design reference book for cantilevered concrete column? 1

[Engineerataltitude]

I work in a very rural area where I am the only licensed structural engineer. I got a request from the local county public works department to engineer a new decorative arch over Main St. They would like the columns on either side of the road supporting the arch to be concrete. I have never designed a cantilevered concrete column like that before and currently I have no references to identify how best to design the column for torsion, moment at the base, etc.

Can anyone recommend a design reference for something like this?

 

[KootK]

The closest analog that I can think of would be those overhead highway signs that you see sometimes, even though they are done in steel. I'd think that some of the state DOT's would have free information that you could access. I'm pretty sure that I've seen stuff over at the FDOT website. Designing the columns themselves should be pretty straight forward once you have the loads (moment, shear, torsion, K=2.0). It's the determination of the loads, and any vibration criteria, that I'd be most concerned about.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

I got a request from the local county public works department to engineer a new decorative arch over Main St.

I expect they want exactly what is stated - a decorative arch. Suggest contacting a company that specializes in design / manufacture of large precast concrete culverts; an example would be Contech.

They likely can come up with a basic precast arch, which with some imagination can be used to support decorative features which you design.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Engineerataltitude]

I'm attaching a rendering of the kind of arch they are looking for.

I wonder if a precast concrete culvert might work? I'll contact Contech and ask. I'll also try my state DOT website, but I don't recall seeing too many concrete support posts for lights or signs along the highways.

I have design resources for concrete column design, but only for columns supported on both ends. I could use some direction for the kind of reinforcement that would work for the moment at the base, connecting it to the column.

I'm actually hoping I can talk them into making the column steel and then facing it with the decorative rock they are looking for. I like the ductility of the steel column in a high seismic environment and I can just design a nice grade beam as a foundation to attach to.

 

[Ron]

The entire structure could be done in steel with either faux stone covering or real stone covering. The foundation would be drilled piers. As KootK noted, could be designed as you would an overhead highway sign or traffic light stanchion. If you decide on steel, be sure to carefully waterproof everything to protect the steel from corrosion.

The other possibility is to drive two precast, prestressed concrete piles and leave the butt of the pile as the top of the structure. Connect with a fabricated steel arch truss as shown and you're done.

 

[hokie66]

In cast in place concrete, just design them as cantilevered beams. For the footings, you could either use rectangular pads or a pair of bored piles joined by a ground beam.

 

[dhengr]

Engineerataltitude:
Any stone masonry cladding or veneer around a steel cantilevered column and arched truss system will be very stiff w.r.t. the steel structure. And, it will take a significant lateral loading of its own, from either wind or EQ’s. I would let some good stone masons have at it in building a stone masonry veneer exterior layer (wythe), and then design the interior as a solid grouted and reinforced masonry pier. It appears to be about 25' high to the top of the piers and I would put a nice decorative concrete cap on each pier to shed water to the outside. It appears that the piers are about 3-4' round or square at the top of the base structure, and I would grout the core solid in 6' lifts or some such. See what kind of lifts a truck load of ready mix will produce, and maybe let that be your basis for lift heights. Clean out any mortar which has fallen into the core during the lay-up of the stone masonry. Provide vert. column rebars and ties, and also some cross ties btwn opposite sides of the masonry. It looks like you should have no trouble finding nice building rock from the mountains in the background. The foundation could be a spread footing, below the frost line and a little larger than the base structure shown in the sketch.

 

[TehMightyEngineer]

I worked with Contech on an arch project right before Christmas. They've got a good engineering staff from what I saw and they're definitely masters at getting the most out of the concrete. Though they're prefabricated arches are more like culverts and it looks like you're looking to have cantilevered posts with a steel arch framework according to the attached picture.

I would also suggest precast for the posts as you're going to have a very hard time getting a durable finish that's both appealing and easy to cast-in-place. Much easier to have a precaster cast it laying down and crane it into place. Most precasters can cast stone finishes into the concrete and make it look great, we do that for many of our precast stairways.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Engineerataltitude]

Excellent advice all the way around. You've all given me several good avenues to pursue.

Thanks as always to a great group of generous professionals!

 

[DETstru]

Given the steel arched truss they have going across the road, this whole thing screams steel to me. I would just do steel columns and use welded masonry ties to secure the stone veneer.

Reasons you can tell them to go with steel:
[ul]
[li]No reason to deal with all the concrete formwork when you've already got a crane on the job to set the truss. This will save money.[/li]
[li]Simpler connections from the steel arch to the columns.[/li]
[li]Economy in scale - all steel vs. half conc/half steel[/li]
[li]A steel column will be smaller in cross section than a concrete column and will give them more flexibility in how big they want the overall dimension to be once the veneer is in place.[/li]
[/ul]

 

[KootK]

Nicely done DETstru. I'm convinced.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DETstru]

Thanks Koot

 

[hokie66]

DETstru,
I'm not convinced. The picture shows something big for the columns, and the gap between steel and stone would require a lot of filling in unless you use a big steel fabrication. Concrete seems the way to go to me, with three choices: 1) cast in place to the size required. 2) cast in place within prebuilt stone as suggested by dhengr. 3) Precast like a lighting tower. But the choice will likely depend on what is most convenient/economical in the area.

At any rate, this is not an arch. It is just a steel frame spanning between two verticals.

 

[DETstru]

hokie66,

Truss, not arch. No big deal.

I still think steel is the way to go. If they do want something substantial in size (like 3'x3' overall) he could do a steel column with CMU backup behind the veneer.

Casting a 2'x2' column is way overkill for this application and will end up costing more. With all the formwork, limited scope, and mobilization it'll easily cost upward of $600/yard. Steel will easily be less, especially since they're already coming out to set the truss.

Precast wouldn't be a tremendous savings due to the low quantity. If we had 100 of these I'd say precast would be more economical but not for two.

 

[Engineerataltitude]

sorry out of town for few days...

The biggest problem with pre-cast for this application is that it likely will have very high transportation costs to get to this rural location. This location is more than 350 miles from the nearest manufacturer. With the columns being so large I'd suspect the transportation cost would make them much higher cost than either steel or cast in place concrete or masonry.

One of my biggest concerns about the design they've shown is being able to get a decent handle on the deflection of the top of the columns. I don't like seeing rock veneer that high up around public walkways on a structure that that might have enough sway to start to crack the mortar over time and allow rock veneer to fall on people. Like many public projects, its not likely to have rigorous maintenance on it over time.