Wood Pergola 1

[spats]

I'm designing a wood pergola on the oceanfront in Florida. It will be built using standard pressure treated SP dimension lumber, with 6x6 wood posts embedded in concrete. The "roof" will be 2x4s laid flat at 6" on center, supported by joists at 2' on center. I'm dealing with the Florida Building Code, which is basically IBC/ASCE 7-10. Question is, do I need to design the "roof" for 20 psf minimum construction live load? It's not truly a roof, and nobody is going to be stacking construction materials on top of it. Is there anything in the code that would allow less than 20 psf over the entire surface area (treating as solid)?

One of the reasons I ask is that the positive (downward) wind loads are killing me. I don't know what else to do besides treat it as an open building (ASCE7 Section 27.4.3). And then there's the fact that the "roof" surface is only about 60% solid. Would it be proper to apply the prescribed plus/minus wind loads to only the solid surface? I'm getting into 3Xs, which are not readily available in PT, and not readily available in the lengths that I need.

 

[MacGruber22]

Did you check your local building code? The model building codes do not call out pergolas, but your local code might. If they do not specifically address pergola "roof" loading...you may have to just wait and see.

I have never had an issue with residential pergolas going through plan review. I put a CA note that says the construction loads are not permitted on the pergola structure.

Then again, it should depend on the size of the pergola, particularly the height - can the workers install the purlins from the ground or a reasonable ladder? How close is the pergola to an existing structure - say, a house? Could workers stand on the pergola to work on the adjacent structure?

Slightly unrelated, depending on the use group/occupancy of the primary structure in which the pergola is acting as an accessory structure to, your local building code may not permit the use of unprotected wood (or even wood at all). We have designed pergolas for elderly care facilities, and wood pergolas were a no-no.

To me, structure live load had little or nothing to do with wind classification of open, etc. If you had a 40-ft tall open lattice structure (i.e. electrical tower), you could expect maintenance live load of 300 lb on the horizontal elemental.

In the end, I think this all boils down to good rationale, CA notes, and your local building code.

What happens if you put 20 psf (or 300#) on the joists only (not the purlins)? Is that a problem for you?

 

[woodman88]

I would say yes the the 20psf roof live load.

You might check using the Lattice Frameworks section of the ASCE 7-10 to reduce the wind loads.

Per ASCE 7-10 page 313

"Other Structures All Heights
Figure 29.5-2 Force Coefficients, Cf Open Signs &
Lattice Frameworks....

....Notes:
1. Signs with openings comprising 30% or more of the gross area are
classified as open signs.
2. The calculation of the design wind forces shall be based on the area of
all exposed members and elements projected on a plane normal to the
wind direction. Forces shall be assumed to act parallel to the wind
direction.
3. The area Af consistent with these force coefficients is the solid area
projected normal to the wind direction.
4. Notation:
∈: ratio of solid area to gross area;
D: diameter of a typical round member, in feet (meters);
qz: velocity pressure evaluated at height z above ground in pounds
per square foot (N/m2)."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[manstrom]

I use self weight plus maybe 5 psf. There is no reason to design for more. It is supposed to be airy. I'm surprised you need a permit.

 

[MacGruber22]

I ran some numbers on the 2X4 SPF #2 spanning 24" over at least 3 continuous supports, flat-wise. A 300# point load works all day long with that span.

I'm getting into 3Xs, which are not readily available in PT, and not readily available in the lengths that I need.

Are you talking about the joists?

 

[spats]

Thanks for the replies. The pergola is freestanding, well away from any buildings. It is 30' x 19' in plan with 6 posts and is about 11' tall. Using wood is not an issue.

Several of you mention "other structures" and "lattice framework". Those provisions are primarily for horizontal wind loads. I was using "open buildings" for the vertical loads. I planned on using ASCE7 Section 29.5 to evaluate horizontal wind loads on the vertical projection of framing members, but these loads are small in comparison. The primary wind effects will come from downward and uplift pressures, in my opinion.

I agree that a 300 lb. point load is appropriate... and yes, in some situations, somebody is going to hang a bunch of plants off the structure. I just have a problem with applying 20 psf construction live load to the entire surface, especially having to do D + 0.75(Lr + 0.6W) for an Exposure D hurricane.

Manstrom, I don't know how you justify 5 psf, but it seems like a reasonable number. The local building official does require signed & sealed drawings for permit. As far as they're concerned, this is a "shed/accessory structure" which needs to be properly anchored for a 120 mph wind load. The 20 psf construction live load seems to be required per the minimum live load table in Chapter 16 of the FBC. Only fabric awnings are allowed to be designed for 5 psf.

 

[spats]

MacGruber22,

I'm talking about the joists and beams being the problem, not the 2x4s. A basic layout is attached.

 

[MacGruber22]

Got ya.

You know, 2X4's at 6" o.c. laid flat seems like quite a tight spacing. Pergolas are usually much more "airy" than that. I can envision future maintenance of the structure, where they replace the 2X4 purlins. They might stack a pile of them up on the pergola while a worker replaces the older ones. So my new vote is that you need a live load of sort.

 

[woodman88]

The point of the

"Other Structures All Heights
Figure 29.5-2 Force Coefficients, Cf Open Signs &
Lattice Frameworks"

information is that it has information about reducing the effect of wind on open/lattice frames. This information is, in my opinion, allowed to be used on your type of structure.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[spats]

MacGruber22,

This pergola is on the ocean side of mid to high-rise condos, and is meant in part to hide pool equipment from their view from above. That is why the less "airy", but I'm thinking of putting them at 8" o.c.

woodman88,

Note 2 of Figure 19.5-2 says it's specifically for exposures normal to the wind direction. In any event, using Figure 27.4-4 for MWFRS and Figure 30.8-1 for C/C for open buildings are both giving me coefficients of +1.2, -1.1 and Figure 29.5-2 is giving me a coefficient of 1.6. No help there.