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Old 11-11-2004, 09:27 PM   #1
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Blind floor bolt option?

I have been working on a complete replacement of the floor on my 1973 31' AS. I have decided to use a product called Polyboard instead of plywood. But that is a topic that I have mentioned in other postings. What I have in mind here is to propose an alternative type of fastener for holding the floor to the frame. The factory original type of preffered fastener is the elevator bolt. There has been a lot of discussion about using self-drilling, self-tapping screws installed from the top of the floor. Check out the following post for some of what has been said to date:

http://www.airforums.com/forum...=elevator+bolt

In my installation I have not removed all of the belly pan and would prefer not to have to take it all off. That means that I need to use at least some blind fasteners. I have been reflecting on what options I have besides screws. It occurred to me that I might be able use a particular type of expansion bolt mounted from the top. I have included a sketch and a photo of the type of bolt I mean and how it would look installed. The bolt in the photo is not exactly the right size but I show it for identification of the type I mean. I would pick one that has a bolt diameter of 1/4" (the same as the elevator bolts). I would have to drill a larger hole through the floor and the frame to install it. It also does not have as large of a head as an elevator bolt. The question is would it be enough better than screws to be worth a try?

Malcolm
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Old 11-11-2004, 09:31 PM   #2
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Let me try again with the drawing...

The drawing was a bit too big so I had to re-size it and try again...
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Old 11-11-2004, 09:46 PM   #3
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Hey, Malcolm, I was just looking at your floor replacement pics and wondering how it was going for you.
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Old 11-11-2004, 09:54 PM   #4
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Malcolm,

I can think of three concerns and I would advise against it.
1. Where you have a floor seam, the fasteners will need to be staggered across the seam. The row of fasteners that are on the side nearest the web of the crossmember will be too close for the legs of the fastener to open fully. Kind of the same problem with installing Olympics too close to a rib.
2. The 'nut' on these fasteners is not a full thread form. I think if you put enough torque on these fasteners to get good clamping pressure, you might strip more than a few. And they can't be removed.
3. Unless you can find some with a hex head, I would think it might be easier to remove than belly skin than it would be to put all those fasteners in with a screwdriver. That's a close call, though.
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Old 11-12-2004, 03:43 AM   #5
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I agree not enough clamping force and probably will loosen up from the vibration.

How about rivnuts (might be easier to remove the belly pan) or floor screws that are used in semi trailer floors? They need a hole drilled but are self tapping and fine thread.

John
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Old 11-12-2004, 01:47 PM   #6
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I agree with the concerns...

Removing all the rest of the belly pan would indeed be easier in some respects. It is not just an issue of removing the belly pan though. The fresh water tank takes up two cavities of the under-floor area. I don't really want to have to remove it just so I can get to the bottom side of bolts. In large part its just the principle of the thing with me. I resent the idea of having to take things apart just so I can get to the nuts on the bottom side of the floor. I have no reason to take off the rest of the belly pan except for that. Also my driveway where my AS is parked has a slope of about 8" or 9" per 8'. I have the trailer jacked up so that it is level. This puts the back bumper at about 3' up in the air. The front part (where the belly pan is still installed) is at its more normal elevation. A bit too tight to work under comfortably. I don't really want to have to jack the trailer up any higher just so I can use elevator bolts unless there is no reasonably alternative. My jack stands are pretty much at their limit at the back end of the the AS now.

I agree with the concerns mentioned about there not being enough room for the expansion bolts at the floor seams. I also do not like the idea of having to drill such large holes through the top of the cross-members. I have also personally experienced frustration with this type of expansion bolt in the past where the nut does not seem to be all that sturdy. Doesn't anyone make one with a better nut? As far as the screw-driver slot is concerned one thing I have done in the past with this type of expansion bolt is to simply replace the original screw with a bolt that has a head of a type I can live with. Doing this of course would drive up the cost per fastener. I do think that the fastener is not all that likely to loosen up over time simply because the nut would be constrained from turning by the flange and teeth into the floor. If this were the only conern with the approach I would think a little lock tite or maybe some glue under the head of the screw would take care of it. Maybe even a lock washer under the head of the bolt would work. The bigger problem in my mind is about the hole size and the lack of room for the expansion.

