PEMB's & Upgrades

[ToadJones]

I know this has been beat to death here, but do you guys handle engineering/ upgrades to PEMB's?

I have a customer who wants to upgrade crane capacity to a a crane runway that is on brackets on a PEMB moment frame.
I have no experience to PEMB frames.

I can handle everything up to and including the bracket but from there should I inform the owner that he must contact the PEMB manufacturer?

I want nothing to to with analyzing the PEMB frame (not the first time I have been down this road either!)

Thoughts are much appreciated.

 

[SAIL3]

The new col's may not have to lign up with the existing col's thereby, avoiding the existing col fdn. Also may have to provide
independent bracing for the new col's as the existing bracing on the PEMB would be probably marginal at best for this type of application..

 

[ToadJones]

Not having the columns line up would really muddy the waters....

I agree about the bracing...I would provide independent bracing which will also help take torsion out of the PEMB frame.

 

[Lutfi]

Toad, you can always make the columns fit. The owner/user may have to accept loosing few inches of crane coverage. I use brackets on PEMB for small cranes (max of 10 tons) otherwise, PRMB will have to use hot rolled sections and they lose their competiveness due to leveling of the playing field.

Trust me, I am not a fan of PEMB; But they have their place in life.


Regards,
Lutfi

 

[ToadJones]

crane coverage? not sure I follow you.

 

[Lutfi]

Hook travel on the floor.

Regards,
Lutfi

 

[ToadJones]

Chances are good that the trolley approach takes the hook no where near the columns now anyway.

 

[Lutfi]

That might be very true.

Regards,
Lutfi

 

[dhengr]

Toad:
You have an existing crane and bracket support on the existing PEMB, whatever the existing crane and PEMB might be. And, the PEMB has taken, probably was designed for, external gravity and lateral loads over and above the existing crane loads. And, for the existing crane you can calc. vert. and lateral loads, moments, shears, etc. on the bracket. Whatever they may do to the PEMB we might never know. But if you don’t exceed those conditions at the bracket with your new scheme, it would seem to me that all is well. The trick is to be able to say, with some degree of confidence, but without complete reanalysis of the PEMB, that the elimination of the gravity load on the frame legs, allows the bldg. to carry this few/small percent increase in lateral loading from the bigger crane. Under the new crane loading conditions we are loading the PEMB less severely than it was originally loaded, so we have done nothing to reduce its strength as the enclosure.

Put new crane columns in at each frame line and attach the col. top to the bracket (maybe modified to some extent) to take the lateral loads into the PEMB, but no vert. load. You have relieved the original frames of the gravity loads, so the bldg. should be able to take the slight increase in lateral loads from the new crane. Put a new pier on the existing ftg. and if needs be, underpin the inner edge of the existing footing with a 3'x8'x1' ftg. (or some such, the 8' dim. parallel to the crane girder) to pick up the new eccentric column load.

You are correct about your “bad sketch,” it could use more detail, better scale/proportioning and dimensions to really know how the details will work out. Save hairpins and the like to be cast back into the new slab which is shear keyed into the remaining slab. Don’t do all frames at once, or under max. snow or wind loadings. Tie the new pier and ftg. to the old ones with some simple dowels, ties or some such.

 

[dcarr82775]

Can you simply add a column directly below the bracket that essentially transfers the crane load off the existing frame?

 

[dik]

... and you can't operate your crane if there's more than a 15mph wind <G> and/or more than 6" of snow on the roof...

The problem is that there is little reserve or the design loading. If you change that, you can affect the moment diagram for the frame. If your building frame profile reflects the elastic moment diagram, and you proportionally overload it (based on the design loads), the entire frame goes plastic... not room for much redistribution... If you change the loading and/or introduce new ones, you may have an overall impact on the building frame...

Any warrantee the original owner had may be voided, and you accept full responsibility for the structure... PEMB' are fugly and I try to recommend against the use of them for any industrial application. Loads can be introduced as a result of a process modification, etc.

