blake92
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How does the orientation affect how much a steel tube can hold? or does it not really have an effect?
eistein1994 said:How do you determine the Wall thickness for square steel tubing with a length of 5300mm that must be able to bare a work load of 10-12ton?
Baluncore said:Firstly ,as height gets greater you will reach a point where column buckling occurs spontaneously.
http://en.wikipedia.org/wiki/Buckling#Columns
Secondly, what side loads such as wind are expected?
Thirdly, how will you erect it without failure of the structure?
Baluncore said:Find the total cross sectional area of material in the leg wall.
Find the minimum yield strength of the material and so compute the maximum safe compressive force.
Divide by g to convert that force to a weight.
Don't forget to subtract the weight of the leg and the frame above from that maximum.
I am not at all surprised that you are not sure.blake92 said:but my answers never made any sense so i wasnt sure what i was doing wrong.
blake92 said:i did most of that,
A=5.75in^2
σ=30.36ksi
I=31.75in^4
L=5ft
radius of gyration= 2.349in
K=.5 (i just assmued fixed ends since its secured at the top and bolted to the floor)
i then used this equation- Pcritical = (∏^2)(E)(I)/(KL)^2
but my answers never made any sense so i wasnt sure what i was doing wrong.
SteamKing said:The critical buckling load of the legs of your structure may not be the limiting feature of your frame.
I refer you to this thread, where the buckling of a column similar to your frame was discussed:
https://www.physicsforums.com/showthread.php?t=763056
In addition to the strength of the legs, you also have to check the upper part of your frame if loads are applied on the horizontal members. In general, you want to examine the longest horizontal members in bending and shear, then check the shorter members, until everything is checked. Once you have identified the horizontal member or members whose loading is most critical, then you can check the loading in the legs to see if any problems arise there.
Because it appears you are using open structural tubing in some of the horizontal members of your frame, a more detailed investigation of the tubing structure may be required, depending on the max. loading of the frame.
Once you have the loading of the structure determined, then the weld design can commence.
Open structures generally do not attract much in the way of wind loads, unless you are designing the frame to withstand a tornado or some other type of cyclonic wind.
If you expect some significant side loading, say from seismic activity, or if the frame is to be mounted on a moving platform, then additional investigation will have to be carried out, to check the strength of the frame under combined vertical and lateral loading.
Don't worry about the units for your calculation so much, as long as you use consistent ones. Converting units back and forth just provides another source for errors to creep into a calculation.
blake92 said:I determined the allowable load for each of the beams so i know how much they can hold individually assuming the load was evenly distributed on their top face.
I also determined that each of the four vertical beams can hold 131,000lbs before failure( assuming the load is applied directly straight down and no other forces are acting on it) i think that is correct but not 100% positive. I feel it makes sense but I am not sure.
I would they like to know even if the 131,000 is wrong how would i go about determining the max load for the entire structure? would i just say its roughly 131,000*4 (for each leg) or is there more to it than that?
SteamKing said:Great! What are the allowable loads you determined for these members? From your sketch, it doesn't look like the horizontal members are all the same cross section. Can you provide details?
The max. load in the vertical supports may not be the limiting feature of this frame.
There's probably more to it than just the max. allowable load in each leg, but I can't say because I don't have all the details of your structure and your calculations.![]()
blake92 said:The following are the max allowable UNIFORM loads on the horizontal beams:
*assuming a weight is applied evenly across the top surface to each beam*
HSS 6x6x1/4in Square Steel Tubing
for the four equal length 8ft beams (purple if the color shows in the picture):
i got an answer of 24kips or 24,000lbs for each.
for the baby blue beam 12ft that is perpendicular to the first four:
i got an answer of 16Kips or 16,000lbs
i felt this made sense bc as a beam gets longer it would be able to hold slighlty less.
Lastly the front beam (dark green). This beam is actually an 12ft I-beam so i had a harder time figuring out the uniform load but i was able to determine a max point load at the mid point of 8.6kips so i know it can at the very least hold that much.
i even went on to determine the weight of each beam which is about 19.02lb/ft for the square tubingAlso it might be hard to tell in the picture but the four purple beams are notched and fit inside the I-beam where they were then welded.
