Suspended Canopy Design

[SinStrucEng]

Good day everyone,

A client I am working with has requested a suspended porch canopy over the main entrance to their home. Snippet below. I am yet to do something of the sort and was hoping to get some tips on the design, construction and generally "accepted" approaches. The canopy is conceptualized to be about 5 feet wide and project 6-8 feet from the face of the building. Our unfactored loads are 1 kPa (live, roof access), 2.44 kPa (snow), and 0.72 kPa (dead).

I was planning to have the cables (2 to 4) through-bolted to a steel ledger at the outer edge of the canopy, and then secured to the roof joists above (which are 64x302 I-joists). I have concerns about the upper connection and also not sure which cables are appropriate/commonly used for this application.

Thank you in advance.

 

[JLNJ]

A couple of questions: What is the force in your hangers and what do you see as the load path to resolve the force? Do you have any net uplift in the canopy? Does your snow include drift?

 

[SWComposites]

have seen rod or cable suspended canopies on commercial buildings. probably have seen them on residential buildings.
but the end connections are critical, and need to be designed so that the cable or rod is easily replaced.
you will need some sort of fitting attached to the structure at both ends, to spread out the load, and have adequate connection capability.
probably best for those fittings to have lugs on them, to which the cables or rods attach with a single bolt thru an eye on the cable or rod.
and the fitting connections to the structure need to be adequately sealed/flashed/something to prevent water ingression.

 

[Eng16080]

Is this just a roof, or a deck that people will also stand on?

You should ensure that there's adequate redundancy in your design. If the entire thing is depending on two rods to avoid collapsing, I would consider that a bad design. If this is a deck with railings, I would anchor the railings into the building for additional redundancy. I would also use deck hold-down connectors connecting the exterior framing to the interior floor joists. The design won't rely on the strength of the hold-downs. Rather, they will (probably) keep it from collapsing/ falling off in the event that the rods fail.

That is at least how I approach these things. Exterior decks/balconies/canopies scare me a lot, and suspended framing scares me even more, so I tend to be cautious with this stuff. Keep in mind that your design will be exposed to the elements and we all know it's unlikely that any maintenance or inspection will ever be done once the building is occupied.

 

[EdStainless]

Eng16080, I think that your question is 'Would it ever be possible for a person (or persons) to stand on it.
Rods are a much better option that cable.
Cable will end up with moisture between the strands and could develop all kinds of issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Eng16080]

That's a good call EdStainless. I see now that OP mentioned cable. I was thinking rods in writing that response. Yup, cable seems worse.

 

[EdStainless]

Indoors SS or brass plated cable looks cool.
But outdoors it is asking for trouble.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SinStrucEng]

Good morning folks. Thank you for replies thus far.

@Eng16080 This is not a canopy for assembly purposes, nor will there be any direct access to it (windows or doors). The live load is considered only for service requirements. As such there are no railings, etc. I recognize the risk if only two tension members are installed and for that same reason I said 2-4. I think that 4 rods/cables is ideal as it would provide structural assurances whilst still maintaining the aesthetics the client desires.

@All I forgot about the existence of rods when originally writing my post, lol. In principle what I am asking:

[ol 1]
[li]What style (grade, type, etc.) of rods are used for this? We are in Canada.[/li]

[li]Your advice in the actual rod end connections at the roof. The roof is framed perpendicular to the rods using 63x302 I-joists at 12" o.c. I was originally thinking that spreading the end reaction across multiple joists (perhaps) using a long threaded rod (with nuts/plate washers at each joist) was a good way to disperse the loads into the roof diaphragm but I am not so sure now. In essence the threaded rods would function like a big tieback. Perhaps if the roof is also strapped across all blocking/bridging... Or maybe this is all crazy talk and there's a better way.[/li]
[/ol]

Personally I 100% hate this and tried to convince the client multiple times to just install a column to two for me to use as the basis of the canopy. But they're going for an ultra modern look....

 

[XR250]

I would use rods of proper diameter to resist uplift - if any, not cable.
I would put a double rafter 4 ft back and then ladder frame them out to the gable
Then use a double band fastened to the rafters with Simpson DTT1z.
Weld a plate to the end of the rod and lag that into the double band.

 

[Eng16080]

Well, I read 2-4 to mean that it could be 2, but regardless, I think we're on the same page concerning the need for redundancy.

With the I-joists framed perp. to the rods, I don't see how they can reasonably resist any force applied in that direction, unless the force is being transferred directly into the top chord. I would add blocking between the joists going back 3 or 4 bays with a tension rod transferring the load to the furthest blocking. Ideally, the roof sheathing should be fastened to the blocking.

I also hate this, but you should be able to make something work.

 

[TORCHMAN]

Hanging canopies in Canada are tough to design because of the snow load. JLNJ made a good point about the drift. That should be accounted for.

We are building a 5 storey building in my area with the same hanging canopy. In locations where we have a block wall, the engineer spec'ed a cast in plate to be cast in and designed a bunch or reinforcement in that area of block. In the area where there is infill studs, we added a steel post. Difference is that we have hollow core slabs that we fill at post location which provides a very strong connection and rigid diaphragm.

For your design, I think you will find that the end forces on your rods will be too high to connect to any wood member. Is there a way that you could have a column at the back of the canopy (in the wall) and make a moment connection at the base and design a pier + pad footing at the basement?

 

[Eng16080]

Also, I agree with XR250 if this is new construction. I was under the assumption that the canopy is being added to an existing building.

 

[XR250]

With the I-joists framed perp. to the rods, I don't see how they can reasonably resist any force applied in that direction, unless the force is being transferred directly into the top chord. I would add blocking between the joists going back 3 or 4 bays with a tension rod transferring the load to the furthest blocking. Ideally, the roof sheathing should be fastened to the blocking.

Ah, did not catch they were I joists. They are drawn as dimensional lumber.
Well they could use LVL in those areas if they can afford it.
This is another situation where I hate I-joists (more than I do in general)

 

[SinStrucEng]

Honestly, I am now of opinion to simply decline this design request and inform the client that we can go with a post (or he can find someone else to take the design responsibility). Too much "ick".

 

[Eng16080]

SinStrucEng, can you clarify if the building is existing or if this is all new construction?

Personally, I would try to give the client some idea what it would take for this to work, unless of course, you just don't want to deal with them! To design this stuff right is always more complicated than it seems, especially to the client. They usually think we can just attach a bracket to the outside of the wall and that's it!

 

[SinStrucEng]

This is a new build home and the client has been more or less argumentative about almost everything. Their open concept has caused quite a few headaches thus far and this canopy was a late addition (after all framing specs were finalized).

 

[XR250]

They usually think we can just attach a bracket to the outside of the wall and that's it!

Probably all that happens in many cases!

 

[Eng16080]

SinStrucEng, I think there's probably enough good advise here to design this.

In terms of the client, I'd probably send them a change order and a brief explanation of how this is more work than it appears. For a problem client, I'll have them pay the change order up front. Charge what you think it will take plus some extra for having to deal with them. I know you were only asking technical advice here, but that's my two cents on the business side of things. I hate dealing with clients like this. Don't give them anything for free.

 

[lexpatrie]

I'd be at least somewhat concerned with uplift, and the durability of everything in a snow/ice/rain region is of concern since it's going to collapse onto a walkway when it lets go.