Terciary beams, Is this even possible?
Terciary beams, Is this even possible?
(OP)
I got myself into a bit of a mess, I was approached to design a system with beams of very long spans (40 feet) supporting a second level(not a roof). I pointed out that it was most likely not possible to do the 40 feet span.. and the arch suggested the following system: Simply support the 40feet beam at the middle on another beam that spans in the perpendicular direction from column to column... It seemed a good idea on paper and I have seen this things before... Now the 40feet span beam (secondary) is resting on a 26feet span beam (primary).... to support the steel deck I need of course more beams... so lets call this ones tertiary beams... they span 13 feet and are supported by the secondary beam... I calculated everything and everything checked out for ultimate and serviceability conditions. However for the tertiary beams I only checked for deflection of that beam and I did not account for deflection of the secondary beams... Basically I got a beam with differential settlement on both ends... My question.. how should I calculate the deflection of this thing?.. I want to avoid any damage to non structural elements on top of the deck. This is still in the design stage, so there's no danger involved.
I attached a sketch to make everything easier to understand. The sketch only shows the section where the primary beam supports the secondary.
I attached a sketch to make everything easier to understand. The sketch only shows the section where the primary beam supports the secondary.






RE: Terciary beams, Is this even possible?
And I agree that this is a silly layout and dumb way of addressing the problem.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
I would suggest you pre-camber and ensure that you also check the vibration characteristics of the floor. If this is beyond your ken, you can have a look at ATC 93 or Design Guide 11. If this is still too much, contact Dr. David Allen (one of the authors of those documents) who has consulted for us before. Fantastic, a dream to work with, and very reasonable rates.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
Your architect's concept is commonly known as a "sky hook"....doesn't work. If you have enough vertical room to do that kind of crap, you have enough room to put a logical purlin/beam/girder system in place to take the loads.
RE: Terciary beams, Is this even possible?
Don't think I've seen it on a floor. But camber, as stated above, may help, though not with vibration. Offhand, I'd expect your tertiary members to realize almost full live load at some point, your secondaries not so much, and your primaries rarely if ever.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
How is this situation different from what is happening to me right now?.... Provided I can get deflections below the L/360 for live load (which I just managed to do by decreasing the spacing of the tertiary beams) why would this be such a bad idea. I am trying to play devil´s advocate... I would still use a different system if possible.
RE: Terciary beams, Is this even possible?
1. effect on tiles and finishes - I believe that your should be concerned with differential settlement between the ends of your tertiary beam, since the maximum differential settlement is 2.3cm (3.5-1.2) then your finishes would likely be damage because difference between the levels is beyond 2.0cm, which is the maximum allowable deflection for some codes to prevent damage for finishes.
2. Serviceablity in terms of visual perception - I think that for your tertiary beam you should take into consideration the Span as 26ft. My reason for this is people would look at the system as a whole, like those of a flat slab, since the middle support is also deflecting. However one other concern for you would be vibration since the deflection is relatively significant.
RE: Terciary beams, Is this even possible?
What about honeycombed beams? What about WWF? There has to be something commercially available to you which will do this span; 40' is just not that crazy! By my "back of the envelope" you need a heavy W27 beam, which are not at all uncommon, and possibly a tension cover plate or built composite with a concrete floor.
What loading are you designing for that you can't get this to work clear-span?
RE: Terciary beams, Is this even possible?
@Enhineyero: Thanks, those values were for dead load.. for live load the absolute deflection at midspan of the tertiary is 1.6cm.. the differential would be around 1cm. I am concerned on the visibility part.. and also I would not want anyone walking on the floor to feel any unevenness. The dead load deflections are rather large... around 1.5 cm, but I am assuming that any deflection due to the weight of the concrete on the steel deck (wich is most of the dead load)will not count towards this perception since the floor will deflect as the concrete is being poured and the final surface will still be level. Deflection due to steel deck is about 1cm, deflection due to surface finish is about 0.5cm.
Could you please explain more on the 26ft for visibility... this is the one area were I am mostly worried.. the tertiary spans only 13feet.. resting on the secondary which spans 40feet and then the tertiary which spans 26feet. Both the 40feet and the 26feet deflect within L/360 in their respective directions (for the 40feet I checked for both 40/360 at mid-span and 20/360 at quarter span).
