Cantilever or not? 1

[RHTPE]

Good morning all.

The uploaded .PDF shows a condition for which I have to review the design. The dbl C5 is being suspended from an existing precast box beam and will support the bridge deck concrete placement.

I maintain that, since the dbl C5 cannot deflection upward between the hanger rods, the design model shown at the bottom is correct. The left hanger rod carries all gravity load.

If one were to assume a support about 1" to the right of the left hanger rod, then the shear between it and the left hanger rod becomes astronomical, as does the tension in the hanger rod.

Am I missing anything?


Ralph
Structures Consulting
Northeast USA

 

[rb1957]

and the beam would become a simply supported beam, with an overhang. and this'll have bigger deflections (and smaller reactions) ... depends on what's important to you

 

[MaddEngineer]

It's amazing how this seemingly simple assembly is so difficult to analyze.
The spring stiffness of the threaded rods, the bearing stiffness of the concrete to keep the backspan from arching up, the stiffness of the beam, the pretension (clamping) loads in the bolts, etc.

Seems the easiest solution is to have the contractor install two bolster pads (timber or steel) adjacent to the left and right rods and then you have a clearly defined backspan.

The other alternative is to load test the assembly.

Also, the aluminum shoring joists will also distribute load to adjacent undersung beams based on spacing. We don't know all the information concerning the assembly so it is difficult to fully comment... i.e., spacing of underslung carrier beams, total load imposed, size of clamping rods, etc.

The contractor probably uses this detail all the time and probably would think we're all nuts for questioning it anyway.

Just one more comment... BA may not be that off by stating to use the full backspan, since, the clamping force in the rods may actually cause the underslung beam to camber downwards between the left and right fixings and the canteliver to camber up initially, just a little more kindling to add to the conversation.

 

[RHTPE]

Okay folks, I caved and revised the detail to better match a sensible design model. Please see the sketch. The left shim between the top of the channels and bottom of concrete beam is to be removed prior to placing concrete on the deck - it is only there as a erection aid.

If I had the resources, I'd love to do a load test to determine failure mode.

To respond to some of the comments:
- Deflection is not of concern, either as a "pure" cantilever, or as a simple span + cantilever.
- Realistically, the contractor's guys will no doubt tighten the S#%& out of both hangers, thus eliminating rod elongation as a concern IF it were a "pure" cantilever.

It's the simple ones that we sometimes look act and realize that it ain't always so simple. If we only knew as much as we think we do ....


Ralph
Structures Consulting
Northeast USA

 

[BAretired]

Maybe the shims should be moved to miss the chamfer.

BA

 

[RacingAZ]

One of the very first thing I learned when I was just starting out as a structural designer is to avoid any hanging connection whenever possible for primary structural elements. Hanging connections where the bolts are in tension should be a last resort type of thing that needs to be scrutinize thoroughly. It's very easy for the fabricator to fabricate those bottom plates with an oversized hole and you'll be in trouble.

Maybe a better connection is a steel angle bolted or epoxied to the sides of beam 1 to pick up the channels.

 

[RacingAZ]

I didn't realize this is just a temporary structure so my concern might not be valid at all.

 

[rb1957]

what's the moment on the left hand end ? sure there's some structure shown, but isn't there a down load associated with it ? so the (what looks like) two fasteners attached to the beam react load and moment ??

"it is only there as a erection aid." ... "it" refers to the shim or the beam ?

IMHO, the shims at the hangers only simplfy the analysis; i think the guys building this are going to say "wtf?". are there alignment issues ?? does the beam have to be at a certain height to do it's job ? could this be a reason for shims ??

also IMHO, don't "cave" to our opinions. we could be a bunch of lunes. if you accept our advise, that's different "good points guys, things i hadn't consiered" but not "ah geez guys, i'll do it your way 'cause you'll only carp endlessly if i don't".

 

[spats]

I agree with rb1957's assessment. I think everyone is over-analyzing this with the "beam can't deflect up" arguement. All you need is a resisting couple to make the mechanism stable. The left rod balanced against the channels pressing against the bottom of the beam provides this. Using shims concentrates the compression on a smaller area, which may/could overstress the concrete in bearing. I think it's much better being tight against the bottom of the beam, and distributing the load.

This can't be as complicated as everyone is making it. As engineers, we make design decisions every day based on assumed mechanisms and structural models. As long as the mechanism exists, it's OK to use it, even if it won't necessarily act exactly the way you assumed. If you design your assumed mechanism properly, it can't fail.

On the other hand, I'm surprised no one is concerned about torsion in the concrete beam.

 

[miecz]

My mentor here used to say to me: "Don't spend a lot of time to solve the problem, just eliminate it." With the channel flush up against the bottom of the box, the system is very difficult to analyze. In fact, I couldn't do it. One simply needs to size a shim so it doesn't overstress the concrete in bearing, and you have a system that's easy to analyze, build, and defend.

