Skylight Problems 3

[Lion06]

I have a situation where a skylight is being installed on a 35 degree pitched roof. It's a VERY large skylight, and the load along the slope is fairly significant. I can use steel to frame it out, but the skylight sits on a curb 9" above finished roof, and I have to keep steel below finished roof for thermal reasons.

The HSS would sit on top of existing trusses and be skewed such that it's longitudinal axis is twisted 35 degrees (i.e. the top face matches the slope of the skylight).

I'm concerned about the following: If I keep top of HSS below finished roof, and build up a curb on top of it (approximately 9" high), how is that load going to get into the HSS? It's not a very wide HSS, and the wood plates put on top would want to twist off. The small moment arm wouldn't prevent that.

I know I can't be the first person to deal with a skylight issue like this. What have others done?

 

[Ron]

SEIT...can you post a sketch or photo? What are the trusses made of...wood, steel? How large is the skylight?

The skylight curb will likely bridge any deflection you have in the structure, so you'll have to attach the curb at regular intervals or continuously along the HSS. I'm assuming the skylight curb is pre-fabricated.

 

[paddingtongreen]

SEIT, this is not quite your case but I had to cover for a bidders choice of three different manufacturer's skylights with slightly different dimensions. I welded angle brackets at intervals on the hidden side of the HSS and relied on the HSS to resist any torque resulting from load on the angle brackets

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

Could you place the HSS with top horizontal, using a triangular seat at each existing truss?

Alternatively, if the top of HSS must remain parallel to the roof, center the curb over the c.g. of the HSS to eliminate torsion.

BA

 

[Lion06]

I don't have a scanner at home, so I'll see what I can do about a sketch.

The skylight is 16'-0" x 9'-6" (the 16' dimension is the sloped dimension, so these fit just in between the trusses).

The trusses are made of wood (sloped top chord @ 35 degrees - approximate size is 10"x12") and steel bars (for the bottom chord). It's really just a big triangle - no web members. There are purlins (4x8) that frame into the truss top chord at roughly 2' o.c. - they bear on top of the trusses, they don't frame into them. They are skewed 35 degrees such that the bottom rests flush on the top of the truss member, and there is blocking below each purlin to keep it from sliding down the truss chord.

Now comes my part. The new skylight wants to sit on a curb (that we provide) that sits 18" above (orhtogonally) the top of the truss, but I'm only allowed 8" of structure because the finished roof is 9.5" above the truss member.

The HSS I am providing will work for strength and deflection at a very small size (4x6), but even using a 4x8, I'm not comfortable getting that load into the HSS with 10" of blocking over an 8" HSS.

Am I missing something or does this just seem wrong?

I'll see what I can do about a sketch asap.

 

[Ron]

Will the purlins be cut out where the skylight is going? Why are you not bearing the skylight curb on the purlins rather than the truss chord? What type of decking is provided? Plywood?

Why not consider using a wood curb instead of HSS? Deflections and load distribution would be more compatible. Rest everything on top of purlins.

 

[Lion06]

Ron,

There is an existing skylight (that is supported completely differently than the one that is being put back), so no purlins would need to get cut out. The HSS's that I was providing span the same direction as the purlins (truss to truss). I originally designed it as wood, but then I found out that the skylight needs to sit 18" above the top of truss, so I figured 1) that a 4x18 might be hard to come by and 2) how to connect it to the top of truss and keep it from rolling over.

I have a good connection for the steel option - a 12" wide bearing plate with a V-shaped end plate lag bolted to the truss. This gave a decent moment arm to take out the toppling force I'm worried about. I honestly can't think of a good way to accomplish the same thing with a wood connection unless I try to make a similar connection out of wood. I looked for a Simpson Connection, but didn't see anything for a beam over beam skewed connector.

The decking is 1x6 T&G spanning over the purlins. I don't know how much of a consideration diaphragms were over 100 years ago when this was built, but I don't think that we can count on this decking as a diaphragm. Even if I could count on it as a diaphragm, the curb sticks 10" above it.

 

[Ron]

SEIT....Thanks for the additional info...I have a better picture now.

Your approach is good. As for the "toppling" force, you will need a cricket above the slope (for roofing purposes), so why not use one downslope as well...not a big deal from a roofing standpoint. Provide a cant support to your HSS and let the roofer provide appropriate flashing.

I still think your support level is at the top of the purlins, not the top of the truss...am I missing something?

 

[hokie66]

SEIT,

This is a large skylight, not something off the shelf. You should be able to have a member built into the skylight frame along the 9'6" sides to span between the trusses. That way, you only have to resist the force from the skylight by shear along the trusses, not by torsion or overturning resistance.