Could rivnuts be installed from the blind side with the floor panel in place? Wouldn't they have a similar problem with the size of the hole that would have to be drilled? Also it seems like the sleeve of the exansion bolt does at least fill the hole drilled in the floor panel. I would think that the bolt into the rivnut would not if the hole in the floor had to be drilled large enough to clear the rivnut.

How about the relative merits of large rivits compared to elevator bolts or screws? I found some at a local hardware store that carries all sorts of hard to find items. I am attaching a photograph of two different ones that I sampled. They are both 3/16" in diameter. The one with the large flange head is aluminum and the other one is steel. These two are long enough to be used where the floor sits directly on top of the cross-member but would not be big enough to connect through the floor and shim down to the cross-members that sit 5/8" lower (where the plywood seams are). They are also available in 1/4" diameter but I am not sure if their stem would fit my air-powered rivit gun. If I were to use these rivits, where the floor seams occur my intent would be to fasten the shims down solidly and then use glue and screws to fasten the floor to the shims.

I did run across the type of screws mentioned for holding down truck floors. The salesman said that they are generally intended for connection into thicker material than the cross-members are made of. He said that the material would be something that would typically be thick enough to drill and thread a hole. He had some concerns about whether or not they would hold reliably enough in the thinner metal. I used Tapcon screws for much of my ill fated installation of a preasure treated plywood and they seem to hold reasonably well. I think they are intended for connection to thinner metal such as steel studs. My general installation policy was to install them at about 6" on center. I suppose I could install a strip of thicker metal under the top flange of the crossmembers to give more to attach to.

I do intend to use elevator bolts in at least some of the locations. Of course I can use them everywhere that the belly pan is off. That is the entire back half of the AS behind the fresh water tank. For the other part what I had in mind to do was to start installing my floor from the front and work back. I can use elevator bolts at the seam between sheets to hold down the first sheet and the shim below it. If I then glue and screw the edge of the next sheet to the shim that might very well be enough for that joint. I could also use Tapcon screws at that joint on the second sheet that are long enough to go through the shim to the cross-member for extra hold. I could probably reach far enough under the edge of each sheet to be able to put elevator bolts at least at the end of the middle outriggers. Maybe I can reach in far enough to install some in the middle part of the sheet as well. At least I can except where the fresh water tank is not in the way. So I would have elevator bolts holding down the sheets around mjost of the edges - assuming that glue and screws work at the seam for the second sheet. So then the issue is more one of what to use in the middle of the sheets if there is not access to the underside of the floor.

Stepanie - In answer to your question I have finally made the decision to use Polyboard on the floor. I have all the sheets on hand and am ready to go for the install pending what I finally decide about fastening the floor down (and the availability of enough time). I still have to take out some more of the preasure treated plywood - about 4 sheets are still in place. As soon as I get a little further along in the process I intend to publish the details. I have been taking photos for that very purpose. Installing a new floor is easier the second time around except possibly for the removal of the preasure treated floor. Since you live in the same general part of the world as I do you are welcome to come by and take a look sometime if you want. For that matter so is anyone else that would be interested. Just give me some advance warning to be sure I am here.

Malcolm
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Old 11-12-2004, 09:12 PM   #7
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I have a similar challenge for a different reason. I will be sealing the belly pan to reduce critter entry. Every opening will be sealed. Then I'll insulate. Then put the floor in. I'm planning on using self drilling wafer head screws,hopfully I'll find 1/4" and I'll epoxy with a filler between the frame and floor. No more "trailer" smell for me. On a 59 all the frame members are flush and I'm having extra cross pieces added by the welder.

Wafer head screw. http://www.stainless-fasteners.com/s...ing_wafers.htm
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Old 11-13-2004, 10:09 AM   #8
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Screws

This page may be helpful for those needing to blind attach flooring.
http://www.compassintl.com/section2/dartsrule.htm
Note the greater than 1/2 issue and the solution.

and this

http://www.compassintl.com/section9/...rts.htm#k-lath
or how about this.
http://www.grabberman.com/slfdrill.htm

While those with restoration values will want elevator bolts I think there is potential here for those of us "rebuilding".
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Old 11-16-2004, 01:20 AM   #9
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How about the long rivits?

