Concrete form deck too short 3

[RattlinBog]

I feel a bit ridiculous asking this, but I could use a sanity check.

Has anyone dealt with a situation where concrete form deck (non-composite) was ordered too short and one of the ends won't be able to bear on a support? Beams are 7 ft OC, but 13 ft form deck was ordered. I can provide a sketch in the morning when I'm back at my desk.

Would it ever be acceptable to arrange the form deck in a way that the end of one deck laps the end of the next deck between supports (in mid-air)? The lap joint would need to be connected together. I've crunched some deck bending stress numbers already but wanted to see if anyone has even come across something like this before... (P.S. Slab is designed to be self-supporting. Form deck is for construction only. Area is constantly wet, so I expect the decking to rot away over time.)

 

[KootK]

You can do anything that you can design to work. You will, however, be the new owner of ab exotic construction safety issue, So make damn sure that it does work.

Maybe something like allow the floating lap where the supporting cantilever would be 18" or less but shore it otherwise?

What kind of form deck are you using and what are the overall dimensions of the suspended slab? How wide are your beam flanges?

 

[RattlinBog]

Agreed. I'm the designer and the owner of this one. I work at the plant that this is being constructed. We're rebuilding the floor of an existing conveyor gallery. Old slab was falling apart. Contractor is working around the clock this week to get this done. I'm helping address problems as they come up. Only learned about the short deck earlier this afternoon.

I made a sketch and ran some numbers before I left the office. I'll attach tomorrow when I'm back.

Beams are W8x18 (5.25" flange). Form deck is 1.5C-36 (20 gage) Gr. 50. Slab total thickness is 5"-6" (low point in the middle for drainage). Slab is roughly 11.5 ft x 42 ft.

 

[KootK]

That's pretty small area. I'd be inclined to just have the contractor shor the laps where they land as I mentioned above. I guess you'll need some way to attach the deck to the slab at the joints to keep it from dropping at the laps in service.

You could also construct some temporary wood thing between the beams to support the floating lap joints. Just inspect it yourself prior to por if things are moving fast.

 

[RattlinBog]

I think something like that would be doable. I was able to forward my crude gallery elevation sketch to my laptop. Attached

I was thinking this arrangement could potentially work. The middle form deck would overhang on both ends, and the two outer decks would overhang on only one end and be supported by the middle deck.

The middle deck is overstressed when I place an equivalent point load at the end of each cantilever to mimic a reaction from the two outer deck loads. That's assuming there's no connection at the lap joint and the other decks are just dumping load to the middle one. Perhaps if I join the lap joints sufficiently, the decking could go back to behaving like a continuous piece.

 

[RattlinBog]

Here are the rough calcs I did last night. After some sleep, I realized I doubled up on the applied load / overhang length. The two deck pieces will lap 2 ft, so my point load on the middle deck should act closer to 1 ft away from the support vs. 3 ft. That will help. I only checked dead load last night...still need to add construction live load and sharpen my pencil. Contractor is planning to stich the laps together and slam some angles below to provide some additional edge support. One of the iron workers I talked to was on a bridge job that had several mid-air deck laps, and they just used metal deck screws at those joints without issue.

 

[KootK]

Considering the situation, I think that your arrangement is well thought out. Take care to provide deck uplift connection capacity for partial load situations during construction that are likely/possible.

Sure, you probably get some manner of moment connection at the lap joint when fastened by two rows of fasteners spread out. I think that's a pretty lousy moment connection though, particularly for sheet with very shallow flute webs. It will rely on the pullout capacity of some screws in thin sheet which doesn't feel great to me. I'd let that be just bonus, feel good capacity rather than what I'm hanging my hat on.

Contractor is planning to stich the laps together and slam some angles below to provide some additional edge support.

I struggle to imagine what good the angles would do but, at the same time, I'm also not clear on what the proposal would look like in practice.

 

[KootK]

You put together a handsome calculation package Mr. Bog. You'd not be impressed with my work I fear.

 

[RattlinBog]

KootK, thanks for the help and kind words. To put this to rest, attached are my updated calcs with a more accurate 1 ft overhang instead of 3 ft for the middle deck, which is no longer overstressed. The outer decks are slightly overstressed in positive bending with 75 psf DL + 40 psf LL, but I'm not too concerned. DL is technically a little lighter since it's not a true 6" slab.

 

[SlideRuleEra]

RattlinBog - You are wading into design of "means & methods" of concrete forming which is very different from design of permanent structures. From that viewpoint there are wildly over optimistic assumptions and glaring omissions in the calcs. Stop before this project ends with a pile of crumpled steel, wet concrete, and injured (or worse) workers. In 20 years on Eng-Tips I've never said something this dramatic... but I will today. Nitty-gritty details if you want to go there, but to start:

How do your calcs justify that loading of 115 psf (75 psf + 40 psf) on "butchered" deck is acceptance when Vulcraft rates intact deck at 93 psf (simple span) or 91 psf (two-span continuous)?

