Aluminum sections used in concrete formwork - problem

[RHTPE]

Merry Christmas & Happy Holidays all.

The attached pdf shows an application of an aluminum extrusion used in concrete formwork & shoring. My client has asked that I evaluate the application and comment on methods to make this work as depicted. This has been used as shown underneath 6 deck placements. During the 7th placement, the assembly (without the fitted sideplates) rolled over and dropped the deck about 2.5 inches.

I have devised means to counter the roll-over by installing bridging between 2 of these members adjacent to their supports. This member is continuous over 4 or more supports, and maximum moment is negative at the first interior support. I have determined that the permissible shear strength for ONE member is 4,200 lbs (from the 2005 Aluminum Design Manual, and the calculated shear for the assembly under design load is approximately 9,520 lbs. The maximum shear can be reduced somewhat from the 9,520 lbs, but probably not less than 8,000 lbs or so.

The goal is to devise an acceptable (and reasonable) means to strengthen the member as depicted for the one (out of many lesser) case where loads are highest. Remember this is a concrete formwork table used many times, BUT it not permanent construction.

Question 1: Since we cannot consider this completely "composite", most properties are simply the sum of (2) joists - do you agree?

Question 2: Since the assembly is not truly composite, and since the "shared" load must be transmitted to the shoring leg through the lower joist, we can only consider the shear capacity (maximum reaction) to be that of only ONE joist (the lower one) - again, do you agree?

Question 3: I initially suspected lateral torsional buckling (LTB) of the assembly to be the cause of the roll-over. Now, while I suspect that while LTB may have contributed to the problem, I am just as concerned about the member's shear capacity at the shoring legs. Do you think that installing a vertical plyform "side plate", fitted between the top & bottom flanges, on both sides, would dramatically increase the permissible reaction of the "stacked" joists? The side plates would be held in place by (2) small self-drilling/tapping screws thru the plyform into the wood nailer "box".

My goal is a workable solution that can be easily implemented and easily understood by those constructing it.


Ralph
Structures Consulting
Northeast USA

 

[beton1]

Hi Ralph, Merry Xmas and ...:

My initial thought is to install the plywood to the upper
member as well; and then one deeper plywood section to each side which would enable the two sections to be fastened and act in a more composite manner. Not sure if this works dimensionally or how practical it is. You are basically stating no other section is a viable substitute, you are tied to working with what is available and built?
You may want (or need) to add a thicker section of lumber or engineered lumber at supports where more critical.

 

[BAretired]

Question 1: Since we cannot consider this completely "composite", most properties are simply the sum of (2) joists - do you agree?

No. It is less than the sum of two joists because the top chord of the lower joist is not laterally braced.

Question 2: Since the assembly is not truly composite, and since the "shared" load must be transmitted to the shoring leg through the lower joist, we can only consider the shear capacity (maximum reaction) to be that of only ONE joist (the lower one) - again, do you agree?

No. It is not that simple.

Question 3: I initially suspected lateral torsional buckling (LTB) of the assembly to be the cause of the roll-over. Now, while I suspect that while LTB may have contributed to the problem, I am just as concerned about the member's shear capacity at the shoring legs. Do you think that installing a vertical plyform "side plate", fitted between the top & bottom flanges, on both sides, would dramatically increase the permissible reaction of the "stacked" joists? The side plates would be held in place by (2) small self-drilling/tapping screws thru the plyform into the wood nailer "box".

I wouldn't call it LTB. I would call it buckling of the top flange of the lower joist.

Vertical plyform "side plates" would likely have some beneficial effect, but they do not prevent lateral buckling of the top chord of the bottom joist, so they cannot be considered a valid solution to the problem.

BA

 

[Ron]

I see several issues with this configuration:

1. There is a hinge at the neutral axis, so when subjected to torsion, fast failure is more likely.
2. There are no shims shown between the plywood and the bottom narrow extrusion section. As the device gets more and more use, this will cause the plywood to lose its effectiveness.
3. Contractors treat formwork pieces like crap! The repetitive use of the device will give it a short life.
4. The "clamps" are obstructions to proper bearing, and create localized concentrations if the formwork bears on them.
5. Aluminum is very strong, but also subject to localized deformation. Be careful of connections.
6. I believe section to be poorly conceived for the application.
7. There are not enough fasteners to make the plywood work well and repetitively.
8. The failure on the 7th use is a tip-off that the system weakens with repetitive use or gets damaged with repetitive use contributing to failure by the high margin loads.

