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Ladders Rungs - Fixed or Pinned? 2
- Thread starter Deutero
- Start date
DaveVikingPE
Structural
If you're talking about the rungs themselves, they're cantilevers, i.e., fixed.
Theoretically they are fixed since the ends are welded all around. However, if you want to be conservative, assume them to be pinned.
Why are you considering this? Are you designing a unique ladder? Most of ladder designs are standard and are well proven.
Keep it simple.
Regards,
Lutfi
Why are you considering this? Are you designing a unique ladder? Most of ladder designs are standard and are well proven.
Keep it simple.
Regards,
Lutfi
Yes, OSHA standards are at on their website 1910.27
One reason for asking the question is that 3/4" rungs are commonly used (and specified) on welded steel ladders, but calculate out for marginal strength. The "correct" way to do it would be to treat the stringer and the ladder together; don't just assume the stringer is an inflexible support. I also suggest looking at strength design, not allowable stress design, as well.
When checking into OSHA standards on ladders, also hunt up their proposed standard- be aware of upcoming changes.
When checking into OSHA standards on ladders, also hunt up their proposed standard- be aware of upcoming changes.
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1
- #9
You can assume that the weld "fixes" the rung, but fixity isn't determined always by the connection alone, but also what it is connected to. If your ladder rung was connected to a 48" thick steel mass, then it would behave pretty much as a fixed connection.
If, however, your rung was welded to a 28 gage steel plate, or perhaps (getting silly) - a 3" wide wet noodle, then it would approach a simple span condition.
With a typical 3/8" x 2 1/2" steel stringer bar on each side, you most certainly would have to model the steel to see, but you'd be really wasting your time. Just design the durn thing as a simple span.
If, however, your rung was welded to a 28 gage steel plate, or perhaps (getting silly) - a 3" wide wet noodle, then it would approach a simple span condition.
With a typical 3/8" x 2 1/2" steel stringer bar on each side, you most certainly would have to model the steel to see, but you'd be really wasting your time. Just design the durn thing as a simple span.
- Thread starter
- #10
In light of the "Complexity of Engineering" thread, I prefer to keep things a s simple as possible. But, this is why I ask:
In the past, I have figured ladder rungs to be required to support a 200 lb concentrated load at mid-span for a conservative pinned support. Similar to what JStephen stated, even with the required minimum 16" rung span, the required Sx (w/o safety factor)for this condition is equal to the actual Sx of a typical A36 3/4" rung, based on Fb=0.6Fy.
According to "Each step or rung of a fixed ladder (not implying the connection) must be able to support a load of at least 250 pounds applied in the middle of the step or rung." I ask, is this a revision to a former standard? Further, when researching pre-fab stair treads, a load table in a McNichols catalog states: "Maximum tread length based on 300 lb concentrated load ... at center of tread length." Is McNichols suggesting a revision? In my opinion, the reality of this situation is that when combining engineering with an apparent societal condition, at least in the United States, people are getting larger and thus heavier. I believe we need to consider this.
Considering this, the 3/4" A36 does not calc as acceptable for a pinned connection. Am I wrong? Am I splitting hairs? Am I inducing complexity in engineering? (Please, I do not intend to elaborate that thread)
Thank you all very much for your comments. Any more thoughts?
In the past, I have figured ladder rungs to be required to support a 200 lb concentrated load at mid-span for a conservative pinned support. Similar to what JStephen stated, even with the required minimum 16" rung span, the required Sx (w/o safety factor)for this condition is equal to the actual Sx of a typical A36 3/4" rung, based on Fb=0.6Fy.
According to "Each step or rung of a fixed ladder (not implying the connection) must be able to support a load of at least 250 pounds applied in the middle of the step or rung." I ask, is this a revision to a former standard? Further, when researching pre-fab stair treads, a load table in a McNichols catalog states: "Maximum tread length based on 300 lb concentrated load ... at center of tread length." Is McNichols suggesting a revision? In my opinion, the reality of this situation is that when combining engineering with an apparent societal condition, at least in the United States, people are getting larger and thus heavier. I believe we need to consider this.
Considering this, the 3/4" A36 does not calc as acceptable for a pinned connection. Am I wrong? Am I splitting hairs? Am I inducing complexity in engineering? (Please, I do not intend to elaborate that thread)
Thank you all very much for your comments. Any more thoughts?
If you're just trying to comply with the OSHA standard then, as you've pointed out, a 250 pound mid-span load is fine.
However, you're wise to consider that your actual loading may exceed that.
If you're expecting that the workers will be ferrying tools and equipment up the ladder, I'd definitely increase the load.
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How much do YOU owe?
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However, you're wise to consider that your actual loading may exceed that.
If you're expecting that the workers will be ferrying tools and equipment up the ladder, I'd definitely increase the load.
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How much do YOU owe?
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- Thread starter
- #14
For simplicity, I start all of my basic calculations for mild carbon steel at Fb=0.6Fy. Even if I apply 0.75Fy per AISC 9th (F2-1), the conservative 300 lb point supported by a pinned end connection, still exceeds that Fb that a 3/4" dia A36 will provide.
Through the use of the intent of this forum, considering the section props of the side stringers, I have concluded that, with all-around fillets I will consider the end connections as fixed, which, by far, exceeds what is Fb.
Last thoughts; Yes, ladders have long been considered a proven miscellaneous structure. I appreciate that. However, considering previous comments, we tend to be conservative for various reasons. For those who may want to consider wider ladders with longer rung spans, not necessarily wanting to increase the 3/4" bar size, having formerly worked for a fabricator, I have seen ladder rungs inserted into holes in the siderail stringers and then either filleted on the inside or butt welded on the outside. Something to consider ...
