strength calculations for a step

[Ernest Christley]

I can't figure this one out.

I want to add a retractable step that will stick out of the fuselage at
a 40 degree from vertical angle. It sticks out 10" and is made of 3/4"
square, .065 thick, 4130 tube. Assuming no weakness in the support,
will this support a 300lb man (me with all my luggage on my back), and
how can you tell?

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

[Richard Lamb]

Ernest Christley wrote:
> I can't figure this one out.
>
> I want to add a retractable step that will stick out of the fuselage at
> a 40 degree from vertical angle. It sticks out 10" and is made of 3/4"
> square, .065 thick, 4130 tube. Assuming no weakness in the support,
> will this support a 300lb man (me with all my luggage on my back), and
> how can you tell?
>

I think it's the "assuming no weakness in the support" part that has
everybody tongue tied.

The square tube sees healthy enough.

It's the "rest of the story" that will make a difference.

[Smitty Two]

In article et>,
Richard Lamb > wrote:

> Ernest Christley wrote:
> > I can't figure this one out.
> >
> > I want to add a retractable step that will stick out of the fuselage at
> > a 40 degree from vertical angle. It sticks out 10" and is made of 3/4"
> > square, .065 thick, 4130 tube. Assuming no weakness in the support,
> > will this support a 300lb man (me with all my luggage on my back), and
> > how can you tell?
> >
>
> I think it's the "assuming no weakness in the support" part that has
> everybody tongue tied.
>
> The square tube sees healthy enough.
>
> It's the "rest of the story" that will make a difference.

Well, if I'm tongue-tied, it's mostly because I don't know why he wants
a step that sticks out from vertical at some angle other than 90 degrees.

[Richard Lamb]

Smitty Two wrote:
> In article et>,
> Richard Lamb > wrote:
>
>
>>Ernest Christley wrote:
>>
>>>I can't figure this one out.
>>>
>>>I want to add a retractable step that will stick out of the fuselage at
>>>a 40 degree from vertical angle. It sticks out 10" and is made of 3/4"
>>>square, .065 thick, 4130 tube. Assuming no weakness in the support,
>>>will this support a 300lb man (me with all my luggage on my back), and
>>>how can you tell?
>>>
>>
>>I think it's the "assuming no weakness in the support" part that has
>>everybody tongue tied.
>>
>>The square tube sees healthy enough.
>>
>>It's the "rest of the story" that will make a difference.
>
>
> Well, if I'm tongue-tied, it's mostly because I don't know why he wants
> a step that sticks out from vertical at some angle other than 90 degrees.

yep!

[Ernest Christley]

Richard Lamb wrote:
> Smitty Two wrote:
>
>> In article et>,
>> Richard Lamb > wrote:
>>
>>
>>> Ernest Christley wrote:
>>>
>>>> I can't figure this one out.
>>>>
>>>> I want to add a retractable step that will stick out of the fuselage
>>>> at a 40 degree from vertical angle. It sticks out 10" and is made
>>>> of 3/4" square, .065 thick, 4130 tube. Assuming no weakness in the
>>>> support, will this support a 300lb man (me with all my luggage on my
>>>> back), and how can you tell?
>>>>
>>>
>>> I think it's the "assuming no weakness in the support" part that has
>>> everybody tongue tied.
>>>
>>> The square tube sees healthy enough.
>>>
>>> It's the "rest of the story" that will make a difference.
>>
>>
>>
>> Well, if I'm tongue-tied, it's mostly because I don't know why he
>> wants a step that sticks out from vertical at some angle other than 90
>> degrees.
>
>
> yep!
>

It's retractable. It has to have someplace to retract to. It is
attached to the outboard end of a delta wing, not a conventional
fuselage. The odd angle is so that I can attach it to structural
members and have it stick out past the leading edge. The height is such
that it has to be lower than the leading edge to be useful.

The plans call for a step welded to the leading edge that does not
retract. This leaves an ugly step dangling in the wind, where it's
turbulanc can do maximum damage. It is also so high that it is barely
useful. You almost need a step ladder to reach the step. It works, and
is simple, but I think I can do better.

The 3/4" tube has a sleeve welded around the top. It slides in a
1"x.065 square tube that has a sleeve welded around the inside at the
bottom. The 1" tube is to be welded to structural members. The sleeves
positively capture the inside tube, and there is 1.5" of overlap in the
two tubes. Both sleeves have several inches of weld.

