Crane Ties

[slickdeals]

I am reviewing some calculations for a crane tie design. The ties are attached to the columns with tie plates, through bolted through the column.

In doing bolt force calculation for moments, the engineer has used a method where he has estimated his "compression block" based on AISC's Case I (Neutral Axis not at CG), Page 7-10, AISC Manual 13th Ed. I contend that he should be using an analysis similar to a base plate analysis and use concrete compression block.

What are your thoughts? FWIW, the design is submitted by an Australian company. I am not sure if there are provisions that allow you do this, although I suspect not.

 

[hokie66]

Not sure what you mean by a "crane tie". A sketch would help.

 

[slickdeals]

See attached.

 

[slickdeals]

Just to clarify, in the attached sketch, the engineer has chosen to calculate the tension due to moment in the through-bolts using equations meant for steel-against-steel rather than steel against concrete (like a base plate). Using HILTI Profis, which I believe does a rigid plate assumption, I get higher bolt forces than the calculations.

 

[hokie66]

I can't really give you any specific advice, as I am not up to speed with either method. Seems to me, though, that for the normal component, the force is steel on steel because of the plate at the back of the column. The concrete comes into play mainly for the component parallel to the face, and if you can justify the V1=200kN force, then it is OK.

 

[dhengr]

Slick:
Obviously, you are imparting those horiz. forces (V & N) into the columns, and they produce bending, shear and torsion, which must be checked. I would also check some max. concrete compressive stress caused by the brackets as you suggest. I think your base plate analogy is a sound approach, although I don’t have the latest AISC 13th Ed. you are looking at. I guess I understand the 545mm from the pins to grid E, but what I would need is the dimension from the pins to the face of the column (or back of the base pl.), so I can draw my FBD for the bracket to the column. I would also wish to see some positive means of transferring the V’s to the column, rather than just the shear in the through bolts.

 

[slickdeals]

@dh
The eccentricity from face of column to center of pin is 250mm.

You brought up another of my concerns, regarding transfer of shear into the column. The M24 bolts are installed in 27mm ID sleeves, and as a result of field tolerances, may cause some bolts to be in contact with sleeve at time of installation, while others have a 2-3mm play. Won't this result in bolt shear overload as the other bolts may not be engaged until the concrete crushes.

Also, if perfectly installed, they are prone to a +/- 1-3mm rattle every time the crane imparts loads, which I don't like. What do you think?

@hokie,
Not sure why you say that for the normal component it would be steel against steel. If you see the attached sketch, under normal compression + moment (clockwise), the top plate will have bearing against concrete at right end and bottom plate at left end due to bolt tension. Am I missing something?

 

[slickdeals]

The other item to note is that the plates have 27mm diameter holes to receive M24 bolts, which means that the bolt is not bearing at the plate. No welded plate washer is specified.

I am normally used to seeing these anchorages tied to the slab, but this is my first time looking at the ones tied to a column.

I will be checking the column for biaxial bending, biaxial shear and torsion along with very associated compression. Also for good measure, check it with very low axial loads.

 

[hokie66]

I was just trying to say that I don't see the N component being an issue, as the forces are not that great.

I would be most concerned about the shear transfer. Those sleeves are shown to be PVC, and I don't like that idea, although I imagine that it done all the time. And you are right...tolerance is a problem. In addition, the sketch shows the sleeves centred apparently very close to the edge of the columns. As I recall, most tower cranes I have seen on concrete buildings were anchored to spandrel beams rather than directly to the columns. That type arrangement overcomes the edge distance problem.

 

[slickdeals]

Apologies, it is a standard hole (not an oversized hole). My metric brain wasn't switched on.

 

[dhengr]

Slick:
I’m confused, your 1st sketch, “Tie Setout Plan LVL 40,” shows the pin plates on the tie brackets in horiz. planes (pins vert.), and it also shows 3 through bolts at each bracket. The “Typ. Section thru Column Tie” shows the pin plates in the horiz. position, but with 4 through bolts. Which is it, or does it vary? Are we looking down on a column section, or at a side view section?

All thread rod is awful for picking up any “V” forces or shear loading, since you have to yield and crush the threads; and yield and crush/extrude the PVC conduit before you have a good shear transfer to the concrete. Furthermore, you have threads in the faying surface/plane, and in bearing on the base plate holes. Could you use a light metal conduit which I assume gets cast into the column, for this future application of the through bolts. Could you pressure grout the conduit void and threads full with an epoxy which would improve short term (and cyclic) shear transfer?

Alternatively, take the bolts out of the shear transfer problem. Bolt the base plate to the column face, then provide some shear lugs which weld to the edge of the base plate and wrap out and around each side of the column. I have a couple ideas, and if you are interested I could try to explain them or send a sketch. But, I’m not real sure how to send an attachment.

 

[slickdeals]

@dh,
Apologies, it is a plan view of a column section. On Level 40, it is only 3 because of column dimensions. I posted the column detail from one of the lower floors; where column sizes are 1m x 1m or 0.8m x 0.8m, 4 bolts have been used.

I had similar thoughts, use a pressure injected grout to enable "instant" load transfer. I also like your idea of welding a couple of plates at the edges and create a "C", which will act as a lug.

You can send your sketches, by scanning them and using the "attachment" feature available at the bottom of the reply to thread box. Choose "upload to engineering.com" and follow the instructions.

 

[dhengr]

Slick:
To take the bolts out of the shear equation, could you cast a channel, toes in, into the face of the column at that tie bracket location, 6"-8" longer than the bracket base plate is high. Then bolt the tie bracket down and weld its base plate to the web of the of the channel? Those PVC conduits through the column cross section are a fair amount of concrete area eliminated as relates to column strength, in a fairly concentrated location. What does that do to the ultimate column design/strength for the building. Does their bolt/conduit spacing miss the rebar in your column design? You and Hokie say you usually see these braces at the floor level/edge and this would seem to be an easier location to deal with the problem and the applied loads. Why isn’t that done on this building, don’t you get any say in these matters? I haven’t dealt with this crane tower problem in years, the ones I dealt with on my building designs climbed up in elevator shafts using the floors and core shear walls for support.

I’m just trying to describe a couple of possible solutions, I’m not actually picking thicknesses and dimensions of the pieces, weld sizes, etc., I’ve haven’t run any numbers. On Detail 1, the 4 shear lugs are shop fabed, shoved up against the side of the conc. col. and field welded to the edge of the Tie Bracket Base Pl. In this case the tight-fit or adjustability is in the length of plate B1. The Tie Bracket can later be easily removed and the lugs gouged off the Base Pl. edges. On Detail 2, the plates B3 & B4 can be preped. and cut to length once you know the height of the Tie Bracket Base Pl., then they are field fitted and welded. Put B3 to the Base Pl. edge and scribe the conc. col. side on its bottom face. This sets the location for B4 and its welding to B3. In this case the tight-fit or adjustability is in the excess width of plate B3.