Terciary beams, Is this even possible? 2

[IngDod]

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.

 

[IngDod]

@AELLC: Wow... that must have taken a while. In the future I will definitely try to steer the architect away from this system... I got out-maneuvered this time.

 

[AELLC]

If you don't have commercial software, the practical way is to set it up on Excel so that it shows the deflections clearly and you guesstimate the loads on the beams until the deflections are equal, of course - I suppose you could automate it 100% on Excel, but I don't have that Excel expertise.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[dhengr]

IngDod:
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.

 

[Archie264]

>>>...you guesstimate the loads on the beams until the deflections are equal, of course - I suppose you could automate it 100% on Excel, but I don't have that Excel expertise.<<<

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.

 

[AELLC]

Archie,

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.

 

[Archie264]

Well, to hopefully not derail the topic too badly but years ago I read in "Engineering News Record" that they considered the number one structural engineering software package to be...Excel. And why not? For the vast majority of what we do custom spreadsheets can be set up and tweaked by us, the end users. It gives us control and customization and helps us avoid having to trust in black boxes. (From an old SNL skit: Unfrozen cavemen engineers like myself don't understand those mysterious magic black boxes that spit out answers, the derivation of which we know not. Ugh.)

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.

 

[AELLC]

Archie,

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.

 

[Archie264]

Sure, I'll give it a try. Having said that, though, I might have to step out soon. It might have to wait till Monday...

 

[Archie264]

In the mean time, one of these might get you started:

http://www.solver.com/product-mix-excel

http://www.ehow.com/how_7480745_microsoft-excel-solver-tutorial.html

http://www.excel-easy.com/data-analysis/solver.html

 

[Triangled]

is there a reason why your secondary bears on, and not hangs from, your primary?

 

[Triangled]

Gotta agree with dhengr on the layout. And put members as close as you need, even doubling the 20 footers if that's what it takes. If they complain about weight, recommend standards shapes.
Good luck.

 

[IngDod]

@dhengr: I followed your advice... I am getting secondary beams that are commonly used as primary, but I have a conventional structure again. By doing this I also managed to get rid of most of the moment connections. I am very grateful for your help.

@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.

 

[Ron]

Ingdod....what about something like this....

 

[Triangled]

IngDod, been there done that. Congratulations for getting it straightened out.

 

[IngDod]

@Ron: Thanks, thats the configuration I arrived to with dhengr's advice. I dismissed this in the first place because I thought that the secondary's were going to be too large (beyond available sections).. turns out that it is doable... although its gonna cost an arm and a leg.

 

[CELinOttawa]

Just not your problem.... With your hands tied behind your back (available sections, demand for few columns) that's what they get. Make sure they know it.