Roof sag

[MIStructE_IRE]

A client had a builder in to convert her attic. The attached sketch shows the original trussed roof, and the new roof system - classic attic conversion.

The roof has now bowed slightly, even though I’ve checked the beams, which are in fact oversized.. The rafters were not strengthened and don’t need to be as in my view the moment associated with span A-B and B-C remains as it was before. The load and span length remain unchanged so the rafter stresses should remain the same. In fact, the axial is probably reduced somewhat.

All the numbers stack up.. so I’m a little stumped here! Anyone come across this?

It could be simply the initial deflection of the steel beams, the overall change in stiffness of the structure and everything has shifted as it settles into its new state of equilibrium.. Any thoughts appreciated as always!

 

[phamENG]

Where is the sag noticed? Ridge line from the street? Cracks on the interior? Cracks in the ceiling below?

 

[sbisteel]

The rafters were not strengthened and don’t need to be as in my view the moment associated with span A-B and B-C remains as it was before.

Seems to me like they've been changed from axially loaded members to shear/bending propped type cantilevers.

 

[Harbringer]

Seems to me like they've been changed from axially loaded members to shear/bending propped type cantilevers.

Yeah, if your drawing is at all to scale you have some pretty long cantilever rafters with a short backspan. I'd expect the connection at the ridge to have opened up significantly at the bottom.

 

[dhengr]

MIStructE_IRE:
In the original case, you have a truss which is a stable structure, and which spans and deflects as a truss. In the second/new case you have a structure which acts much more like a beam/rafter and ceiling joist system, a predominantly bending system. But, in addition you have likely added some attic loading and the new blue structural elements are likely hanging some of that new load off of the rafters (the old truss top chord) which is now spanning half the bldg. width instead of acting as a truss top chord. The new attic jsts. may improve the attic floor strength and deflection, but the ceiling joists play a big part in how that whole ceiling spanning element deflects/settles and loads the rafters.

 

[dik]

The horizontal tie is the only thing that takes care of the thrust caused by the roof rafters... also if truss, the roof rafters are likey slender. Maybe the horizontal tie isn't working as it should... this would cause the ridge to drop...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[IRstuff]

Was anything else changed, like the roofing material itself? If the roofing material was changed from shake or shingle to cement, that would have been accompanied by plywood sheathing, and the roof load would have increased by about 3 lb/sqft. Even if the roofing was unchanged, did the inside get get finished walls, either plywood or drywall?

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[dik]

and, they don't often remove the old roofing... just shingle over the existing...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MIStructE_IRE]

Thanks guys, for clarity the top chords was always loaded with a UDL, so the bending moment remains the same.

Roof material remains the same and there’s no visible cracking (ceilings etc were removed and not yet rebuilt). There’s a visible say in the roof tiles viewed from the street.

 

[dik]

The downward compression was resisted by the web members that are 'gone'... I suspect the force has been transferred to the horizontal member and may contribute to the sag...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Craig_H]

With the removal of the diagonal webs, the number of connections joining the top and bottom chord have been drastically reduced to only the heel connection. I would look closely at that connection, as it may be overloaded depending on the degree of load sharing between the original members and the new framing added by the contractor.

Your ridgeline deflection is largely due to the lack of vertical support at the ridge, and also the lack of webs to transfer shear from the top chord to the bottom chord. The two webs that previously connected to the ridge supported that connection; now that they are gone, this ridge joint is free to move up and down without engaging the bottom chord. What's been done is analogous to removing the web from a beam and leaving only the top and bottom flanges. Is that beam still as stiff as the original?

 

[LittleInch]

Have A and B moved down due to increased loads on the beams due to the new joists and the more direct loading coming from the roof from points A and C?

What's the plan view - i.e. what do those new I beams rest on which support the vertical members you identify as "stud"?

Has the ridge beam moved sideways? as you've lost virtually all your horizontal stiffness by removing those truss elelments?

where exactly is the "bowing". Any pictures?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[XR250]

I'm going to disagree with most of you. The long diagonals are in tension under gravity loads and they support the compression of the shorter, removed diagonals and support the bottom chord (which does not require vertical support anymore). Since the shorter ones have been replaced by the i-beams, the long ones are not necessary. Run it in a 2d truss program and you will see. The peak of the truss does not require support.Could be that the i-beams are deflecting or the supports for the I-beams have settled.

 

[phamENG]

I agree with you on the bending moment. If we pull our heads out of the textbooks and realize that these trusses aren't loaded at the nodes but continuously along the chords, it's clear that the chords are not axial only members - they are combined axial and flexural members. When you removed the web members that connect at B you did not change the flexural behavior of those chords, but they were not zero force members and the load had to go somewhere. That somewhere was back into the opposing "rafter". We typically ignore axial load in traditional rafter setups as being insignificant. But here you've taken a truss (which depends almost exclusively on axial loading) and turned it into a hybridized system. So your analysis must account for axial load in the "rafter". Did it? What were the results?

You didn't mention adding collar ties to the upper portion of the attic to tie the two sides of the ridge together. With would have added fixety to joint B and helped minimize outward spread of the rafters that could be causing your problem.

How are the beams deflecting? What's the beam span? How does the deflected shape of the roof correspond to the deflected shape of the beams? Steel is being used, so I'd imagine it's a pretty good span and/or large loading. I think your idea here is spot on and, if it isn't the primary reason, I think it's going to contribute significantly. You went from a nice, deep (read: stiff) roof structure that was spanning across the building the short way, and swapped it for a more flexible system supported on what are probably very flexible beams running the long way. The overall stiffness of the entire system is likely way down, and it's manifesting itself in the roof sag.

 

[XR250]

@pham,

The chords continue to be axial force members as the bottom chord is still continuous (from what I can tell from his diagram).

 

[phamENG]

XR - I agree, but the removal of some webs and the addition of the beams changes the overall distribution of the loading. I was just curious about the level of analysis that was done and to what extent they were redistributed.

You brought up a good point about the orientation of the force in the removed members under the considered case. It makes some of my first paragraph less important now that I think about it. I still think the issue is the beams and the global change in system stiffness.

 

[XR250]

I still think the issue is the beams and the global change in system stiffness.

I agree

 

[Enable]

What was the order of operation? If the roof was loaded throughout the restoration effort great care would have needed to be taken to ensure the rafters were not unduly stressed.

For example, if the steel joists and flooring were put into place after the rafters were secured to the girders then the weight of the joists + flooring would be superimposed causing deflections throughout. Or if the builder didn't support the rafters on the entire floor system prior to removing the truss members additional sag would result.

 

[XR250]

Right, if the webs were cut prior to installing the beams, it would def. be sagging.

 

[Tomfh]

Where is the sag occuring?

A? B? C?