No one has said anything in answer to my question about using large rivits to hold down the floor. Does anyone know if there is a chart around that compares the reletive merits of the different kinds of fasteners I have mentioned here and that we have discussed from time to time in the variuous postings about floor repairs? Information on the holding power and pull out strenths for each type of fastener would be interesting. I, for one, would also be interested in knowing if anyone has any information as to what kind of design loads the fasteners have to resist in doing their job of holding the floor in place. Noing that it would be easier to decide how many fasteners would be needed of each type.

I also discovered another type of blind fastener that could conceivably be used to hold down the floor. Take a look at the following website:

http://www.toggler.com/newproducts.html

This brand of fastener has evidently been around for about 30 years. The one shown on the above site is just an improved version. They are available to take a 1/4" diameter screw or bolt (you supply the screw or bolt of your choice). The catch is that you need a 1/2" diameter hole to install them. I saw a chart that said that the 1/4" diameter model had a 1080 lb tensile pull out value when used with concrete block walls. It is also available in stainless steel. What do you think?

Malcolm
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Old 11-16-2004, 08:15 AM   #10
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Malcom,
Those are what the fire extinguisher guys use. They hang some big extinguishers with them. Those with truss head bolts/screws would be a great idea!!
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Old 11-16-2004, 09:18 AM   #11
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Malcolm,

I'm not sure you will be able to find information that is specific to your application. I have looked in several places for that type of information, especially in articles on aviation (FAA has some good articles on plywood airplane construction) and boats (Coast Guard regulations).

I think you don't want to depend too much on construction techniques for buildings, they are not subject to the same dynamic loads you will see in a trailer.

With that said, I don't think that pull-out strength is a useful metric in this application. What you are interested in is the clamping pressure that the fastener applies. Can your fastener apply the necessary clamping pressure, without stripping out? How does the fastener lock in place so that it doesn't loosen up over time and with vibration?

The factors that work well with plywood might work poorly with polyboard. I used 1/4" elevator bolts on 6" centers, with split washers, in plywood. This works fine on plywood, where the plywood compresses slightly. With polyboard,you may need to find a method to make ensure the toggles don't loosen up over time.

If you decide to use the toggles, maybe you could use them in combination with self tapping screws? That way you are not putting all your eggs in one basket.

Another method, which is more work but might be cheaper, is to make a nut-plate for the cross members. In this method, you get some 1 1/2" x 1/4" steel strap, and mount it under the top flange of the crossmember. Drill lead holes through the polyboard, crossmember, and strap. Take the polyboard up, tap the crossmember and strap, and drill a clearance hole in the polyboard. Then you can screw it down with grade 5 screws and loc-tite.

Just an idea.
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Old 11-16-2004, 03:55 PM   #12
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Rivits vs. bolts, etc.

Don,

There are obviously lots of ways to go for fastening down the floor arn't there?

Is it your oppinion that rivits would not have enough hold-down strength?

Maybe I am a bit naieve about this but isn't the pull out strength related to the hold down? Do you think the Polyboard needs to be pulled down really tight or is it OK to just pull it down snug with a fastener that will not let it pull away up to some pull out force?

I thought some about your idea of putting a plate under the top of the cross-member top flange. I think that 1/8" thick strap would probably work OK. I am thinking that because of my experience with the 2" x 2" x 1/8" angle that goes across the middle of my fresh water tank. I am using self-drilling/taping screws there because I did not have enough clearance to get a nut in place for the elevator bolts without dropping the water tank. That is one tough piece of steel! It is hard to drill the hole to begin with and the screw really grabs and pulls down tight. If you want to thread the hole you could probably do it from above without removing the plywood or Polyboard couldn't you? Also the screws that I saw at the hardware store that are typically used for fastening down truck trailer plywood to the frame of the trailer are pretty beefy and come with a self-drilling/taping tip. It seems like as long as one picks the right type of self-drilling/tapping screw that it should not be necessary to do a separate drilling and taping operation.

If I envision a process where I drill holes in the Polyboard and then remove it to tap holes or whatever then presumably I could add some sort of nut plate or captive nut to the existing cross-member couldn't I? That could result in a nice weight savings over a full size strap. Maybe squares of 1-1/2" x 1/4" steel would be good for this. Mark where the holes are somehow when drilling the Polyboard so as to not confuse them with old holes, take off the Polyboard, clamp the square in place and drill/tap the hole. Either hold the plate in place with a rivit or perhaps some type of glue. I think there are also nut inserts available that could be attached to the cross-member if you know where to put them. I must admit, though, I am not anxious to move the Polyboard sheets around any more than necessary.