RE: Terciary beams, Is this even possible?
What if you added another "secondary" assuming that is your 40' span, so that you had two secondaries supported at third points on your "primary", the 26' span. Now your tertiaries are spanning 26/3' instead of 26/2', your secondaries are supporting less load and your primaries, though supporting more load are supporting it at (perhaps) better points for deflection.
RE: Terciary beams, Is this even possible?
With your arrangement, the maximum deflection is at the center of the secondary beam with a very slight chance that it is a short distance from there along the central tertiary member.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Terciary beams, Is this even possible?
@Paddingtongreen: Thanks for your reply, the problem is the that the columns that support this beams (40feet)at their end points are spaced 13 feet... to have more beams spanning 40 feet I would have to reduce the column spacing to half at least... which is very narrow.
The system looks like this... Its a rectangular building, with three lines of columns... the first and last have columns spaced every 13 feet... half of this columns carry 40feet beams and half 20 feet beams. The middle line has columns spaced at 26 feet.. this is the line that carries the 26feet beams that support the 40feet beams.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Terciary beams, Is this even possible?
This thing is ugly, dumb and stupid.. but apparently it can support itself.
RE: Terciary beams, Is this even possible?
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Terciary beams, Is this even possible?
It seems to me that your last three threads all pertain to the same structure, and that the Architect is asking you to do an awful lot (fairly complicated structure) with a very little selection of available materials, shapes and quality construction and fabricating/welding talent. All put together by a fairly young and inexperienced Engineer to boot. If the Architect and Owner want you to do the next to impossible, they should at least be willing to pay for good quality fabrication, and material grades and appropriate sections/shapes suited for the job.
Why not 20' long primary beams on every other grid line and btwn cols., and 26' long secondary beams btwn. the primary beams, spaced as needed for your decking. This might fit your available structural sections better and add some simplicity and uniformity to the framing. You might even eliminate every other exterior col., or you would have 13' perimeter spandrel beams. I would eliminate the field moment connections of the beams to the cols. if I could, given your faith in the local welders.
RE: Terciary beams, Is this even possible?
AELLC,
If I've understood the nature of the calculation correctly an iterative calculation like that is rather easy to do in Excel using the "Solver" feature. Having said that, though, I'm guessing you might lose something in the process. Currently you have a "hands-on" aspect to your calculation; once it's fully automated it becomes yet another "black box" spitting out answers. But, if you created it it would be your "black box" software so that might not apply. In any case, using the "Solver" feature is how you might approach doing that, if you're so inclined.
RE: Terciary beams, Is this even possible?
I am a bit worried that this thread is going off-topic but IngDod informed me elsewhere that he uses Excel as a tool.
I have been using Excel for a long time, but I have seen only very few "engineering examples" done by others.
I am very rusty as to using the more advanced techniques. I don't understand Solver, Pivot Table, or any use of macros, VBA, UDF, etc.
The most important thing is to start with a sensible framing system, such as dhengr has posted just above, then worry about Excel vs. hand calcs, etc.
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Terciary beams, Is this even possible?
As for the "Solver" feature, all it does is iteratively apply small incremental changes to a calculation until it finds a peak or a valley. So, in your case, you could just set up a cell subtracting the deflections from each other and have Solver minimize it. Or something along those lines.
RE: Terciary beams, Is this even possible?
Can you post an example (or mock-up) of that in the Excel forum? Does not have to be regarding retaining wall or dams, that would be helpful. I have another thread going in Wood Design and Engineering where I illustrated a method of solving continuous beams.
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
http://www.solver.com/product-mix-excel
http://www.ehow.com/how_7480745_microsoft-excel-so...
http://www.excel-easy.com/data-analysis/solver.htm...
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
Good luck.
RE: Terciary beams, Is this even possible?
@Triangled: Thanks, I managed (with dhengr´s advice) to get the layout of a proper structure again.
This is a lesson I will never forget. Thanks to everyone.
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?
RE: Terciary beams, Is this even possible?