 

[msquared48]

SEIT:

Although a 4X18 is a special order, a 3.125X18 or 5.125X18 Glulam is not. Should be off the shelf. If not, look at a parallam or microlam.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

Ron-

Excuse my ignorance here, but what's a cricket? The main support members of the actual skylight structure are along the 16' dimension. These are real deal spanning mullions. They sit on a curb, but only for flashing purposes. There is a soft joint provided between the jamb mullion and the "curb" to ensure that the skylight jamb is taking all of the load. There are 2 "rafters" in the skylight frame spanning head to sill, so that the max glass span is roughly 3'. The head and sill of the skylight span the same direction of the purlins, but don't sit on a purlin. They'll be sitting on whatever structure we provide (i.e. our structure will be identical to a purlin in span and support conditions, it will just be a heavier, deeper member). But this heavier, deeper member is spanning truss to truss which is why I'm seeing the support level at the top of the "new purlin" (which is this heavier, deeper member). The skylight support is at the top of this "new purlin".


Hokie-

That's a good idea, except that the skylights are quite as wide as the truss clear dimensions, and the support sits a full 18" above top of truss chord.

Mike-

As soon as I find out what a cricket is, the glulam idea might just work.


All-

One last question since I'm considering a wood option again. If I take the skew out of the equation, it's like a horizontally spanning member with a strong axis load, and a weak axis load at the top of the member. Does the weak axis load at the top of a wood member give anyone concern for tension across the grain? My first reaction is to say no because the load is applied at the top over the span of the member, not at the support. I believe that by the time it gets to the fixed support that it is truly in the member as weak axis bending and torsion. Any thought?

 

[msquared48]

A cricket would be like a small ridge emanating from the top middle of the skylight and dying into the sloping roof framing, with small rafters in either side dying into the roof diaphragm. The purpose, like overframing, is to redirect the rainwater away from the skylight - to either side.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

SEIT..Mike's description of a cricket is good. It is generally done on the "high" side of large penetrations in the roof, such as skylights, chimneys, etc. The same concept can be used downslope to prevent sliding or torsion on the laterals.

Your description is more of storefront or curtain wall "sloped glazing" than a typical skylight.

You seem to be analyzing this as if you have a 16-foot span supported on each end by the head and sill. Doesn't any load transfer happen along the 16-foot jambs? If so, then part of the load of the interior mullions is picked up at the jambs. That leaves two point loads at the head and sill, transfered as a distributed load to your "new purlin".

Given that, I like Mike's idea of an LVL modular section. Use a cant section (like a gusset) at the ends to prevent rotation. You could also use a small strut to do that.

 

[Lion06]

Are you saying to use the cricket to provide insulation around the structure to allow it to stick above the finished roof? That makes sense.

The jambs (of the actual skylight frame) provide support for the glass. These jambs, along with 2 "rafters" span to the head and sill, but the head and sill of the actual skylight frame are not structural, and rely completely on our structure to do that work.

What I'm considering right now is this. Provide an 8" HSS spanning truss to truss with the original connection I mentioned (bearing plate with a V-plate lag bolted to the truss). This will keep the structure below the roof line between the end of the skylight and the truss. Then in the 9' section of skylight provide another 8" HSS (with blocking on top for the skylight connection) welded to the top of the workhorse. I think that should take care of the issue.

It was originally brought to me as a thermal issue. I believe that is true, but in talking through this here, I believe there is another issue. There are multiple skylights of this size in adjacent bays. If I have multiple HSS's sticking roughly 10" above finished roof level for a distance of roughly 50' could cause a dam of water to build up. Something like a cricket could help, but is it unusual for the water to drop 10" like a waterfall over this barrier?

 

[Ron]

SEIT...you're not concerned with the water coming off the skylight...that's negligible. You're concerned about the impediment to drainage behind the skylight (upslope). That's why you need a cricket...otherwise water will pond against your skylight head and cause problems.

Also, you should insulate your HSS sections, otherwise you might get a thermal fatigue issue in your steel-to-wood fastening system.

 

[Lion06]

Excellent point about the thermal fatigue - wish I could give you another star for that one.

I was not concerned about the water on the skylight, just the water that would be dammed, like you said. I figured the cricket would handle that typically, for water directly against the skylight. I was more concerned with the idea that the only way for the cricket to work would be to have it "waterfalls" over the new purlin 10" down to the roof, since the cricket would need to get it over the new purlin if I provided a single purlin from truss to truss - because there are multiple skylights side by side.

 

[hokie66]

SEIT,
What does your architect say about all this? Multiple skylights side by side are a recipe for a very leaky roof.

 

[Lion06]

Another part of our company is acting as architect (so to speak). Our company has several different parts - one of which is a branch that deals with exterior and facade renovations of (mostly very old) buildings. There's no architect on the job, and one of our guys from the other group is doing all the detailing that I don't much care for (and whatever other kind of stuff I the architect usually does - but I guess we'll see how pretty it looks).

 

[Ron]

SEIT...agree with hokie66 on this...this is a potential roofing nightmare. Depending on how close together these skylights are, you might not be able to appropriately flash them....inviting a lawsuit!!

 

[Lion06]

There is approximately 3' clear between them. There are 5 in a row. I'll bring this up as a potential problem.

Thanks very much for your input, gentlemen!