SteamKing said:Because of the layout of the horizontal members, you can't assume that all of the loads are evenly distributed. For example, the two purple beams in the middle of the frame, which connect the two 12-ft beams, impose point loadings on the longer beams at the points of connection. Even though the loads on the purple beams may be evenly distributed, the same cannot be said for the longer beams. You have to analyze the longer beams with a mix of point loadings and distributed loads.
BTW, which section did you select for the I-beam?
blake92 said:i realize the point load part, and in my hand drawn sketchs of this set-up i made the two middle purple beam have a point load on both end beams. The one thing i was not sure about is how large that point load is? let's say i assume the two middle beams can hold a max of 24kips (uniform load) then does that mean i can split the difference between the front and back beam? for example each middle purple beam would apply a point load of 12 kips on the front and back horizontal beam.
also i used a W 6x20 A36 I-beam
SteamKing said:Yes, if you create a free body diagram of one of the middle beams with a distributed load, you'll see that the end reactions of this beam are each half of the total distributed load. These reactions must then be applied to the long beams so that they can be analyzed in turn.
SteamKing said:Yes, if you create a free body diagram of one of the middle beams with a distributed load, you'll see that the end reactions of this beam are each half of the total distributed load. These reactions must then be applied to the long beams so that they can be analyzed in turn.
SteamKing said:I'll take an in-depth look at your calculations and get back to you.
SteamKing said:I'll take an in-depth look at your calculations and get back to you.
blake92 said:I apparently made a mistake when drawing up the design and there are these extra supports in the back as displayed. These new vertical beams are directly beneath the two middle purple beams.
because of this, could i assume that where those vertical beams come up is the end of the beam, and solve for a beam with two equal loads on above where the reactions are at the end?
SteamKing said:You still have to analyze the longitudinal I-beam for max. load, if for no other reason because it is supported only at the ends.
The longitudinal HSS tube in the back has supports under all points where the cross beams are welded to it, so it is not as critical as the I-beam. However, the vertical supports for the middle cross beams appear to be quite a bit smaller than the end supports, so they should be checked to determine max. loading as columns. Because the two cross members in the middle of the frame are supported only a the ends and have the largest span, they still need to be checked for max. distributed loading.
blake92 said:For the point load on the I-beam
W 6x20
A=5.9in^2
S=13.4in^3
depth(d)=6.20in
thickness(tw)=.260in
wt=20lb/ft=.02kips/ft
Mallow= Sσ
M= (13.4in^3)(30.36kip/in^2) = 406.8kip*in
Mp= Pa
Mp= (12kips)(35in) = 420 kip*in
Mw=406.8-420 =13.2kip*in
w=8M/L^2
w= (8)(13.2kip*in)(12in)/(12ft)= 1.1kips/ft
Vmax= P+wL/2
Vmax= 12kips+(1.1)(12)/2 = 18.6 kips
Average Shear Stress = Vmax/dtw
shear stress = 18.6/(6.20)(.260)= 11.5ksi
11.5 is within the allowable stress of 14.5ksi for A36 steel
i think i did this all correctly, the only part was when i solved "Mw" i didnt make my answer negative because since its a moment i thought sign only meant direction and i didnt think that mattered and i only needed the value.
SteamKing said:The W6"x20# beam is A36 steel, so the allowable stress will not be 30.36 ksi. At best, it will be 0.66*36 = 23.76 ksi in bending, according to AISC.
blake92 said:i take that back. it doesn't work.
am i using the correct equations to determine the uniform load on the I-beam?
SteamKing said:blake92:
I've taken a quick look at the die rack and made some hand calculations of the maximum loading which it can support. These calculations are attached below:
View attachment 71733
If the two HSS tubing cross-members in the middle are loaded to 250 lbs./in, and there is no distributed load applied directly on the I-beam, the bending stresses at the ends of this beam reach the allowable bending stress for A36 steel. (the self-weights of all members have been neglected)
Of course, these calculations are extremely rough. If you are concerned that the rack may be overloaded, I would recommend that a structural model of the entire rack be analyzed by an experienced structural engineer.
As I have mentioned in a previous post, mixing HSS and mild steel members in the same frame means that you will load up the mild steel member to its stress limit before the HSS members.