 

[RHTPE]

Sometimes we fall into an over-analyzing trap. For this condition, the principles concerns are:

1. Falsework safety. Will this temprary structure safely sustain all anticipated loads? Of particular concern is the tension load in the left hanger rod, as it will likely exceed the vertical loads on the cantilevered end of the dbl C5s.

2. Falsework performance. Primarily deflection that will be reflected in the permanent structure - the cast-in-place bridge deck overhang.

Both of the above are directly affected by the actual behaviour of the dbl C5x6.7s. Certainly modeling the C5s as a "pure" cantilever will give better deflection & stress numbers than will a model as a simple span + cantilever. Our role as engineers is to ensure that our analysis & design model can constructed and will behave as modeled.

Yes spats, torsion could be a concern. Analysis of that issue needs to rest with the bridge designer, not necessarily the falsework designer. But what is not clearly reflected in my sketch is that the load applied to the right edge of the precast beam by the stay-in-place metal deck will mostly offset the CCW rotation imposed by the suspended falsework.


Ralph
Structures Consulting
Northeast USA

 

[rb1957]

i don't understand your terminology "simple span + cantilever"

as drawn (the 2nd time around) the beam is a simmply supported beam. as drawn originally (and what prompted yr question in the 1st place ... aren't you sorry now !?) i think it is a cantilever, with a distributed reaction along the bearing face.

 

[hokie66]

rb1957,
In terminology used in buildings, it should be called a propped cantilever. A simple beam (or simple span) has a pinned support at one end, and a roller at the other.

miecz,
I strongly, strongly endorse the argument made by spats. Analyze the right support anywhere you want to, but the beam will deflect less if placed hard against the concrete soffit. I don't think that violates the principle which your mentor was trying to impart.

 

[rb1957]

odd and odder ... the original sketch looks nothing like a proped cantilever to me, the later one "could" depending on what's happening on the left end of the beam ...

 

[miecz]

hokie66-

analyze the right support anywhere you want to

Where exactly would you analyze it?

 

[MaddEngineer]

Actually a propped cantilever has one built in support and one simple support (Remember the Fixed End Moment Equations), basically it is a cantilever with the free end propped.

According to the AISC Beam Diagrams & Formulas they just call it a beam overhanging one support.

But now we are getting into semantics, have you ever done a structural plan and noted the overhanging end as a CANT. on the drawings. So in practice I think it is understood that it is considered a cantilever.

A true cantilever is a perfect assumption anyway and may not actually be attainable in practice, since the bedding material will most likely deform somewhat and allow a slight rotation and shift the reaction away from the true face of the support, even if ever so slightly.

I think the OP was looking for a consensus of opinion or confirmation regarding the modeling assumptions.

I think one of the predicaments that we as structural engineers have to deal with is that we (often) don't get to field test and prototype our designs. They have to work right out of the box based on our best educated guess and hope you didn't miss anything (this becomes more precarious when designing something that there aren't many documented and peer reviewed procedures for or when the assembly deviates from typical tried and true practice).

I don't think you can just arbitrarily pick a resisting moment arm out of your hat and hope to sleep at night when they are loading the formwork with concrete, MEN and materials and hope for the best.

I do a lot of work with temporary as well as permanent structures and I find the temporary structures a lot more challenging at times because they have to be light, easy to carry, easy to install, easy to disassemble and they typically will actually carry the full design loads (which again may not be well documented).

Whereas with permanent structures you can often engineer the Sh** out of it since they will be using cranes and other heavy equipement to safely place the items (or working off of the temporary structure) and then weld/bolt the hell out of it so it doesn't go anywhere, and often they carry less than 50% of the design loads anyway.

 

[hokie66]

miecz,

I said I agreed with spats, so .85d. But I would be happy with anything in the right half of the beam. .5d is probably overconservative, 1.0d is unconservative.

 

[hokie66]

MaddEngineer,

Thanks for gently correcting me about the definition of a propped cantilever. You are indeed right. Must have been a senior moment, or two.

 

[DTGT2002]

Ralph, looking at what you have drawn, I think your approach is reasonable. While internally the waler has fixity at the "cantilever," you don't really have absolute fixity at the support, even when the carpenters wail on the bolts of the hanger rod to get it tight.

So a pin-pin analysis seems reasonable, especially in light of the addition of the shims. From a practical standpoint, the shims make sense as it is likely the beam soffit and the waler will not have identical profiles.

Gut check says while shoring loads often approach or exceed design loads, the bridge here likely has a much higher service capacity that what you are applying and for this type of construction, I would anticipate some understanding on the designers part about methods of construction, but that is a fair point to verify.

As for the location of the other shim, I prefer my hold down reaction to be spaced at least as far out as my last applied load. Easier said than done here, but why not check where it can be and come up with a minimum spacing from the rod carrying the load. If the loads/capacities are ok with the shim at 0.75 dist to load, then so be it. Hopefully your load at the shims is low enough for scrap lumber to be used and avoid steel plates.


Sorry to be slow to respond - I was in Honduras when I got your note.

Daniel