 

[RattlinBog]

SRE--thank you for getting real with me. I've always appreciated your critique. I'll try my best to give a full account below. Apologies for the long-windedness, but I'm trying to be as thorough and transparent as possible. See attached too. I don't disagree that this is getting into means and methods; but I am trying to help rectify an unexpected situation, and I'm not sure who else to turn to.

Some background on this project: this is the floor of an existing conveyor gallery. Roughly 11'-6" x 42'-0", sloped. Surrounding this gallery is a crushed rock stockpile that has just gone through crushing and transported by other conveyors. It is impossible to work safely near/below this gallery unless crushing operations are shutdown and rock isn't being dumped onto the pile. It is also impossible to work safely inside the conveyor gallery/on the gallery floor (this project) unless that conveyor is also shutdown. Both the crusher operation and this particular conveyor have been shutdown this week (10/9-10/13). That is the window of opportunity I have to work with. Contractor is working 24 hr days.

1. Conveyor gallery was constructed in 1970s. Floor is hosed frequently for material buildup and freezes in the winter. De-icing salts are used (and have been used for years before I was hired).
2. May 2023 - a 3 ft diameter of concrete slab broke free and fell 20 ft below. Now there is a big hole in the slab. We closed off access to the gallery.
3. Over the next few weeks, I designed a replacement slab, appealed to management the need for additional, unplanned infrastructure budget, and began construction bidding. I kept the replacement slab similar to original design but went through the appropriate calcs.
4. For metal form deck, I originally required non-composite 1.5C-36 (18 gage) Gr. 50, assuming two-span condition. (I looked through the same Vulcraft tables you included above and also used Vulcraft's unshored construction span deck design aid.) Looking back through my notes, I used DL=66 psf (accounting for avg thickness with 1.5" deck) and construction LL=20 psf. I forgot about the smaller value in my more recent calcs. I tried to be a bit conservative since I haven't encountered an overhanging deck condition before.
5. Contractor was selected, and contractor ordered materials. Neither of us worked with a separate party to complete a detailed design of the form deck. I assumed contractor would order what I called out, and I assumed form deck sheets would come in 14+ ft lengths.
6. September - A few weeks before the one-week construction window was scheduled, contractor informed me that the 18 ga. deck was taking very long to procure and was concerned about it arriving in time. However, 20 ga. deck was available immediately. I double checked my calcs/notes from May and checked Vulcraft and saw that 20 ga. appears to be ok for 7 ft span in a 2-span condition. I informed contractor that I would still prefer 18 ga. but the 20 ga. would be ok in a pinch.
7. October (this week) - Mob and demo began Monday, 10/9. Contractor is working 24 hr days. I check on them once or twice a day. Contractor found some additional structural steel corrosion that we repaired.
8. Tuesday, 10/10 at around 6 pm - I learned that the form deck is only 13 ft long. Contractor said they plan to arrange it to try to keep the overhanging sections as short as possible. I didn't provide any feedback/recommendations at that time. I first called a former colleague with 15+ years experience. He hadn't personally dealt with a situation like that but gave me some advice on what steps I could follow. He suggested calculating the actual required moment vs. available moment using section modulus values from Vulcraft tables.
9. Same Tuesday evening - I did some reading from literature before diving into calcs (SDI Manual of Construction with Steel Deck 2nd Ed; SDI NC-2017). The SDI Manual of Construction has one example on pg. 38 (see attachment) on penetrations that is somewhat similar to my case. They're just checking fb = M/S.
10. Using the section properties for the 20 ga. deck (unsure of what actually came on site), I went through the calcs that I attached in my earlier reply. I used DL = 75 psf and LL = 40 psf, which I believe may both be a bit conservative.
11. Tuesday around 9 pm - I sketched a rough arrangement of what I believed would be the best overlap condition for the 13 ft long deck sections. I walked over to the contractor's overnight crew and provided sketch to discuss. We talked through potential issues. One of their more experienced iron workers mentioned having recently worked on a bridge that had some mid-air lap splices. For that, they worked with an approved and inspected detail that had several deck screws installed at the lap.
12. Without me bringing up any further ideas/requirements, contractor proposed installing deck screws at every rib at these mid-air lap splices. Also proposed taking an angle (L4x4x3/8 or similar) and installing below the exposed edge of each lap. Angle would be oriented perpendicular to deck span and welded to truss bottom chords. No, an angle is obviously not the greatest flexural member, but it's what we had on site to work with. While not being able to take much serious load, I believe the angle would still help with limiting deck deflection at that mid-air lap. I thought their proposed ideas would be helpful.

Side note--the following morning, I checked on the deck gage with my main contact for the contractor, and he said the 18 gage actually did arrive in time. I asked for the receipt/slip on the decking, which I should be receiving soon.

This reply is already too long, but I'll gladly answer other questions. SRE, you are correct about 75 psf DL + 40 psf LL exceeding the allowable for 20 gage deck in Vulcraft's span tables. Like I mentioned above, I originally selected the deck size (18 gage) in May based on 65 psf DL (5.25 avg slab thickness) + 20 psf construction LL, but in my haste this Tuesday evening, I had forgotten that and used 40 psf LL instead. The calculations I did on Tuesday took into account actual vs. allowable bending stresses (f = M/S). I did not consider the Vulcraft span tables because I figured they were no longer applicable given my unique situation. I've been working 12-14 hours days this week to try to stay on top of issues as they come up and keep the workers safe. I have no interest in cutting corners or doing something unsafe. The parties involved on this project are myself (owner and designer), the contractor, and a certified inspection firm. Who else should I turn to? I haven't reached out directly to Vulcraft, but perhaps I should.

