First Concrete Joist Spacing for One-Way Joist Slab System

[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.