
#1
Oct2113, 04:11 PM

P: 9

I'm looking to replace a 6x6x12 pressure treated post with 4x4 steel tubing.
I was looking at a steel handbook and according to the "design strength in axial compression," a HSS4X4X1/4 for an "effective length" of 12ft is 72 Kips (72,000 lbs). Of course you still need to consider bracing, buckling, and other engineering aspects Im not expert in. Still, am I reading this correct? By chance can purchase this HSS cheap and for that kind of strength I'd go with it instead of the PT wood. Thanks and forgive me if I'm posting in the wrong section. 



#2
Oct2213, 02:04 PM

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Cruizer: No, that would support 244 kN, not 320 kN. (And the 244 kN value assumes the tensile yield strength of the square tubing steel material is Sty = 315 MPa.)




#3
Oct2213, 09:05 PM

P: 9

Thank you. Yes, yield strength is 46 ksi. 244 kN is still more than I would need and the price is unbeatable. Thanks again. 



#4
Oct2313, 08:50 AM

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HSS 4X4X1/4 Compressive Strength
Cruizer: How will your square tube be constrained on each end, in each of the two horizontal directions? Will it be pinned on each end, in each direction? Or fixed on the bottom end, free on the top end? Or what? And what will be the actual distance (length) between these two end constraints?




#5
Oct2313, 09:37 AM

P: 9

The tube will be welded to a 1/2" thick plate and anchored to a concrete footing. Not sure if this qualifies as a fixed connection but it will restrain it in both directions. As for the top, I will have a 1/2" L flange which will hold 22x12 beams and connected with 2  1/2" bolts. In this case the beam will only be restrained in the direction of the beam span. The tube will be a 12' 4x4, but I may also replace a 15' 6x6 with a 15' 4x4 tube.




#6
Oct2313, 10:28 AM

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Cruizer: At the upper end of the square tube, are you saying the square tube will act like it has a pinned restraint in the beam lateral (horizontal) direction, not free to translate laterally? Or will the square tube upper end be free to migrate laterally (which is called free), in the beam lateral (horizontal) direction, like a flag pole?




#7
Oct2313, 11:28 AM

P: 9





#8
Oct2313, 08:12 PM

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#9
Oct2313, 08:49 PM

P: 9

The other horizontal direction will be indirectly restrained. By this I'm referring to the joists (one every 16"), which run perpendicular to the 22x12 beams. The joists rest on top of the beams and are connected with hurricane ties to the beam, and connected with joist hangers to the ledger board.




#10
Oct2313, 10:58 PM

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Cruizer: But look at what happens if your entire system migrates horizontally at member BC, in figure 1 of the attached file. Notice the difference in the shape of the columns in figures 1 and 2. In figure 1, the columns look like flag poles (very weak); but in figure 2, they do not. The columns in figure 2 are much stronger than in figure 1. One method of creating a frame like figure 2 is by adding the green diagonal member, for each of the two frame horizontal directions. (I currently show only the frame horizontal direction parallel to the joists, in the attached file.)
Therefore, we need to find out if your whole building can migrate in either of the two horizontal directions, like figure 1, before we can determine the column effective length (Le). It can make a big difference in the capacity of the columns. 



#11
Oct2413, 09:38 AM

P: 9

I see. Can you tell me the compressive strength of the 15' 4x4 tube if the connection at the top is treated as a free connection? Depending on the answer and could add bracing to make the member stronger. I appreciate your great input.




#12
Oct2413, 10:45 AM

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#13
Oct2413, 11:04 AM

P: 9

According to that result it looks like I should stick with the 6x6 pressure treated wood, which is rated to be used without lateral support up to 20 ft. Not to mention it can generally support 80100kN of axial loading, in ideal conditions. Would you agree with that assessment?




#14
Oct2413, 11:22 AM

P: 9

Actually, I should rephrase that. Would I be better off remaining with the 6x6 post or subbing in a 4x4 steel tube?




#15
Oct2513, 08:24 AM

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Cruizer: If the square wooden post is fixed at the bottom end, and free on the top end, then Le/b = [2.1(4572 mm)]/(140 mm) = 68.6, which exceeds 50, which is not even allowed for a rectangular wooden column (b = crosssectional width). Therefore, I currently have nothing valid to compare, there.




#16
Oct2513, 09:17 AM

P: 9

Thank you Nnv. You've been extremely educational. I've gone with the steel tube, which although it's not as strong as I'd hoped it is still more than I need. I will definitely add bracing too.




#17
Oct2513, 12:52 PM

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Cruizer: If you brace the frame (in both directions) such that you have pinned ends, as shown in figure 2, then Le/b for your square wooden post would then become 32.7, which would then mean your wooden post could support an axial compressive force of 37.3 kN, if the only force applied to the post is a concentric, vertical, axial compressive force, assuming the post is southern pine (SYP), grade number 2.



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