Cantilever Canopy Addition to Existing Rigid Frame PEMB

[Bhotar3]

The client wants to demo an existing Veranda over the entrance to a Rigid Frame PEMB, and erect a 30 ft canopy that extends over and covers parking spaces. The column lines are 28 ft apart, and are the main supports for the existing veranda. The would prefer not to drop columns and pour footings, so I would need to design this canopy to connect to the existing tapered columns. For me, I'm not comfortable attaching to the metal building unless I can do an analysis on the entire structure. Only problem is, after contacting the original Architect and EOR, no structural drawings with any existing member sizescan be found,(Shocker, right?), so the only way to determine existing member sizes would be field discovery. Regardless of existing member sizes, I don't think a 30 ft cantilever is feasible, and would most certainly require - at a minimum - columns near midspan (They can't have columns in the parking lot).

What are your opinions of this? Would you feel comfortable designing something connected to existing columns, or would you want to drop 4 columns with 2 adjacent to the existing structure and leave the PEMB untouched??

I have attached sketches for reference, with the existing veranda hatched out.

 

[msquared48]

For a veranda that large, I would not cantilever it off the PEMB. PEMB's are designed to the gnat's eyebrow to save money. Personally, I would recommend a minimum of four columns to avoid having to do a lateral and partial vertical for the main structure.

If the client absolutely wants the cantilever, have him pay for a new building designed just for the cantilevered canopy.

Mike McCann, PE, SE (WA)

 

[Coty]

I started answering this two different times, but the more I think about it, the less confident I am in you being able to gather all of the information you would need to not drop columns 10-12 feet from the building. If your soils allow, drop in some pier foundations to limit excavation and utility interference and design a propped cantilever.

Gathering the plate sizes for the PEMB frame isn't difficult, just time consuming determining every weld size and stiffener plate thickness, etc. The frame analysis wouldn't be too difficult as long as you are familiar with tapered member frame design. However, I think it would be pretty difficult to determine the capacity of the moment connections of the beam/column connections. If you don't drop columns, that cantilever will induce significant moment in the frame. 14' trib, 30 psf D+L, 30' long cantilever would be about 190 kip feet per frame. The uplift combo would likely be even greater. If the existing moment connections are welded, determining if the weld is good quality and whether the weld is a PJP or CJP is difficult. Adding a canopy that large will stress those joints, which are typically optimized just for the loads required. They may or may not be developing the full capacity of the beam and column sections. That's a lot of time gathering all the info you need for proper design, plus one mistake could be be bad news.

If you dropped columns down on the sidewalk, most of the load ends up in your new columns. You would end up putting a few kips of load back into your PEMB frames. If you are feeling crazy, you could even skew your columns out a few feet to reduce your cantilever size while keeping the columns out of the parking area.

 

[Bhotar3]

The client agreed to pay for field discovery, but dropping columns near the edge of the sidewalk would be more cost effective. Nothing about tying a large cantilever back into this building makes me feel comfortable at all, considering it was built nearly 30 years ago.

 

[Bhotar3]

I have a feeling that if I were to recommend adding columns, the next question I will be asked would be "What's the maximum length of cantilever that we can have?" Answering that question would still require discovery and design.

 

[Coty]

True, it seems to be cheaper in the long run to add the columns. Installing a large cantilever like that would require a good amount of shoring. With the propped cantilever approach, the columns can be guyed off in order to install the beams. That discomfort is a good thing. I would be more comfortable analyzing some additional shear in the PEMB than the large moment. The existing veranda scheduled to be removed will offset the new vertical shears a little, especially for uplift.

 

[Coty]

They might, but rationally speaking, they would likely only want to cover the parking spaces and that is what I would steer them towards.

 

[Ron]

Consider a free-standing aluminum cantilevered canopy such as those by Dittmer Architectural Aluminum

Dittmer

I agree with Mike and Coty....not a good idea to hang this off a PEMB

 

[WinelandV]

IMO, you should avoid trying to tie into the building at all. The field discovery is time-consuming, but you will likely spend just as long analyzing the frame, and that's assuming you're extremely proficient with whatever analysis software you use. If your cantilever section is as shown in "C.1 Demolition Section", then you're going to have drifting, which will add additional load to those PEMB columns. As msquared48 said, the building is already at the gnat's rear end.

I'd communicate to ALL the parties that it is HIGHLY UNLIKELY that you can tie-in without reinforcing the existing building, and that any reinforcement will be time-consuming and expensive. Correct approaches are what Ron is showing, or to drop four (4) new columns, and let the architect deal with making sure everything is water-tight.

 

[ajh1]

Seems like a lot of work to cover at most 3 parking spaces. I would also vote for a stand-alone canopy. A couple of columns just off of the building and two more at the edge of the sidewalk. Canopy should slope towards the parking to avoid drainage and snow buildup problems. Perhaps even better would be a couple of small rigid frames set parallel to the side of the building with a ridge running perpendicular to the building. That would drain and snow to the sides rather than out into the parking lot or back against the building. You still have a cantilever issue depending upon how far out into the parking area the roof is supposed to go. I would avoid tying to the existing structure given its age and the complexity of the reinforcement required.

 

[Ron247]

The maximum standard cantilever that most PEMB offer is 10'. In some cases, especially heavy snow load areas, they use of standard 10' cantilever canopy could lead to a different rafter size. The chance that a 30' cantilever will work without serious reinforcing is ZERO. In the PEMB community, the gnat's eyebrow tolerance MSquared mentioned is considered wasteful.

I would review as an option connecting a pinned-end rafter to the column at a lower elevation than the eave (say 5' lower) and placing columns on the other end. I would consider letting the new rafter cantilever 10' past the column. There is no way I would fixed-end connect to the existing rafter without extensive measuring and analysis that I feel sure will yield "No Can Do". You could also span a beam between 2 columns webs and then connect to it. That would share the new load with 2 frames per rafter rather than one and help align the new columns to the parking spaces. You could also batter the new column back towards the building to lessen interference.

Years ago, a building may have the same frame at every column line. Thanks to computer analysis and CadCam, you could have different frames at different locations. Field measuring is also less refined than mill ordering of material. PEMBs do not tend to order say 1/4" plate. They sometimes subtract the allowed mill tolerance (say .009) and then order "minimum .241". That is what you battle when you field measure. You will not measure too thick, so your reviewed frame stands a good chance of not "checking out" for the amount of load you are looking at adding.

 

[structSU10]

My first thought is to build a new column structure inside the building, adjacent to the existing columns. If they have the space to accomodate it, the separate structure may be able to cantilever the full distance, but you may need to do trussed columns or something, that would be braced back to the existing building.