Monorail Beam Flange Reinforcement

[ibrahimdemir]

Dear all,

In case the secondary stresses due to trolley and the load on it is high and trolley flange needs reinforcement do you use a flat bar underside of the flange to increase the stiffness? Is there a supporting document to see what has been considered for the reinforcement calculation, and if available how can I reach that document? Does have specific weld requirement?

I have a BHP brochure on Monorail Beam Design which gives a type of reinforcement which consist of two bar ( rectangular or square ) elements. These stiffener bars are welded under the flange, and each one is located underside of the trolley wheels and continuously welded. All these works are based on BHP tests. However, this document does not support the full bar stiffener and beams with tapered flanges.

If you know the availability of this document please let me know ASAP. If the document is reasonable in size ( for you ) could you please forward on my e-mail address below.

Thank you in advance.


Ibrahim Demir
Design Engineer
Clyde Babcock Hitachi
idemir@cbh.net.au

 

[edba]

For safety purposes, I personally would replaced the monorail beam (I suppose its removable) with a new one that can take the load.


KISS - Keep It Simple and Safe

 

[dbuzz]

Do you need to:
1. increase the moment capacity of overall section for strength,
2. increase the stiffness of overall section for deflection control, or
3. strengthen the the bottom flange for local bending due to the trolley wheel loads

If it is the latter, don't make the mistake of having a single flange "doubler" plate that spans between the lange toes, i.e.:
_____
|
|
__|__
-----

In this arrangement the flange "doubler" plate won't contribute to resisting the cantilever bending moment due to the trolley wheel loads.

 

[49merc]

You may be able to replace the trolley with one that has more wheels to spread out the load. This works pretty well with relatively light systems.

 

[ibrahimdemir]

Thanks for your replies.

Unfortunately, we are not able to replace the section size. The cross section (U beam)is the most suitable section for bending to a small radius in the weak axis direction. I tried to use Universal column sections which does not require the stiffening. However, the manufacturer can not bend to the working radius that the client requires. Additionally, built-in cross section becomes very expensive with bends. Therefore, we need to use universal beam which have less flange width that does not cause problem during the bending. However, these beams do not have the required flange thickness.

I have made this kind of reinforcement by using the BHP reference earlier. However, we are trying to reduce the cost down, and additionally two stiffeners under the flange require more space to overcome the secondary stresses. This time, we wanted to try to use a single flat bar. However, we do not have any supporting document or tests at this stage. I am still trying to obtain it if available.

The BHP document I have has some references but I am not sure if they are suitable for my purpose. I am just guessing; if they were, the BHP would use the same method instead of using two bars under the flange.

Thanks again.

Ibrahim Demir
idemir@cbh.net.au

 

[dbuzz]

Torsional effects on a monorail beam curved in plan will further stress the flanges.

Refer to Bradford's Design of steel I-beams curved in plan to AS4100 (Australian Journal of Structural Engineering, 2000;3(1-2):85-98).

 

[ibrahimdemir]

dbuzz,

Thanks for your response. However, I disagree with your statement on the zero contribution of the doubler plate under the flange for the trolley wheel load. Basically the resistance comes from the flextural stiffness in the longitudinal direction not in the transverse direction of the bar(s). The action is localised under the wheels. Therefore, the remaining portions both side of the bars are stiil unloaded locally and have contribute by resisting flexurally in the longitudinal direction.

Thanks again

 

[ibrahimdemir]

edba and 49merc,

Thanks for your response. However, the loads are very high and we need to keep the bend radius as I explained earlier. The only option we have is to reinforce the bottom flange by a method that I was explaining earlier.

Our target is to use a single flat bar ( doubler plate ) underside the flange at this stage. Otherwise, we will be forced to use the BHP reference for reinforcement with two bars.

Thanks for your contribution.