Dik

 

[MiketheEngineer]

M^2 probably hit it right on the head.

Anytime I have tried to add an ounce of load - they blew out!. NOT always - but usually. If the span is over 40' I can almost guarantee there is no "FAT" in the system!!

As suggested - look for an independent system!!

 

[ToadJones]

dhengr-
I apologized for the sketch ahead of time because I knew you might be waiting to pounce
I like your logic and I agree with it 100%. New columns would only improve the building system IMO.

The other issue is that I do not like to design or analyze crane runway brackets for reactions greater than 50 kips. I suspect these brackets are already over that load.

The crane lateral loading to the structure will not increase much and is usually never an issue since the load combinations with lateral loading don't usually control the design of the lateral system.

 

[dhengr]

Toad:
At my age, I get most of my exercise pouncing on things like your sketch, and I needed the exercise. I rarely bother the gal next door next door, since unless she’s in the right mood, she runs too fast, and I refuse to run after it any longer. Although she does walk over here fairly often, and there’s no question about moods then. Did you hear about.... The old bull and young bull were standing up on the hill, and the young bull said, let’s run down there and get us one of them thar heifers; and the old bull said, let’s walk down and get em all.

I was merely suggesting that the existing ftg. (6.25' sq.) maybe only go into the bldg. 3' - 1.5", but your col. center line might be into the bldg. 4' - 1.5" or more, apparently/maybe not even on the existing footing unless you tie the two together to take full advantage of both acting together. And, we can’t see yet how the piers would fit together, or if you could use the existing pier, probably not. I might use a little bracing btwn. the existing frame leg and the new col. weak axis to improve that situation. With a little study, having removed the bracket bending moment and axial load from the frame leg, should make that possible. My new col. web would be parallel to the crane girder.

I would modify the existing bracket to take only new col.-top lateral loads into the PEMB and then maybe have to take a bit of a look at the existing x-bracing, but essentially no vert. crane loads into the existing frame legs. And, I agree that increasing the crane cap’y. will probably not change the controlling lateral loads. That’s a fairly small percentage increase, in either direction, in a load condition which has already been factored into the PEMB. In the one direction, into x-bracing in the plane of the ext. wall, and in the other direction into a frame leg which now has a whole bunch of unused cap’y. Actually, in the plane of the PEMB frames, you might tie/lace the existing frame leg and new col. together (a standing truss of sorts) to take the lateral loads from trolley movement. All of these assumptions and studies would be done on a fairly local basis, not thinking about what this might do to some purlin half the bldg. away. Rather, since I’ve removed bracket bending moment and vert. load on the frame leg, can it now take these new col. bracing loads, or the slightly greater lateral loads through the bracket into the x-bracing, or in bending in the leg and at the haunch joint.

 

[ToadJones]

Dhengr-
If I have time tomorrow morning I'll sketch the section up in CAD and post it.

If you look hard at the bad sketch and assume that the footing is centered on the main PEMB frame center line then you can deduce that the center line of the new proposed new column would not be over top of the existing footing but would be exactly 12" off to the inside.
I think I could justify pouring a new footing down to the elevation of the bottom of the existing and then pouring a portion over top the existing and somehow tying the two together with dowells.

I will suggest this to the customer.

Many of your other suggestions are pretty much standard operating procedure as far as column orientation and tying the crane column back to the building frame. I prefer to stay away from "trussing" the crane column to the building column in a situation such as this as interaction between the two is what I am trying to avoid (only because it is a PEMB). In this case I usually just use simple beam tie backs to cut down the unbraced length of the crane column in the weak axis.
This might seem far fetched, but since I will be all but eliminating the bracket moment on the PEMB frame and any lateral force associated with that moment, I think can certainly justify slightly higher side thrust loads from the crane.

I might also persuade the owner that we can design the new columns for more future upgrades.
You'd be surprised how often companies want to re-rate cranes.

I have seen runways that were originally designed for a single 5 Ton crane that now have four 10 Ton cranes on the same runway.