My biggest concern is the inside tube not being strong enough and taking
a slight bend over time. Then it will no longer slide. Hence my
question. If it were a straight beam, I think I could run the
calculations, but how do you analyze it when the cantilevered beam isn't
straight?

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

[Smitty Two]

In article >,
Ernest Christley > wrote:

> Richard Lamb wrote:
> > Smitty Two wrote:
> >
> >> In article et>,
> >> Richard Lamb > wrote:
> >>
> >>
> >>> Ernest Christley wrote:
> >>>
> >>>> I can't figure this one out.
> >>>>
> >>>> I want to add a retractable step that will stick out of the fuselage
> >>>> at a 40 degree from vertical angle. It sticks out 10" and is made
> >>>> of 3/4" square, .065 thick, 4130 tube. Assuming no weakness in the
> >>>> support, will this support a 300lb man (me with all my luggage on my
> >>>> back), and how can you tell?
> >>>>
> >>>
> >>> I think it's the "assuming no weakness in the support" part that has
> >>> everybody tongue tied.
> >>>
> >>> The square tube sees healthy enough.
> >>>
> >>> It's the "rest of the story" that will make a difference.
> >>
> >>
> >>
> >> Well, if I'm tongue-tied, it's mostly because I don't know why he
> >> wants a step that sticks out from vertical at some angle other than 90
> >> degrees.
> >
> >
> > yep!
> >
>
> It's retractable. It has to have someplace to retract to. It is
> attached to the outboard end of a delta wing, not a conventional
> fuselage. The odd angle is so that I can attach it to structural
> members and have it stick out past the leading edge. The height is such
> that it has to be lower than the leading edge to be useful.
>
> The plans call for a step welded to the leading edge that does not
> retract. This leaves an ugly step dangling in the wind, where it's
> turbulanc can do maximum damage. It is also so high that it is barely
> useful. You almost need a step ladder to reach the step. It works, and
> is simple, but I think I can do better.
>
> The 3/4" tube has a sleeve welded around the top. It slides in a
> 1"x.065 square tube that has a sleeve welded around the inside at the
> bottom. The 1" tube is to be welded to structural members. The sleeves
> positively capture the inside tube, and there is 1.5" of overlap in the
> two tubes. Both sleeves have several inches of weld.
>
> My biggest concern is the inside tube not being strong enough and taking
> a slight bend over time. Then it will no longer slide. Hence my
> question. If it were a straight beam, I think I could run the
> calculations, but how do you analyze it when the cantilevered beam isn't
> straight?

OK, now that's a little more clear, but we're still talking about a
support member. It's good to say what we mean, I think. A step support
bar isn't a step anymore than a spar is a wing.

Engineers spend too much time thinking. Clamp the piece of steel to
something solid and jump up and down on it. You'll have your answer in
the time it would take you to find the back of an envelope and dust off
your slide rule.

[Capt. Geoffrey Thorpe]

"Ernest Christley" > wrote in message
...
<...>
> My biggest concern is the inside tube not being strong enough and taking a
> slight bend over time. Then it will no longer slide. Hence my question.
> If it were a straight beam, I think I could run the calculations, but how
> do you analyze it when the cantilevered beam isn't straight?
>
Divide the load in to two componants - the load normal to the beam (bending
force) and the load parallel to the beam (tension). The compression load on
the bottom face of the beam will be somewhat reduced by the tension and the
tension load on the top face will be somewhat increased and the sides of
your box will carry shear plus tension.

If your beam is 40 degrees from vertical, the bending force would be the
load (I think you mentioned 300 pounds) times the sine of the angle and the
tension force would be 30 times the cosine of the angle (I'll let you do the
math). (assuming, of course, that I am drawing the picture correctly in my
head) It would be reasonable to assume that the tension force is carried by
the entire cross section of the square tube so divide the force by the cross
section ((width + height) * thickness) and add that stress to whatever
stresses you come up with from the bending.