Ibrahim Demir
idemir@cbh.net.au

 

[ibrahimdemir]

dbuzz,

Thanks for your reference. However, that is not the one I can use for the calculation for the resistance cantribution of the doubler plate to local bending.

Thanks

Ibrahim Demir
idemir@cbh.net.au

 

[dbuzz]

The trolley wheel loads are transferred to the beam web from the point of load application by local bending of the bottom flange.

The BHP design guideline uses longitudial billets, which are quite stiff, to increase the effective width of flange resisting the local bending moment.

I would be wary of assuming that a flange doubler plate, of similar thickness to the bottom flange, will achieve the same result. It simply isn't stiff enough.

The Bradford reference is not a monorail design guide but it does advise on how to calculate the combined stress in the bottom flange from bending and torsion. This effects the required thickness for your bottom flange for transfer of trolley wheel loads. AS 1418.18-2001 requires that if the horizontal radius of curvature is less than twice the distance between the supports, then the beam must be analysed as a curved beam.

 

[ibrahimdemir]

dbuzz,

Thanks for the details. I accept your concern about the beam with curvature. I do calculate the combined action on the curved beam and on the bottom flange. This is not my concern.

I understand that you know about the BHP guideline about stiffening. If you follow the examples in that booklet, you will see that those longitudinal billets have a contribution to the resistance against the localised wheel loads in addition to the overall bending. They act as a beam on an elastic foundation. If you write a spreadsheet for this calculation, you will see that you are able to play with the width and the depth of these billets.

So, my concern is to replace these two billets with a flat bar ( a doubler plate in your term ). This doubler plate will still act as the billets act against the load. My question is how? And, is there any supporting paper or test?

Thanks for your input. I still believe it is very valuable for me.

Ibrahim Demir
idemir@cbh.net.au

 

[dbuzz]

I think we are arguing the same point but just differ on the effect of the flange plate.

As I understand it:
- The trolley wheel loads, located near the toe of the bottom flange, must be transferred to the beam web
- The effective width of the flange resisting the local bending moment is based on the flange’s transverse and longitudinal stiffness. The transverse stiffness is based on the bottom flange bending like a cantilever beam from the unsupported flange toe to the flange root and the longitudinal stiffness is based on a longitudinal strip of the bottom flange bending like a fixed-ended beam.
-The local stresses in the bottom flange due to the wheel loads can be reduced by placing billets below the position of the wheels. This has the effect of increasing the stiffness and length of the longitudinal strip (i.e. the idealised beam on elastic foundation) thus increasing the effective width of the flange and reducing the local stresses.

I think we agree on this much.

My concern regarding the concept for a single flat bar, welded to the underside of the bottom flange and spaning between the flange toes, is that it is insufficiently stiff to increase the stiffness and length of the longitudinal strip and therefore won’t be effective in distributing the wheel load across a longer section of flange.

 

[prex]

To avoid a risk of misunderstanding, I suppose that what we call here a doubler plate is a plate having a thickness close to that of the beam flange, a width just a little less than beam flange width, and extended over the full beam length (or locally only, if the problem of high wheel load is local).
The use of a doubler plate is a common solution in the conditions described by ibrahimdemir and it appears in an old article I have on the matter (but no calculations offered for this particular condition).
However this method is not very effective in increasing the effective thickness of the flange, as, to do that, there should be a connection to the web to transmit a vertical load: you could obtain that by making spaced holes in the centerline of the doubler plate to be filled by weld. In this case it is necessary to determine the load transmitted by the button welds, but I have not a ready procedure to do that (though it doesn't seem quite difficult) If this is done, you could then use the total thickness (flange+plate) to check flange bending.
If on the contrary ibrahimdemir wants only to replace the two spreader bars by a single plate, then I would say that if the moment of inertia of the added plate is the same as the moment of inertia of the two replaced bars, then you could use that BHP method (that I don't know) to check the reinforcement.

prex

http://www.xcalcs.com

Online tools for structural design

 

[dbuzz]

prex (and others),

The method defined by BHP (now OneSteel in Australia) is about having a stiff longitudinal strip along the edge of the beam's bottom flange, aligned with the wheel path, which distributes the wheel load as a line load, along a length determined by the flange's transverse and longitudinal stiffness, instead of as a point load.