Thank you all for you input ...
Through the use of the intent of this forum, considering the section props of the side stringers, I have concluded that, with all-around fillets I will consider the end connections as fixed, which, by far, exceeds what is Fb.
Last thoughts; Yes, ladders have long been considered a proven miscellaneous structure. I appreciate that. However, considering previous comments, we tend to be conservative for various reasons. For those who may want to consider wider ladders with longer rung spans, not necessarily wanting to increase the 3/4" bar size, having formerly worked for a fabricator, I have seen ladder rungs inserted into holes in the siderail stringers and then either filleted on the inside or butt welded on the outside. Something to consider ...
Thank you all for you input ...
Consider increasing the material yield strength and also design for plastic moment.
Realistically, when you climb on these ladders, they don't feel marginal in strength. A bending failure in the rung just means a bent rung, not a broken rung (the welds on the ends are much stronger than the rung itself). A better reason to increase rung size would be for a better grip.
Inserting the rungs into holes in the stringers doesn't increase the rung bending strength. I have seen this done, but to be honest, I think it got started in the days of riveting and just never died off. I've never actually heard a reason for it other than "That's the way we've always done it!"
Realistically, when you climb on these ladders, they don't feel marginal in strength. A bending failure in the rung just means a bent rung, not a broken rung (the welds on the ends are much stronger than the rung itself). A better reason to increase rung size would be for a better grip.
Inserting the rungs into holes in the stringers doesn't increase the rung bending strength. I have seen this done, but to be honest, I think it got started in the days of riveting and just never died off. I've never actually heard a reason for it other than "That's the way we've always done it!"
- Thread starter
- #16
JStephen,
As you may have noticed, I'm green in determining the difference between a pinned and fixed connection - I may even need to post a thread asking the real difference between the two when referring to a continuous multi-span beam.
I do understand that inserting the bar into the strigner holes does not increase the rung Fb, but, if the holes were no greater than 1/16" dia larger than the bar, would not the fact that the "top flange" of the bar, coming in intimate contact with the top of the hole, along with the all around welding, contribute to the fixity of the connection? I realize I'm going into more detail than necessary for this particular application, but, I'm simply trying to better understand a fixed connection.
Thank you for your time.
As you may have noticed, I'm green in determining the difference between a pinned and fixed connection - I may even need to post a thread asking the real difference between the two when referring to a continuous multi-span beam.
I do understand that inserting the bar into the strigner holes does not increase the rung Fb, but, if the holes were no greater than 1/16" dia larger than the bar, would not the fact that the "top flange" of the bar, coming in intimate contact with the top of the hole, along with the all around welding, contribute to the fixity of the connection? I realize I'm going into more detail than necessary for this particular application, but, I'm simply trying to better understand a fixed connection.
Thank you for your time.
LittleWheels
Structural
A little excess capacity in a design helps me sleep at night, particularly when thinking about corrosion. I've seen 'see-through' ladder components in some process and production facilities I've inspected. Routine maintenance hadn't picked them up as a problem...
Perhaps design them as acceptable when pinned but realise there is 'some' extra capacity, due to fixity? Just a thought.
Perhaps design them as acceptable when pinned but realise there is 'some' extra capacity, due to fixity? Just a thought.
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- #19
You should go back to JAE's post on 24 Sep 06 1:09. He has a good illustration. Fixity at a connection means there is no relative rotation between the members at the joint. If they intersected at 90 degrees with no load, then they remain intersecting at 90 degrees after load. Theoretically this only counts for the infinitesimal amount of length at the connection itself.
However, it does not mean that the connection as a whole cannot rotate. To use JAE's example, the angle between the rod and the wet noodle may not change, but you can certainly imagine the wet noodle bending and rotating as the rod tries to deflect. Then the deflection and moment diagram of the rod will look very similar to that of the rod as a simply-supported member. Depending on the stiffness of the connecting member, some of the moment in the rod may get transmitted to, say, your plate, runner, or wet noodle. Because of this, you will have less moment in the rod, but it won't be the same as if the rod were connected to a fixed support. Remember, a fixed connection and a fixed support are two different things. A support is assumed to have infinite rigidity, whereas your runner or wet noodle at the connection is elastic. When you do your model of the rung then, you will have most likely pinned supports with a rotational spring support. The stiffness of the spring will help prevent the rod from bending but not as much as a fixed support. The stiffness of the spring will depend on the appropriate stiffness of the supporting runner or the ladder.
However, it does not mean that the connection as a whole cannot rotate. To use JAE's example, the angle between the rod and the wet noodle may not change, but you can certainly imagine the wet noodle bending and rotating as the rod tries to deflect. Then the deflection and moment diagram of the rod will look very similar to that of the rod as a simply-supported member. Depending on the stiffness of the connecting member, some of the moment in the rod may get transmitted to, say, your plate, runner, or wet noodle. Because of this, you will have less moment in the rod, but it won't be the same as if the rod were connected to a fixed support. Remember, a fixed connection and a fixed support are two different things. A support is assumed to have infinite rigidity, whereas your runner or wet noodle at the connection is elastic. When you do your model of the rung then, you will have most likely pinned supports with a rotational spring support. The stiffness of the spring will help prevent the rod from bending but not as much as a fixed support. The stiffness of the spring will depend on the appropriate stiffness of the supporting runner or the ladder.
If you assume a short load distribution of about 3" - a shoe width - then the 3/4" pinned end bar is OK. OSHA says it has to hold the load, not that it needs a safety factor. If you use normal AISC safety factors, it should stil be OK, even for higher loads.
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