The Polyboard does compress some when I tighten down the elevator bolts. I am using the nylock type of lock nuts on the bottom side and hoping that will work. I would think that I could use loc-tite on the toggler bolts if it made sense to go that way.

I am still wondering just how much hold-down we need for an AS floor.

Malcolm
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Old 11-16-2004, 06:12 PM   #13
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Malcolm,

I think using rivets might be a good idea, if you used 3/16" steel rivets. I would put in a lot, but they would be pretty easy to install.

About the difference between clamping pressure and pull out, I think the test that they used was to determine when a concrete block failed during pull out. Doesn't apply to polyboard and steel. I just don't think the data is useful. The clamping pressure should be enough so that the board doesn't slip as the frame twists and flexes. That might be tough to do with the polyboard, if it has a low COF/coefficient of friction against the steel crossmember. You might consider using something to improve the COF of the polyboard, like a strip of butyl tape.

About the nut plate, I agree that 1/8", added to the thickness of the crossmember would be enough to get the thread depth you need. The problem, as I see it, is that the crossmember are too thin to get a good thread. Truck trailers use thicker crossmembers. By adding 1/8" to the existing crossmember, you're just trying to get into the range of truck trailer thickness.

On self tapping screws, I would say go ahead and use them, but drill a pilot hole first. The big beefy ones used by trailer manufacturers are installed with some pretty hefty nut drivers. You definitely can do it if you drill a pilot thats the same or slightly smaller diameter as the tip of the screw. If you try to do it without the pilot, the polyboard will lift up and jam against the washer as soon as you hit metal. With a pilot, it will zip right through.

The main reason to tap holes separately is it gives you an option to use whatever bolt hardware you choose. You could use grade 5 or grade 8, or flat head screws, or even hot dip galvanized. Just more choices to look at if you don't care for the style offered in the self tappers.

As far as using a single strap nut plate, or lots of smaller pieces, I would use the long strap because you would only need two rivets to hold it in place and wouldn't need to cut a bunch of pieces. Using small pieces would be cheaper and lighter.

You could drill and tap, then counterbore the polyboard without needing to lift it off. In fact, the polyboard would give you a good way to start the tap, and keep it straight and on pitch when it got to the metal. You could tap these holes with a slow speed drill motor, doing them by hand would be pretty tedious.

As for how many fasteners to use, I used a few more than the original builder. It sure didn't move in 45 years, so I think it stood the test of time.
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Old 11-16-2004, 08:33 PM   #14
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How many fasteners were there?

Don,

How many fasteners were there in the original? My unit seems to have been modified somewhere along the line so I don't have a good feel as to what the origianl equipment was.

Somehow rivits would seem like a satisfying way to go just because there are so many in other parts of the structure. My guess is that AS would have used rivits themselves if they thought they would be strong enough but who knows for sure. In theory the Polyboard could actually be used as the finished floor too if someone wanted to. It could be kind of classy to fasten the whole thing down with neat rows and columns of rivits and let them all show. Maybe 3" on center would be enough.

The Polyboard is indeed a little more slippery. Actually this was a nice feature when it came to slipping the front curved piece in on the diagonal and rotating it into position. The presure treated plywood needed a fair amount of persuasion with a block of wood and a hammer. The Polyboard slipped into place with a lot less. By the way it also seems tougher relative to tapping on the edge than the plywood did. I am also very pleased that there is no splintering around screws. In general I have thought that enough fasteners through the Polyboard will keep it from slipping at least partly because of all that shear value on all of the fasteners. Of course cranking it down tight as much as possible should help too.

I have used some smaller Tapcon brand self-tapping screws in a few places with the Polyboard and they actually seem to do pretty well without a pilot hole. I think this might be partly because I had already installed the elevator bolts along the second edge of the panel and the decking screws and glue along the shims and splines. The Polyboard was already held pretty tight by then. The Tapcon screws also have a couple of small tabs down near the drill bit that I am told are supposedly for the purpose of slightly enlarging the hole in the Polyboard as they are installed. They are supposed to break off when the screw starts to grab. What I wonder is where they end up. The couple of bigger ones I used on the angle iron over the water tank would definitely have profited from a pilot hole. I will give that a try.

By the way, thanks for all the feedback. It is nice to be able talk over the various options and the relative merits of each.

Malcolm
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