How to Tell Where a Typical Detail Applies
Why this matters
A designer cannot draw every hanger, every wall penetration and every curb flashing on a job, so they draw one of each and write TYP beside it. That one drawing then governs dozens of places the set never shows again. The crew that reads TYP as "here" instead of "everywhere like this" installs the detail at the one callout and improvises the other forty. The crew that reads SIM as "same" instead of "adapt it" copies a joist clamp onto a concrete deck. Both mistakes surface at inspection or punch walk, after the ceiling is closed.
The three words and what each one commits you to
TYP (typical). Written beside a detail callout or a dimension, it means the condition shown applies at every similar location on the drawings unless another detail is called out there. One callout stands in for all of them. The designer is telling you they will not repeat themselves.
SIM (similar). Written beside a callout, it means the referenced detail applies here in concept but the condition is not identical. You keep the intent (the components, the sequence, the protection the detail provides) and adapt what has to change to fit: a dimension, a member size, an attachment to a different substrate. SIM puts the adaptation on you, which is exactly why it needs reading carefully.
UNO or UON (unless noted otherwise). Usually found in a general note: "ALL PIPE SUPPORTS PER 3/M-501 UNO." It makes a typical detail the default for the whole sheet or set, overridden only where a different detail or a note is shown at a specific location. Some sets write "U.N.O." on the detail itself; the effect is the same.
The callout itself is the bubble with the detail number over the sheet number. A callout reading 3 over M-501 sends you to detail 3 on sheet M-501. See related: How to Cross-Reference Between Drawing Sheets, which owns following that bubble; this card owns deciding where the detail at the other end governs.
The worked case
A shop has the hydronic piping on a one-story office addition. The scope is a 2 in heating water main and a short 3 in header off the boiler. The mechanical plan and the detail sheet look like this:
M-101 PLAN (part) M-501 DETAILS
OPEN OFFICE (steel joists) 3 TYP HANGER AT
===2in HW=========+===== STEEL JOIST,
| | | | | PIPE 2in AND
(3/M-501 TYP) | SMALLER
| rod - clevis -
MECH RM (concrete deck) beam clamp to
===2in HW====+=3in HDR=== joist top chord
| | | |
(3/M-501 SIM) [no callout on hdr] 7 PIPE THRU
RATED WALL
1-HR WALL ===||=== (7/M-501) sleeve + listed
firestop system
GENERAL NOTE 4: ALL PIPE SUPPORTS
PER 3/M-501 UNLESS NOTED OTHERWISE
Counted from the plan, the run needs 15 hanger locations: 8 on the 2 in main under the joists, 4 on the 2 in main under the concrete deck in the mechanical room, and 3 on the 3 in header, also in the mechanical room. One pipe crosses the one-hour wall between the two rooms.
The steps
Read the detail's own title and limits before you read the plan. Detail 3 says "pipe 2 in and smaller" and "at steel joist". Those are its parameters, and a typical detail only reaches conditions inside them. Write the limits on a sticky note or in the margin: size range, substrate, rating, orientation. Skip this and you will apply detail 3 to every pipe on the job, including the one it was never drawn for.
Find every general note and keynote that points at the detail. General Note 4 makes detail 3 the default support for the whole job, which means it governs every support location whether or not a callout sits there. Search the general notes sheet, the sheet notes on each plan, and the legend. A typical detail with no general note reaching it governs only where it is called out, plus wherever the TYP beside a callout reaches by plain reading. See related: The Legend and Notes Most Techs Skip.
List every location of the condition, called out or not. Walk the plan and mark each place the condition occurs: here, 15 hangers and one wall crossing. Count them on paper. The callouts are sample markers, not a count. Without the list, the 8 joist hangers become "the one by the callout and whatever looked right."
Strike every location a specific detail overrides. The wall crossing has its own callout, 7/M-501, so detail 7 governs there and the support nearest the wall still comes from detail 3. A location-specific callout always beats the default; that is the "otherwise" in UNO. Check the enlarged plans and sections too, because an override drawn on an enlarged mechanical room plan will not appear on the small-scale floor plan.
Sort what is left into three piles: inside, similar, outside. For each remaining location, ask whether it matches every limit you wrote in step 1.
- Inside: the 8 joist hangers on the 2 in main match size and substrate. Detail 3 governs as drawn.
- Similar: the 4 mechanical room hangers on the 2 in main match size but not substrate, and the designer has marked them SIM. Detail 3 governs in concept with an adapted attachment.
- Outside: the 3 hangers on the 3 in header miss on both limits, concrete deck instead of joist and a pipe over 2 in, with no callout at all. General Note 4 says "per 3/M-501", but detail 3 does not cover a 3 in pipe. A note cannot stretch a detail past its own stated limits.