SRE, What other glaring issues do you see? I ask this sincerely; hopefully none of this has come across as an excuse or attempt to dodge accountability. I'm genuinely interesting in learning about what other issues you see, considering my fb = M/S values appear to be below allowable stress given 20 psf construction LL. What have I done wrong? I'm only 5 years out of college, so I obviously have not turned over every rock in structural engineering.

 

[SlideRuleEra]

RB - Just a quick note for now. I will read your post carefully this afternoon, I know you are well qualified and would not mind me being your Dutch uncle... again.

A heads up for now: Look at deflections, you have not done this. Deflection almost always controls on form work for an elevated slab. If deflection is ignored, control of slab thickness is lost, meaning load increases, which can cascade to structural failure.

More later.

 

[RattlinBog]

Understood--thank you

 

[SlideRuleEra]

RB - You are more competent than most engineers I worked with who were 5 years out of college. I know you have time pressure, so I may have to make more than one post. I'm quite familiar with "plant outages", we might schedule a month or two to take a 600 MW base-load generating station off-line for, say, a major steam turbine overhaul. At the same time there would be a half dozen or so unrelated "smaller" mechanical, electrical, structural, etc. projects that needed the down-time, too. I would likely have a Contractor performing one of those in-house designed projects. Bottom line: When turbine overhaul was complete, the rest of us had better be finished... that unit was going back on-line.

The problem you have is NOT your calculations (100 psf total load should be good for this project...reasons later) The assumptions being used are flawed. Designing "means & methods" is different, as I mentioned before:

1) Deflection controls, for the reason stated.

2) Use available material (either on hand, or readily available... you are already dealing with that).

3) Magnitude and Location of loads (like fresh concrete during placement) during construction cannot be accurately predicted. To compensate, form work has to be over designed (from the viewpoint of most permanent structure engineers). In your calcs, concrete dead load is "ideal"...assumed to be uniform and all-at-once in place. No unbalance loading, no provisions for point loads from workers walking on the forming deck or chairs supporting rebar mat.

I doubt 13" long, 1.5C-36, Grade 50, 20 Gauge can be made to work safely (Your 18 Gauge two-span appears to be perfect for this application). Even if cut to use as 7' simple span, deflection will be ridiculous... 100 psf >> 41 psf that Vulcraft gives for L/240 deflection.
If you lap the sheets per your calcs, deflection will ruin the neat point load assumption of the "long" cantilever bearing on the "short" cantilever. Your first set of calcs, allowing for the double loading error you caught, is probably closer to being accurate... and those calc's results are screaming "Failure", if I remember right.

Thoughts?

 

[RattlinBog]

I'm tracking with what you're saying and agree. Trying to keep my responses shorter for the sake of time. I'm going through some deflection calcs now (and of course I have 10 other projects screaming at me). As you've probably already figured out...the decking is installed and concrete pour is tomorrow... I may or may not have lost some sleep this week.

This is what I can say has happened in real life (right or wrong; just telling the truth): the decking has been installed as sketched. I believe it is 18 gage. At these mid-air laps, contractor installed deck screws at every single rib (I believe 6" OC). They also installed the L4x4x3/8 angle I mentioned earlier below the lap to try to take up some deflection. That's what we had on site to work with. I'm planning to run through a bending strength and deflection on the angle to see if it's doing anything. I hate single angle bending, but I'll go through it.

Is there a good way for me to estimate some strength value to deck screws installed 6" OC at the lap?

 

[SlideRuleEra]

For screw strength, use you could use the old-fashion method:

Calculate screw cross sectional area based on its' "minor diameter".

Assume the screw is nothing special and is made out of steel with yield strength of, say, F[sub]y[/sub] = 33 KSI.

Calculate "allowable" shear load on the cross section of the minor diameter. Allowable being 0.4 F[sub]y[/sub].

Of course you could exceed allowable, not have failure, and never know it. But this should give you some idea.

 

[KootK]

What holds up this angle?

 

[KootK]

Is shoring the deck not possible here? Given where your at, I'd be very tempted to just shore this one and sharpen your pencil on the next. Mistakes get made when your working under this much pressure.

 

[SlideRuleEra]

I assume concrete will be pumped.

Place fresh concrete on the simple spans first, as much as possible. Place concrete as uniformly as practical.

Stress to workers the importance on NOT piling a "deep blob" of concrete then spreading it out with concrete vibrators.

Don't allow anyone under the work area.

Only allow workers directly connected to the placement to be on the decking (no "observers").

Vibrate well and wet cure for 7 days.

 

[SlideRuleEra]

In my vocabulary, shoring is not the same is forming.
Shoring is support of forms that are suitable.
In this case, it's the forms that are questionable.