Or, you could clamp the bar in a vice and jump up and down on it.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

[Ernest Christley]

Capt. Geoffrey Thorpe wrote:
> "Ernest Christley" > wrote in message
> ...
> <...>
>
>>My biggest concern is the inside tube not being strong enough and taking a
>>slight bend over time. Then it will no longer slide. Hence my question.
>>If it were a straight beam, I think I could run the calculations, but how
>>do you analyze it when the cantilevered beam isn't straight?
>>
>
> Divide the load in to two componants - the load normal to the beam (bending
> force) and the load parallel to the beam (tension). The compression load on
> the bottom face of the beam will be somewhat reduced by the tension and the
> tension load on the top face will be somewhat increased and the sides of
> your box will carry shear plus tension.
>
> If your beam is 40 degrees from vertical, the bending force would be the
> load (I think you mentioned 300 pounds) times the sine of the angle and the
> tension force would be 30 times the cosine of the angle (I'll let you do the
> math). (assuming, of course, that I am drawing the picture correctly in my
> head) It would be reasonable to assume that the tension force is carried by
> the entire cross section of the square tube so divide the force by the cross
> section ((width + height) * thickness) and add that stress to whatever
> stresses you come up with from the bending.
>
> Or, you could clamp the bar in a vice and jump up and down on it.
>
> --
> Geoff
> The Sea Hawk at Wow Way d0t Com
> remove spaces and make the obvious substitutions to reply by mail
> Spell checking is left as an excercise for the reader.
>
>

Todd was correct. I was talking about the support beam and not the
step. The step actually sticks out forward from the support beam. I
apologize for not being completely clear.

I'm going to work on the calculation, just for schnitz and giggles, but
took the suggestion to heart and rigged up a test stand by nailing a
couple of blocks to the side of a sawhorse so that the outer housing was
supported at the points where it will be welded. Bounced as hard as I
could in the unstable situation, and could not detect any flexing. That
was with junk mild steel that I was using for a mock-up. I think I
might have overbuilt it by just a tad 8*)

This is the last piece to go on the airframe before the fabric.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

[Dan]

Ernest Christley wrote:
> Capt. Geoffrey Thorpe wrote:
>> "Ernest Christley" > wrote in message
>> ...
>> <...>
>>
>>> My biggest concern is the inside tube not being strong enough and
>>> taking a slight bend over time. Then it will no longer slide. Hence
>>> my question. If it were a straight beam, I think I could run the
>>> calculations, but how do you analyze it when the cantilevered beam
>>> isn't straight?
>>>
>>
>> Divide the load in to two componants - the load normal to the beam
>> (bending force) and the load parallel to the beam (tension). The
>> compression load on the bottom face of the beam will be somewhat
>> reduced by the tension and the tension load on the top face will be
>> somewhat increased and the sides of your box will carry shear plus
>> tension.
>>
>> If your beam is 40 degrees from vertical, the bending force would be
>> the load (I think you mentioned 300 pounds) times the sine of the
>> angle and the tension force would be 30 times the cosine of the angle
>> (I'll let you do the math). (assuming, of course, that I am drawing
>> the picture correctly in my head) It would be reasonable to assume
>> that the tension force is carried by the entire cross section of the
>> square tube so divide the force by the cross section ((width + height)
>> * thickness) and add that stress to whatever stresses you come up with
>> from the bending.
>>
>> Or, you could clamp the bar in a vice and jump up and down on it.
>>
>> --
>> Geoff
>> The Sea Hawk at Wow Way d0t Com
>> remove spaces and make the obvious substitutions to reply by mail
>> Spell checking is left as an excercise for the reader.
>>
>
> Todd was correct. I was talking about the support beam and not the
> step. The step actually sticks out forward from the support beam. I
> apologize for not being completely clear.
>
> I'm going to work on the calculation, just for schnitz and giggles, but
> took the suggestion to heart and rigged up a test stand by nailing a
> couple of blocks to the side of a sawhorse so that the outer housing was
> supported at the points where it will be welded. Bounced as hard as I
> could in the unstable situation, and could not detect any flexing. That
> was with junk mild steel that I was using for a mock-up. I think I
> might have overbuilt it by just a tad 8*)
>
> This is the last piece to go on the airframe before the fabric.
>

A little flexing isn't all that bad. The retractable ladder on F-4E
flexed quite a bit yet had no trouble supporting my 220 pounds plus
anything I was carrying.

Dan, U.S. Air Force, retired