For every SIM location, write down exactly what you are adapting and confirm the source for it. Detail 3's rod, clevis and pipe size stay the same in the mechanical room. The beam clamp has nothing to grab, so the attachment changes to an anchor in the concrete deck. That is a change to the load path, so it needs a source other than your own judgment. Open the spec section for hangers and supports (in MasterFormat numbering, 23 05 29 Hangers and Supports for HVAC Piping and Equipment) and look for what it allows into concrete. On this job the spec lists post-installed anchors listed for cracked concrete, so the adaptation is specified: anchor per the spec, everything else per detail 3. Record the anchor type in the margin beside the SIM callout. If the spec had been silent, the adaptation would be an RFI, not a field decision.
Write one RFI covering every "outside" location, with the count and the question that makes the answer usable. "Detail 3/M-501 is limited to pipe 2 in and smaller at steel joists. The 3 in heating water header in the mechanical room (3 support locations, concrete deck above) has no support detail. Please confirm hanger, rod size and deck attachment." The designer can answer that in one line. See related: How to Write an RFI That Gets a Usable Answer.
Mark the truck set so the next person does not redo the sorting. Highlight the 8 inside locations, write "SIM: anchor per 23 05 29" at the 4 mechanical room locations, and write the RFI number at the 3 header locations. Install the 12 covered locations now and hold the 3 until the answer comes back. The marked set is what makes the hold visible when somebody else picks up the job tomorrow.
The tally out of the case: 15 support locations, 8 governed by detail 3 as drawn, 4 by detail 3 adapted under SIM, 3 held for an RFI, and one wall crossing governed by detail 7. 8 + 4 + 3 is 15.
What SIM obliges you to adapt, and what it does not
SIM lets you change the parts of a detail that have to change to fit the geometry. It does not let you change the parts that make the detail work.
Usually yours to adapt under SIM: a dimension that follows the condition (a rod length, an offset, a curb height matching a different roof build-up), a member that follows a different structure (a strap that wraps a wood beam instead of a clamp on steel), the orientation of a detail drawn horizontal and applied vertical.
Not yours to adapt under SIM: anything that changes a rating or a load. A fire-rated penetration done "similar" with a different pipe material is a different listed firestop system, and those are tested as specific assemblies. A support done "similar" into a substrate that cannot take the load is a structural question. A flashing detail done "similar" at a slope the membrane manufacturer does not cover is a warranty question. If the adapted part is what holds, seals or protects, the SIM becomes an RFI.
The test that sorts them: could a reviewer looking at your adaptation say whether it is right without calculating anything? If yes, adapt and record. If they would need to run a number or look up a listing, ask.
When a location is unusual enough to stop and ask
A typical detail is drawn for the common case, and the sets that cause trouble are the ones where a few locations sit at its edges. Stop and write an RFI when any of these is true at a location the typical seems to cover:
- The condition is outside a limit printed on the detail, as the 3 in header was.
- Two typical details both appear to apply and they conflict, for example a typical duct support and a typical seismic brace detail that call for different rod sizes at the same hanger.
- The location carries something the detail does not mention: a rated assembly, a roof membrane, a vibration-isolated piece of equipment, an exterior exposure.
- The structure is different from what the detail assumes and no SIM is shown, such as a section of the building on wood framing in a set whose support details all show steel.
- The typical would land on or through something another trade owns, such as a beam flange already carrying another trade's clamp or a structural member the drawings mark "do not penetrate."
The cost of asking is usually days. The cost of guessing is a hanger, a penetration or a curb torn out after another trade has covered it.
What changes the answer
The general note's reach. If General Note 4 had read "pipe supports in open office per 3/M-501", the four mechanical room hangers would not be covered by the default at all, SIM callout or not, and the question would become whether the SIM callout alone extends detail 3 there. On most sets it does, because the callout is specific. Where the note and the callout disagree, see related: When Drawings and Specs Disagree: the Order of Precedence.
A renovation set. On existing construction, the typical detail assumes the structure the drawings show, and the existing structure often differs. Verify the substrate at each location before treating it as "inside." See related: How to Field-Verify Existing Conditions Before You Order.
Who drew it. Structural typical details (lintels, joist reinforcement at openings) are drawn and stamped by the structural engineer, and a trade does not adapt them under SIM; any variation goes back to that engineer.
How to check you sorted it right
- Your list's location count matches a recount of the plan done from the other end of the run, and the inside, similar and outside piles add to that count.
- Every SIM location has a written note naming the adapted part and where its specification came from.
- Every "outside" location has an RFI number beside it, and nothing is installed there yet.
- Every location-specific callout on the sheet, including those on enlarged plans and sections, appears on your list as an override.
- A second person given the marked set and your limits note reaches the same piles without asking you.
References
- See related: How to Cross-Reference Between Drawing Sheets
- See related: The Legend and Notes Most Techs Skip
- See related: How to Write an RFI That Gets a Usable Answer
- See related: When Drawings and Specs Disagree: the Order of Precedence
- CSI MasterFormat, Section 23 05 29 Hangers and Supports for HVAC Piping and Equipment, as written into the project specification