending when each rafter shifts laterally? But the rafters were welded on top of column so how could they even shift?

You also mentioned the column tops themselves being restrained from lateral movement is a requirement for the 3 methods. But if the column top is already restrained. Would each rafter still shift individually.. is this what you were stating?

 

[KootK]

By selecting stronger beams with larger section modulus in the Wy axis?

Yes.

By selecting stronger beams with larger section modulus in the Wy axis?

Yes

But the rafters were welded on top of column so how could they even shift?

The columns only help if they themselves are restrained laterally at the top. Regardless, the rafters will flex laterally between columns.

But if the column top is already restrained. Would each rafter still shift individually.. is this what you were stating?

Yes, each rafter will still tend to shift laterally between columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[skylite]

[/quote]Yes, each rafter will still tend to shift laterally between columns.[/quote]

By flexing laterally, are you talking mainly about lateral torsional buckling?? If you are describing the wide flange just bending up and down.. but it's not supposed to do that with the light load of roofing.. would it.

In seismic.. would you think trusses are better? What's worse in seismic.. wide flange or trusses?

If they are not restrain.. the wide flange would just fail or break in half? What is the failure mode?

Thanks in advanced for the useful tips.

 

[KootK]

By flexing laterally, are you talking mainly about lateral torsional buckling?? If you are describing the wide flange just bending up and down

No. I'm talking about weak axis bending due to lateral seismic load applied on the beam. It's probably time for you to draw a free body diagram of your rafters with seismic load applied. That should allow you to answer your own questions. If not, post the diagram here and we'll be happy to critique it for you.

In seismic.. would you think trusses are better? What's worse in seismic.. wide flange or trusses?

I have no preference. Both systems can be designed successfully.

If they are not restrain.. the wide flange would just fail or break in half? What is the failure mode?

This just simple beam bending and all of the usual failure modes would apply: bending failure, excessive deflection, shear failure, connection failure.

Thanks in advanced for the useful tips.

You're most welcome skylite.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[skylite]

No. I'm talking about weak axis bending due to lateral seismic load applied on the beam. It's probably time for you to draw a free body diagram of your rafters with seismic load applied. That should allow you to answer your own questions. If not, post the diagram here and we'll be happy to critique it for you.

I'm just studying the work of a design for more knowledge. Here the span between columns is 18 feet. He used w8x21.. with c-purlin distanced 2 foot apart using 2x6" 0.07" C-purlin and corrugated light roofing material on top of c-purlin like the following.

x---------x---------x
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | | | | |
x---------x---------x


Do you have same of rafters with seismic load applied? In your experience, would the wide flange above ok for the span? Thank you.