 

[beton1]

Ralph has stated that he has introduced some horizontal support. Probably, it does need more (as BA has pointed out the lack of top flange support contributes to buckling out of plane).I believe the system worked initially(as a lot of things that mystify me) probably because they used it before but got sloppy or did not check alignment.Inherently it is not the system a engineer would come up with. If the two members get out of line vertically or there is unbalanced load (concrete loaded or dropped more to one side)then the situation becomes one that is prone to buckling or roll over.I think Ralph is looking for a solution to get this job completed. If there are a limited number of pours left, then the idea is to use what is existing and modify it safely. If this is the start of a large project or there are numerous pours left, then I agree a different system is required.

 

[cntw1953]

Ralph:

A plan view for the system with loads indicated will better the understanding.

 

[RHTPE]

beton1 is correct with the presumption that the contractor is looking for a solution to get this job completed. I am not sure on the number of pours left, but the goal, if possible, is to use what exists and modify it so that it will perform safely.

Unfortunately, this concept was originated by field personnel and incorporated by a P.E. into the shoring drawings.

Formwork for horizontal concrete is always a bit dicey in that the loads are never applied instantaneously over any member and imbalance will occur, albeit for a short period of time.

The unique aspect of the placement that precipitated the "oops" is that it is the one that applies the highest loads to all members of the deck table. See the attached for a sketch of the table and the concrete it supports.

Ron's points are very astute and a very accurate critique of the application, but some of intent of the first sketch was not explained thoroughly by me.

1. There is a hinge at the neutral axis, so when subjected to torsion, fast failure is more likely. I agree, but I have a corrective measure to counter this. Sketch to follow for those interested.
2. There are no shims shown between the plywood and the bottom narrow extrusion section. As the device gets more and more use, this will cause the plywood to lose its effectiveness. Perhaps. The plyform side plates will exist only over the supporting shoring legs. I am considering the use of epoxy to help secure the plyform to the aluminum joist.
3. Contractors treat formwork pieces like crap! The repetitive use of the device will give it a short life. In general, I agree. This particular contractor has a better grasp of the maintenance necessary for repetitive use.
4. The "clamps" are obstructions to proper bearing, and create localized concentrations if the formwork bears on them. Not true. The clamps are situated so that they do not interfere with shoring legs or the supported joists.
5. Aluminum is very strong, but also subject to localized deformation. Be careful of connections. Agreed.
6. I believe section to be poorly conceived for the application. Perhaps, but it is what it is.
7. There are not enough fasteners to make the plywood work well and repetitively. See item 2 above.
8. The failure on the 7th use is a tip-off that the system weakens with repetitive use or gets damaged with repetitive use contributing to failure by the high margin loads. Perhaps, but I believe that the higher loads applied during the 7th use are the prime contributor.

My weekend will be consumed by an attempt to come up with a reasonable solution. Part of that solution will be to remove some of the load applied to the table.

I will share my conclusions here. As always, I appreciate the input from everyone who participates here.


Ralph
Structures Consulting
Northeast USA

 

[Ron]

Ralph,
Thanks for the interesting, practical problem. Have fun with your working weekend...I'm doing the same.

I like your idea of the epoxy to stabilize the plyform. It could also be used to fill that gap.

Merry Christmas.

Ron

 

[cntw1953]

Ralph:

I suspect there are two weakness in this built-up beam. The first has been identified - hinge at the N.A. The second I think has something to do with shear deformation.

While the clamped rods hole the assembly (beam) together, they do not contribute, or very little, to lateral deformation. The solution is to provide horizontal braces from beam to beam, if it is not possible, at least you should find a way to box the beam (attach angles to top & bottom plates, then mount side plates). Even scattered throughout the beam, the boxed segaments will provide improved rotational stability.

Hope the above make sense.