Of course, none of this analysis has considered the welding used to fabricate this rack, which is something one can do only be examining the actual rack.
SteamKing said:If the beam is 144 in long, and one cross member is located app. 35 in from one end, then 144-35 = 109 in.
See p.6 of the calculations, Load Case 17 for a layout of the beam and the formulas used to calculate the bending moments. There are two cross-members tying into the mid-span of the I-beam, so I used Load Case 17 to calculate the moments due to the reaction coming from one cross member, and then flipped them w.r.t. the center of the I-beam and combined the moments to get the total load due to the two center cross-members. An example of superposition, if you will, to calculate a complex beam loading from a simpler one found in a table.
On that same page, Load Case 15 was used to calculate the moments and reactions in the
cross members under the distributed loading.
blake92 said:can you also explain what you did here,
Beam Forces and Moments:
P = 12,000 lbs. from loading on cross-member
a = 35 in.
b = 109 in.
R1 = 10,218 lbs.
R2 = 1,782 lbs.
mainly what is the 109in and where is it measured from?
and the significant difference in your reactions at 1 and 2
SteamKing said:The 12000 lb. load was the maximum load which could be applied to the I-beam by the middle cross members, each located 35 in. from the end of the I-beam. R1 and R2 are the reaction at the ends of the I-beam due to the application of a single 12000-lb. load as described. I did not use these reactions further in the beam analysis because the fixed-end moments produced much greater bending stress in the I-beam.
Obviously, the stresses in the beam ends will be greater due to these reactions, but I neglected them to estimate the max. loading which could be permitted on the cross-members without over stressing the I-beam.
SteamKing said:The loads in the legs are not the limiting factor in the capacity of this rack.
The limiting factor is the 144"-span I-beam made out of mild steel (A36). The HSS tubing has a much higher yield stress that the I-beam. If you load the cross members to 24 kips, that's the limit of what the I-beam can support, without any additional load distributed along the length of this beam. (in other words, no 10 kips or 10 kips/ft., or whatever).
IDK what your desired loading is for this rack, but the W6"x20# mild steel beam is not cutting it, IMO. If you want to add additional vertical supports in way of the cross members' intersection with this beam, as is shown on your revised sketch under the longitudinal tube beam at the back of the rack, that would reduce the bending stress in the I-beam. If you wanted to replace the mild steel I-beam with a HSS tube, that would probably increase the allowable load of the rack.
A beam with an unsupported span of 144" (12 feet) can only support so much load.
26 900 N (purple)
89 120 N (purple)
89 120 N (purple)
26 900 N (purple)
0 N (blue)
0 N (I-beam)
blake92 said:okay, I redid the calculations and ... I now get [Mallow =] 321.6 kip*in.
blake92 said:... for a 6 ft section of the tubing, max load is [Pallow =] 131 kips. Does that sound correct to you?
nvn said:blake92: I currently would assume the beams are simply-supported (for checking the midspans). If we blindly ignore dynamic loading, uneven loading, and weldment stresses, then I currently would say the horizontal beams together could hold,
Code:26 900 N (purple) 89 120 N (purple) 89 120 N (purple) 26 900 N (purple) 0 N (blue) 0 N (I-beam)
which totals Ptotal = 232 040 N = 232.04 kN.
Agreed (almost). I currently got Mallow = 36 220 N*m. Next, from your other thread ...
Agreed (close enough). I used your given length L = 1613 mm, and currently got Pallow = 600 950 N, using your assumed boundary conditions. However, I would rather use ke = 2.10 for the leg, which instead gives Pallow = 530 230 N.
Therefore, we see that Pallow does not govern, because Pallow (530 230 N) is much greater than Ptotal/(4 legs) = 58 010 N. Therefore, the governing stress is currently bending stress at the midspan of the I-beam, where Mallow = 36 220 N*m. Therefore, Ptotal = 232 040 N.
However, perhaps they set Ptotal = 88 960 N due to dynamic loading, uneven loading, and/or weldment stresses. Unless you analyze everything carefully, including weldment stresses, etc., then it might be advisable to not exceed the previous load, Ptotal = 88 960 N.