Bending & shear in a round bar

[RHTPE]

I am involved with proposing an alternate connection between an architectural concrete facing and a structural wall, with 2" of EPS insulation and a waterproofing membrane between. This is a cast-in-place "sandwich" wall. Rather than use the double-headed shear studs spec'd by the EoR, I'm trying to integrate the concrete-to-concrete connection with the form ties.

My connection uses 1/2" coil rod screwed into the coil ties used for the structural wall, and then connected to coil ties used for the architectural facing formwork.

Critiques of this design (the sandwich wall) are unneccessary - it is what it is and I cannot influence it.

Each 1/2" coil rod (root diameter = 0.42") carries 586 lbs of vertical gravity load (the weight of the facing). The yield strength of the coil rod is 110 ksi. Assuming the rod to be fully fixed each end by its embedment into the concrete, I figure the maximum bending moment to be 586 in-lbs, or about 0.73 Fy. Shear is of little concern (0.03 Fy). From AISC 8th edition I find allowable bending stress to be 0.75 Fy.

My question for the group: Am I missing anything here?

My familiarity with AISC 13th edition is somewhat limited.

Ralph
Structures Consulting
Northeast USA

 

[hawkaz]

A sketch might help.

If I understand this correctly- you are trying to cantilever these bolts off the structural wall to support the suspended facing panel for gravity loading. If this is the case, you need to treat the connection at the facing panel as free instead of fixed.

If you have a 2" gap, your moment arm would be 2" plus half the suspended panel thickness. If we call this a 2" panel- your moment would be 586# X (2" + 2"/2)= 1758 in-lbs.

If I understand your concept correctly, you will need significantly larger bolts or space them much tighter.

Also, don't forget to check deflection.

 

[ron9876]

Don't forget lateral loas.

 

[RHTPE]

Attached is a picture of the completed anchor.

I still maintain the ends of the rod are fixed.



Ralph
Structures Consulting
Northeast USA

 

[jdgengineer]

I think in general your approach sounds reasonable. Considering how close you are to the capacities, I would make sure to check according to AISC 13th edition to make sure nothing changed.

I agree with others make sure you include lateral loads (both in-plane and out-of plane). Although I suspect the vertical loading may be the worst case loading.

Make sure you have enough embedment to develop this fixity. I think that if you do have enough to call it "fixed", it seems to me appropriate to call it fixed at both ends. It's similiar to a moment frame with a very rigid beam (essentially the concrete panel is the very rigid beam).

 

[dhengr]

RHTPE:
Look at what precast wall panel (tilt-up) people do, many of those panels have a structural back section, embedded foam insul. and face shell as one unit. And, many well caulked movement joints and some face shell crack control reinf’g. I think Hawkaz has it about right if those rods work only as small cantilever rods, and this must be studied as to their shape and bending cap’y., not just .73Fy. If it is all cast as one unit, I suggest two other modes of load and weight transfer: (1) ext. face weight will be transferred in bond and shear through the rigid insul., assuming you can trust the integrity of the bond and foam over the long term; what is the shear and compressive strength of the foam insul.; (2) if the rods act as cantilevers, any deflection quickly brings the face shell into much higher compressive contact with the foam insul., again a shear transfer through the foam to the structural wall section, and now the rods are acting less as just cantilevers, and more as tension struts or ties supporting the weight by pulling the face shell into the foam. Obviously, these tension ties would be more effective if they could be installed at 45°, pointing down and into the face shell. Small square/rectangular panels of this system could be easily tested to gain some confidence in the tie cap’y. Pay some attention to expansion movement of the outer shell w.r.t. the rest of the panel.

 

[msquared48]

RHTPE:

For the record, the connection makes me nervous. I don't know why. It just does. I guess I'd have to have more informatin and run a few numbers to alleviate that feeling. I keep thinking fatigue loading limits and no backup.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RHTPE]

To all thus far:

This is a proposal for an alternate method of construction that attempts to utilize the form ties as the means to connect the architectural facing concrete to the structural back-up member. The perspective is from that of the contractor who has to construct both the structural back-up member and the architectural concrete facing.