 

[doka1]

While I am engineer/superintendent for a concrete contractor and have never tried to use aluminum beams doubled up. Is the contractor using the smallest braces available for the scaffold type frames)...but if i were to design something it would be along these lines...
Use three pieces of coil rod horizontally in conjection with some sort of blocking made out of wood or steel stock. To do so, Nail half inch plywood on top of the first beam, place notches every 4' or so. These notches would allow for half inch coil rod, similar to the one that you are probably using for the clamp. Do the same on the top (the plywood would thereby relieve the bearing concern. The blocking should fit inside the web of both beams. I would probably make it out of 7/8 formply, double it up so the coil rod goes between the two 7/8 plywood molds. You would then have three points that its being stabilized from, essentially creating a "strongback"(formwork term).
In case some you didnt know, the "x" block is actually a plastic/wood nailer piece so you could gain some stability by nailing a block of wood at the end)

 

[beton1]

If I understand the new sketch, the "double" beam is essentially a beam/girder, continuous over 5 spans. This is a stand alone table, ie there are no adjacent similar tables? If there is bracing placed horizontally as stated, it would only resolve 'interior' side buckling; still possible to buckle out. I think Doka1 has a good idea to connect the double joist unit as one, but at this stage can you do this type of remedial work? If you can, perhaps a long continuous coil (scru rod?) can be installed out to out (both beams) but with nuts and washers to clamp both inside and outside of each individual set of double joists (c/w blocking). Still keep bracing on the inside, perhaps at shore locations.

 

[DTGT2002]

Merry Christmas, Ralph!

Sorry to not have replied sooner, though I don't think I have any Christmas miracles to offer.

While I have made some pretty crazy rigs with formwork materials, any time I have tried a clamp like you were showing, it has been solely for lifting purposes of a ganged member, never for an attempt to align flanges for bearing.

Frankly, counting on the upper section of the aluminum top hat to transfer the maximum load gathered when two sections are added together gives me great concern. I have seen the edges of the flange split and separate through typical use (though most likely damaged during stripping operations).

As far as a bandaid, I do think you are headed in the right direction with a plan that makes all efforts to keep the flanges off the nailer aligned. Depending on the manufacturer, there may be sheets outlining the max interior reaction allowable for the top and lower flange independantly. These may just be resolved as the lower bound of the two or even the published allowable shear, but it may be worth following up with the manufacturer.

If this were on one of my projects, I would have them replace the top, nesting the two beams within a shared head, one upright and one inverted and deck back over with a perpendicular layer of purlins to get back to the desired height, BUT you are trying to work through a solution in place.


Ick.

I would guess with a 2 1/2" drop in the deck that the beams slid such that one beam dropped into the nailer slot of the other as the slot opened up and the beams became offset. Arresting this movement is your key, and I might be tempted to secure the flanges of the opposing beams together with self tapping screw to accomodate this in addition to your plywood "gusset" and kickers/blocking between adjacent stringers.

As for capacities, I generally take an arbitrary 5% capacity knock for the simple fact that while aluminum extrusions generally run true, the profile of one vs the other will not match 100% and one member or the other may begin accepting load before the other.


Another thought would be to cut 4x6's or 4x4's the right height to fit between the bottom and top wider flanges (bottom flange in typical installation) and secure these at frequent intervals using endless webbing straps or preforated banding iron, to resist any lateral internal movement of the flanges by forcing the wood to rack in order to shift to the side.

I would also consider adding an "underslung" spreader to the top and bottom flange bolt slots to add another force trying to keep the beams in the desired location.

That said, alignment and capacity of the upper top hat flanges is the key for me and this is certainly a challenge. I am not sure if there is a reasonable, cost efficient solution.

Hit me up if you want any other thoughts.


Daniel Toon

 

[RHTPE]

Okay folks, here is the qualitative solution to modify the existing deck formwork tables to insure better behavior. The exact design calculations will not be shared, as they consist mostly of many iterations of conventional analysis (rather than than a textbook solution), combined with a heavy dose of practical experience and a positive intuitive sense. As well as a reduction in the total load applied to the table achieved by installing additional shoring underneath the slabs.

This is the "almost" final version, subject to alteration because of field input and my final calculations.

As always, I appreciate the input offered and Happy Holidays to all!


Ralph
Structures Consulting
Northeast USA

 

[cntw1953]

Ralph:

Impressive! Happy Holidays.

 

[DTGT2002]

Looks like a reasonable take on it. Enough work to alter the future mindset.


Is the lower purlin spliced at the head without the struts (end of 21' member)?


Showing that you can provide upwards of 2%-5% of the applied dead load at the supports is a good start. I would think locking in the tips of the upper beams at the "splice" would probably be prudent. But, I do think you have a solution in the works.



How many more pours on this thing?


Forensic formwork engineering sounds like quite an adventure.

Sorry I missed your call. I am working to use up vacation days and will be in a half day today and off the rest of the week, barring emergencies.




Daniel