The longitudinal strip is idealised as a beam on elastic foundation, whose stiffness is determined by considering the flange as a cantilever beam. The stiffer the longitudinal strip, the longer the length of the beam on elastic foundation, the more distributed the wheel load.

This is precisely why the doubler plate, even if it has the same moment of inertia as the billets, won't be effective. It does not adequately stiffen the longitudinal strip. Therefore the length along which the wheel load is considered to be distributed does not increase.

Furthermore, the BHP method is based on a fit to the results of experimental testwork for this specific arrangement rather than a theoretical approach to general flange strengthening.

 

[PXC]

Given that the monrail is curved, perhaps you should be fabricating it rather than trying to bend it.

I would also add a comment on the analysis of a curved I-section which is not straight forward. The longitudinal warping stresses are significant. And deflection can be a problem. I wonder how you are addressing these?

 

[prex]

dbuzz
you say no no no, but this doesn't help.
Do you agree that, if the thickness of the doubler plate is the same as the height of the longitudinal strips, then (of course) the BHP method may be directly applied?
Now, if the thickness of the doubler plate is less than the height of the strip, but, as the width is greater, the inertia is larger, how much larger would you consider to be sufficient for an equivalent result?

prex

http://www.xcalcs.com

Online tools for structural design

 

[dbuzz]

In his query Ibrihim says that he is constrained by space, so I expect that a single flange plate, of the thickness required for the billets, would be unsuitable too.

Furthermore the welds each side of the longitudinal billet are important elements in the "built-up" strip along the edge of the flange. I make the analogy with a fabricated plate girder. Would you weld the flange on one side of the web only? Similarly for the strengthening of an I-beam with flange "doubler" plates. Would you weld the doubler plate to one flange toe only?

Remember, you are trying to stiffen a narrow strip along the edge of the bottom flange so that the wheel load is distributed as a line load, along some length of the flange, rather than as a point load.

 

[ibrahimdemir]

dbuzz and prex,

Thanks for you input to this discussion. Now, we all agree on the type of the reinforcement and its contribution to the flange stiffness due to localised wheel loads.

The BHP method has two bars continuously welded on both side, and bars are right under the whell centrelines. So, the wheel load is partly taken by the cantilever action of the flange locally, the remaining vertical load is taken by the stiffener bar through the welds and distributed along the flange longitudinally both side of the wheel ( A kind of beam on elastic foundation). Of course, the full wheel load action is taken by the cantilever action on the flange which is longer with stiffeners. Weld has a significant effect onto the load transfer along the flange.

In case a doubler plate, we lose the contribution of the inner welds to the system. As prex explained, the fillet welds in the spaced slotted holes may provide similar contribution to the system. However, the number, size of fillet welds and the space between holes are the question.

As dbuzz said the required thickness of the doubler plate may have to be equal to the required thickness of the bars in the BHP reference.

At the moment, I can see that we do not have a proven calculation method for a doubler plate with or without slotted hole welds.

Prex, I was wondering if you can send me a copy of the article that you mentioned. I am not sure about the size of it. If possible please send it to my e-mail addrees.

PXC, we are providing very close supports to overcome the issues you mentioned. Additionally the speed of the trolley is almost zero. So, we do not have problem with the centrifugal forces at all. Of course, the fabrication is another option. However, it becomes very costly. Bending the universal beam is very economical even though the bend radius is very small. But, this is not relevant to the universal column sections.

One might suggest to perform a finite element or finite strip analysis. However, this is not an option at the moment.

Thanks in advance.

Ibrahim Demir
idemir@cbh.net.au