The architectural concrete facing formwork requires some form of tie to the substrate, otherwise this will become a 19'-20' tall one-sided concrete form. The goal is to minimize the required perforation of the form panels for the initial structural formwork and the subsequent sealing of those perforations after installing the spec'd double-headed shear studs. In most instances the facing (at its bottom) bears on high-density foam on the 30"+ foundation mat.

The facing is reinforced with #4 at 12" c/c EW.

I'm not thrilled with 5 SF per 1/2" rod. It is likely that I will cut that down to 2.5 SF by cutting the horizontal spacing in half.

dhengr - I understand your thought about the 45° tie, but that's just not appropriate for the conditions of construction. I agree with your 2 modes, but at this stage of the game I'm trying to keep it as simple as possible. Any alternate method will require the EoR's blessing, as well as the Architect's energy consultant. They will likely do an analysis of the heat transfer through the rods as has been done for the 2" thermal break in the 18" foundation walls.

Mike - For the record, it makes me nervous as well. However, I'm in the unenviable position of trying to come up with a solution that satisfies both constructability AND the permanent connection condition. I'm open to any suggestion that will work well with the formwork challenge.

Attached is the sequence of construction as I see it, along with details for supplemental anchors where ties are not required by the formwork.

Ralph
Structures Consulting
Northeast USA

 

[hokie66]

I must say it is VERY unusual to rely on ties to support a veneer, as veneers are always, in my experience anyway, supported by direct bearing or shelf angles off the structure.

If the support has to be done this way, I would want the connectors and rods to be corrosion protected...either hot dipped galvanized or preferably stainless steel.

 

[dhengr]

Ralph:
The cantilevered rods will deflect, and my two conditions will come into play, there’s no getting around it, it’s a matter of magnitude of each condition; and of relieving (sharing through another load path) some of the bending load on the rods, at some deflection level. When the Arch. and EOR make things difficult and ask for the near impossible, they shouldn’t go out of their way to make it absolutely impossible. The real issue is the durability and longevity of the foam insul. and water barrier material, their strength and possible deterioration (creep, crushing?) and loss of strength. I actually only meant the 45° bars to indicate an ideal condition, otherwise I understood they wouldn’t serve both functions. But, if you could place those bars in vert. lines which were also a foam width module, you might just place the foam vertically btwn. these lines. Maybe your outer structural wall forming could be to that same module, simplifying form penetrations of my 45's and your ties. The Arch., EOR and contractor can’t all have everything they might wish for, giving you no or little help in accomplishing what they want. Just tell them to glue another 2" layer of rigid foam over the exterior face of the Arch. conc. facing and that’ll solve their heat transfer problem, and the contractor can give them a break on the exterior finish quality too.

Those coil rods are typically intended to be loaded in tension, although I’m sure they will take some bending too. They do have some nasty stress raisers as relates to bending and shear stresses. And, their bearing stress will be very high right at the conc. surfaces, in transferring the loads, and probably some play in the coil inserts too. Again. whatever you come up with should be fairly easily tested for load and deflection (movement, x, y & z) using a factored test load which gives all parties some confidence. You are not looking for exact magnitudes (percentages) of each load path. You want to show a FoS of 2,3,4 with an acceptable movement. Then get signed copies of the foam guarantees for your file.

 

[msquared48]

In looking at the details further, I would also have to question whether these anchors can be used in a permanent connection application.

Remember that these are form ties - a temporary tie, and the form at the backside of the main portion of concrete wall is removed in the final state, so that the only tension resistance is the embedment and any bond of the anchor into the concrete to hold the Architectural concrete. I would have to check this application out real closely with the form tie supplier.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RHTPE]

Mike - I know exactly what these ties are - I specified them. The detail I proposed does not depend purely on bond between concrete & the tie. The portion of the tie remaining in the concrete will help the coil rod to behave as a headed stud embedded in the wall. My primary concern is the combined shear & bending behavior of th rod.

Ralph
Structures Consulting
Northeast USA