First Concrete Joist Spacing for One-Way Joist Slab System 1

[IanVG]

This is more or less a follow-up to my previous post (thread507-523158), but maybe understanding this point will help me understand the other point. On the one detail for typical joist construction for this project, I see that the first pan is variable dimension ('30", 20", 15" or 10" pans as required'). How was the dimension obtained? Is that only from the beam that it starts from or from anything else? It also seems that the bottom of the joist width for the first joist is variable, indicated by the note: "Where this dimension equals or exceeds typical joist width, reinforce as typical joist." Is there anyway for me to draft/model this system up without verifying every single dimension in field?

 

[IanVG]

Just as further reference, is a picture from on-site showing columns #28 (right column) and #37 (left column), and showing joist systems (from right of picture to left) J25, J29, J34, and J38. You can see (what I think?) is a double-joist near the middle of the picture, and how the joists seem to not be centered on the columns.

 

[jjl317]

If, per your previous post, this is really just a modelling exercise, I wouldn't get too hung up on specific dimensions.

And anyway, in my mind, if you were trying to evaluate an existing building, even if you had every dimension on the original contract documents, you would likely want to have key field dimensions confirmed, as sometimes buildings aren't actually built as originally documented (shocking), and/or are modified after original construction. And I think pan/waffle slabs would be among the most likely structural systems to modified in the field during construction. On jobs like this, we often find we need to "guess" on preliminary layout/design, based on interpretation of the original contract documents, and then identify certain key dimensions to be confirmed, and then modify as needed.

 

[IanVG]

jjl317, thanks! Coming from a mechanical building background, I can understand how mechanical drawings are really diagrammatic/schematic in nature, and how much freedom the mechanical contractors were given to build the system. I guess, naively, I thought the structural side of the building was different and that engineers drew buildings with fairly strict dimensions, spacing, sizing, etc.

Could you provide any insight as to how the contractors actually went about laying out the concrete joists here? Did they begin from one end of the building and move to the other? Or did they start from the middle and go out?

Lastly, why did the structural engineer here have multiple joist spans in parallel that have the same sizing but arbitrarily begin and end along a given run of the building? I apologize if I am not using the right terminology, the old drawings I've been looking at for several years, seem to vary in their vocabulary.

 

[jjl317]

Please don't misinterpret my comments - structural engineers do design building with very specific dimensions, details etc. - my point is that too often the final constructed product doesn't match, either due to known and discussed modifications during construction (through RFI's, etc.) or just deviations/errors in construction.

In terms of why this job might have some areas may not have a consistent layout, my first thought would be that perhaps something specifically is going on above (perhaps a load bearing wall, mechanical unit, etc.).

 

[JAE]

For pan joist systems, a particular bay of joists may have a unique overall dimension that doesn't exactly coincide with the layout of standard pans and joist rib widths.

Contractors would typically start at one side with their edge beam and, with the standard pan edge tongue, start spacing out the joists until they get to the other side, where the standard pan may not fit (i.e. a 30" being too wide they would use a 20").

Engineers don't always care to work out on the plans every pan tongue and layout - we used to instead use similar language to the above or at least call out the last "special" pan width on a bay.
We'd review the formwork shop drawings (always required on pan joist projects) to verify the layout was OK.

 

[lexpatrie]

Bear in mind I'm "not THAT old" so this is not from personal direct experience with designing them, more checking some from say 1960 in 2000. Fuzzy recall warnings apply.

JAE pretty much has this, the layout starts from one side with the pans, this is generally a "one-way" system with some limited connections between them in the perpendicular direction (usually one in a 20' span, more in longer spans, sometimes reinforced with a #4 on the bottom).

Generally one preferred to keep the beams (girders if you'd rather of it it's after 3 pm in Paris), in the wider rather than deeper in the perpendicular direction so the formwork was all set at the same depth. (This is contrary to the current "use a flat plate as much as possible" and avoid beams as much as possible, but if you must, make them deeper than they are wider so long as the reinforcing fits approach).

Joists might suggest an older design as two way slabs started in around 1906 and onward, in the "builder's risk" sense, and though they didn't get the math right until the 1971 code. Anyway.

As to the "makeup" dimension on the one end, it's analogous to some Architects who can't be bothered to fit the dimensions of the modules they use, i.e. Masonry walls that are 13'-2" high, 14'-3" long and whatnot versus 8" increments for normal masonry construction. Or if you had roof trusses at 24" on center and the building is 24'-5" you've got an extra truss in there somewhere.

Alternately, the pan is usually 20" or 30" and the joist width is somewhat selected by the engineer designing them, 4", 5", 6" and the like (speaking extemporaneously here I'm not looking at the tables). They also made tapered end pieces if you had shear problems at the joist/beam (girder after 3 pm in Paris), or you widened the joists.

Usually there are CRSI tables involved, (or when I have to do evaluations of these, and it's been a while, I use the old CRSI tables, and they usually justify the design reasonably well, although Fling [a rather big name, so I gather, in Ohio, although now that I look, he's a past President of ACI, ] and all his cohorts and spin-offs and people who worked for Fling tended to use a lot of moment redistribution so the beams can require a little manipulation of the moments via redistribution to make them work).

To make it a "full" dimension there's sometimes a filler pan or a makeup pan or some other term, it looks like the pans were standardized at some point, and prior to that, naturally, they weren't standardized.






This is "what I have" and it looks like it's a photocopy of something that was cut and pasted into a sheet of engineering paper (hence the "National" on the left margin), and then photocopied at least once more.

Google Image search can sometimes make progress with these, I've managed to trace the occasional unattributed figure using it.

Not convinced we can attach files anymore.

Also, terminlology varies through the country and one man's "footer" is another man's "footing" and so on and so forth, without bothering for the differences between the US, Australia, and the UK/Europe (Canada, if you ask me is a bit of a hodge-podge of US and UK) that's a bit of a blockage even now. We don't find current research from other countries when the search term we use is a "US only" term. Canadians seem to like "hogging moments" would be a good example. Even within a city or state the terms can vary.