Squaring a bay by the tape: what the diagonals said

Two 100 ft tapes come out of the gang box before the decking truck is spotted, because a bay that reads long on Tuesday is a phone call on Wednesday and a crane on Thursday. This is a school gym job, three bays framed at the north end, steel up since Monday, 1.5 in composite deck scheduled to land on the fourth day. The third bay from the end is the one that got the note. Grid is 30 ft by 24 ft, W18x35 beams framing into W10x49 columns, top of steel at 18 ft 4 in. Note F-207, deck level, one bay, started at 7:10 in the morning with the frame still in the shade of the existing building and the tape cold in the hand.

What the number was supposed to be, before any tape moved

The first thing out is not the tape, it is the drawing. The bay is 30 ft by 24 ft on the framing plan, so the diagonal is arithmetic, not opinion: 30 squared plus 24 squared is 1476, and the square root of that is 38.42 ft, which is 38 ft 5 in on a tape that reads in sixteenths. That computed number is what both pulls get compared against. The general notes on this job put the tolerance at 1/4 in on the difference between the two diagonals of any bay, checked before decking. Plumbness and elevation are tolerances in the AISC Code of Standard Practice, and they are read with other instruments; squareness of a bay, in most contracts, is a project number written into the drawings. So 1/4 in here is a contract number, not a law of steel.

How do you pull a diagonal without lying to yourself?

Same reference point at all four corners: face of the column flange, not the anchor bolt centerline, not the corner of the slab. Steel tape, not fiberglass, because a fiberglass tape stretches when somebody leans on it and 38 ft of stretch is invisible. Two people, one at each end, both reading the same edge of the blade, firm steady pull rather than a jerk. Hook it, read it, rehook, read it again. Write down which corner the hook was on and which face the blade was held against, because a diagonal taken between two different reference planes is not the same measurement. Write the time and the sky. Then pull the other diagonal within a few minutes, so the sun does not move the frame between the two reads.

The two numbers as they came off the tape

Long diagonal, northeast column to southwest: 38 ft 4 7/8 in. Short diagonal, northwest to southeast: 38 ft 5 1/4 in. Difference between them, 3/8 in. The computed diagonal was 38 ft 5 in, so the two readings straddle the number and their average lands on it. That is the shape of a bay that is racked rather than a bay that is out of size. Second bay read 3/16 in of difference, fourth bay 1/8 in. The third bay got pulled twice more with the tape rehooked, and both times came back at 3/8 in, so the difference was not the tape, not the hook eye and not the man holding the end.

What a 3/8 in difference does in a frame

It means the bay is a parallelogram, not a rectangle. One corner opened past 90 degrees and the corner across from it closed under 90 by the same amount. The deck sheets then land on the beam flange and come up short on one side of the bay and long on the other, the end closure does not close, and the edge angle has to be dragged into line with a ratchet strap and a spud wrench. The worse cost is at the connections: the clip angles carrying the beam to the column come up half a hole off, and half a hole is a ream or a slot plus an inspector who wants to know why. A 3/8 in diagonal difference on a 30 ft bay works out to roughly a quarter of a degree of rack at each corner. Small number, real money.

When is the difference worth pushing back on?

The line this crew works to, at this bay size: under 1/4 in, write both numbers down and set the deck. From 1/4 in to 1/2 in, pull the bay back before the deck lands. Past 1/2 in, stop and get the detailer on the phone, because a come-along that closes a half inch of rack is covering something that will reappear at the roof line. The 3/8 in bay got a 2 ton come-along from the column base across to the opposite beam, hooked low, pulled slow, anchor bolt nuts cracked loose and the base plate reshimmed after. Re-read twenty minutes later: 1/8 in. Re-read the next morning: 3/16 in. Steel relaxes once the load comes off the pull. The morning reading is the one that went on the note.

What does this note not cover?

This is a diagonal note on one bay at one level. It does not certify the frame is plumb; plumbness is a different reading with a different instrument, and the AISC Code of Standard Practice carries its own numbers for it, read at a stated temperature. It does not check elevation or beam camber. It does not check bolt tension, which belongs to the RCSC specification and to our own bolt-up note: turn-of-nut on the 3/4 in A325, or a calibrated wrench brought to the torque written in that note, verified with a torque wrench rather than by ear. It says nothing about whether the anchor bolts are in the right place, nothing about the welds, nothing about the deck, which carries its own note, and nothing about the other bays or the level above. It also does not cover steel temperature. Steel moves about 6.5 millionths of an inch per inch per degree Fahrenheit, so a 30 ft bay swings roughly 3/32 in across a 40 degree day, and a bay read in the morning shade will not read the same at three in the afternoon in full sun. Neither reading is wrong. They are two different hours in the same steel.

What a GC or an owner can ask for

Ask for two numbers rather than the word square. Ask what the drawing said the nominal diagonal was, because a crew that cannot name the nominal never had anything to pull against. Ask which face of the steel the blade was held on, what time it was read and whether the frame was in sun or shade. That is three questions and about a minute of somebody's time. On this bay the answers were 38 ft 4 7/8 in and 38 ft 5 1/4 in, read at 7:10 in the morning with the frame in the shade, and both numbers are on the back of the deck plan with the time next to them.

The gesture, before the truck is loaded

Before the decking truck is loaded, walk out with a tape and ask the ironworker to show you the last bay's two diagonals, then ask what the number on the drawing was. Two pulls, a stub of soapstone and a difference written in the margin cost about twenty minutes. Finding that same 3/8 in later, after the deck is screwed down and the roofer has already been on it, costs a good deal more than twenty minutes.