Monorail crane design - bottom flange loading

[ak1990]

I was wondering if anybody had any experience with the design of overhead traveling monorail cranes, with the crane trolley loading the bottom flange of the crane beam? I am looking for some deflection limits for the tips of the bottom flange, and how to calculate stress allowances on the bottom flange as a consequence of this loading. I am fairly unfamiliar with loading on the bottom flange and I am generally unsure how to check this aspect of my design and would greatly appreciate any advice. The monorail crane in question is supported by the roof beams which carry its load.

 

[dhengr]

Ak:
Right click on the eye glass symbol at the right end of your thread title, and see what you can find. That subject has been pretty much been beat to death, although I’m sure there is always a new twist to the problem. I also think there are more threads on the topic, if you search a little further. Then come back with any further questions.

 

[ak1990]

Thank you very much for the prompt response.
I am currently following your advice and looking through previous threads. Most of these threads are focusing on checking the beam for lateral torsional buckling etc and these are calculations I have performed. The design codes I am familiar with are the Eurocode, which offer very little guidance with respect to bottom flange loading.
I will give you a little more information about my problem;

I am designing a 28m long monorail crane beam for a hydro turbine house which has to be able to carry the load of a 25 ton turbine ( I am aware that beam lengths of greater than 12m are fairly uncommon meaning three smaller beam sections will be spliced together, assuming they behave as a continuous beam). This beam is suspended from room beams above which are connected to RC walls, which then transfer the load through the structure down to ground level (the monorail crane beam has no direct connection to the RC walls).

The roof beams are at 4m intervals along the 28m length of the crane beam. These roof beams have been designed and checked and are ok with respect to LTB. At the points where these roof beams meet the crane beam additional lateral bracing is also present, meaning that at these points only, there is horizontal and vertical restraint. I have then analysed the continuous crane beam taking into account the carry over moments at the restraint points and calculating the "worst case senario" in terms of which load position provides the highest bending moments etc. I have then designed a crane beam to cater for this and performed all other checks such as beam deflection.

I am now looking to check how the beam responds to the load on the bottom flange (admittedly I should have designed this first), a problem which I have not really encountered previously. I have attached a simplified sketch of the problem with an exaggerated sketch of how the beam could possible deflect under loading (noting that I have approximated the load as two point loads on either side of the beam web). I am looking to calculate the deflection at the far right and far left tips of the bottom flange. I am also unsure how to deal with the other possible beam behavior that I have circled in red.

Any ideas or direction with regards to possible resources would be greatly appreciated.

 

[dhengr]

Ak:
You would do better (I would see it better) if you showed your attachments as pdf files, more people could see them and they print easier for me, at least, for study or redlining. Are you fabricating this trolley beam? You might use a higher strength bottom flg. plate than those for the web and top flg. You want a thicker and narrower bot. flg. to control the bending stresses and deflections. Your sketch shows the bot. flg. the right way, but the top flg. will be a std. bolted structural detail without the same type of flg. tip deflection. You could also use a WT section for the top of the beam and then just add your heavier bot. flg. You should select the proper size trolleys first, at least two (?), since they will dictate the width and shape (slope?) of the bottom flange. The wheel tread shape and wheel spacing are important considerations for your design. You might actually have a spacer beam btwn. the two trolleys which also acts as a spreader beam that your rigging actually hangs from.

Well..., it’s a continuous rail, but not a continuous beam at the two joints. Minimize the number of joints, they are potential trouble, and place them right under the roof beams where you have the best chance of supporting the joint and the two beams. The extra lateral bracing of the roof beams is good, because there are lateral loads and loads along the length of the rail involved too. There are earlier threads on the wheel loading, combined stresses and deflections of the bot. flgs. and there have been discussions about the wheel loads and deflections right at the flg. tips at a joint too. One way to search this would be to click on my name, then look at Mech. or Struct. forums, and click on the number showing my replies to that forum. You’ll see a listing of threads I’ve responded to, and you can scroll down through these and find threads on underhung crane beams, etc. It’s been a while since I’ve been involved in one of these discussions, so other than your thread, scroll down 6 months to start your search. Also, try the search feature at the top of each forum when you bring it up, but I’ve had rather mixed luck with that. I think there are some resources mentioned in some of those earlier threads.

The flg. acts as a cantilevered plate, some length long (engineering judgement here), off of the radius btwn. the web and the flg., or the toe of the fillet weld btwn. the web and flg. It is loaded by two wheel (point) loads, 8 - 10" apart, whatever the trolley. You have the beam bending normal stress, plus this canti. pl. bending stress, as a combined stress to deal with. The flg. deflection is another engineering judgement and experience calc. And, one of the big problems is always one wheel near a joint and the relative deflection of the two beam flg. tips. If you fab. the beam, you don’t often need a full pen. weld btwn. the web and flg., but you want a good fillet without any undercutting at the toe of the weld. For this reason a rolled section might be better because of its generous radius btwn. the web and flg. “I beam” shapes sometimes fit the wheel tread slope on some trolleys and the sloped flg. improves bending stresses. Some people make these trolley beams with two rolled shapes welded together some distance up in the web, a less critical weld.

 

[chicopee]

You may want to do a search of underslung cranes eventho their lifting capacities are on the low side. They are prevalent in manufacturing shops, so there should be a fair amount of material on the subject.

 

[zekeman]

Ak,
The reason you are getting so few answers is that you have provided no readable drawing.
We are not mindreaders.

 

[renderu]

Maybe this will help.

 

[homostructural]

Eurocodes take into account the bottom flange loading of a beam through the expression of the critical moment for LTB (Mcr).

All about Eurocodes:

http://www.DesignToEurocodes.com