Building a Steel Square Tube Deck for 10ft Pond Filter Pit

[King Tut]

I am building a new deck - from wood tables I know that my joists need to be 2 x 8 to span the 10 feet of my pond filter pit that is built below the deck. I do not want to give up 8 " of head room while working under the deck - so I am considering steel of some shape or form to use for the joists and would like to stay at 2" or less height.

Any help in deturmining some material that would do the job would be helpful. The smallest if found was 1 1/4 by 1 1/4 square tubing with 1/4 inch walls. The other dimension is 16' so the joists need to go across the 10' side. I can put them as close as 12" oc.

Thanks - New member - old engineer - IE '71

 

[nvn]

King Tut: Assuming the 2 x 8 joist spacing you are referring to is 406.4 mm (16 inch), and let me know if that is not true, then to have equivalent deck strength, your steel square tube joists would need to be one of the following options, where s = tube spacing, on center.

(1) 31.75 x 6.350 mm square tube, s = 150 mm (5.900 inch).
(2) 38.10 x 6.350 mm square tube, s = 235 mm (9.250 inch).
(3) 44.45 x 6.350 mm square tube, s = 350 mm (13.75 inch).
(4) 50.80 x 4.763 mm square tube, s = 400 mm (15.75 inch).
(5) 50.80 x 6.350 mm square tube, s = 480 mm (18.90 inch).

Let us know if you prefer a tube size different from any size listed above, and I will give you the corresponding spacing value.

 

[King Tut]

King Tut: Assuming the 2 x 8 joist spacing you are referring to is 406.4 mm (16 inch), and let me know if that is not true, then to have equivalent deck strength, your steel square tube joists would need to be one of the following options, where s = tube spacing, on center.

(1) 31.75 x 6.350 mm square tube, s = 150 mm (5.900 inch).
(2) 38.10 x 6.350 mm square tube, s = 235 mm (9.250 inch).
(3) 44.45 x 6.350 mm square tube, s = 350 mm (13.75 inch).
(4) 50.80 x 4.763 mm square tube, s = 400 mm (15.75 inch).
(5) 50.80 x 6.350 mm square tube, s = 480 mm (18.90 inch).

Let us know if you prefer a tube size different from any size listed above, and I will give you the corresponding spacing value.

The 2 x 8 was 16" spacing for 10' and a 2 x 6 was 12" spacing for 10'.

The option 3 size of 1 3/4 square by 1/4 thick walls seems fine - because of my decking material - it will need to be 12" on center spacing. Can you give me the maximum span length that this size tube with the 12" spacing will support?

 

[nvn]

The values I posted above only consider equivalent strength, but not deflection. I am now noticing deflection is quite high, and greatly governs. Do you mind if your floor is slightly bouncy? I will try to look back into this within a few days.

 

[nvn]

King Tut: I made a mistake in my above numbers; but it turns out, it does not really matter, because, as I mentioned, deflection greatly governs, not strength. Therefore, the following steel tube size and spacing currently appears adequate for a span length of 3050 mm, and gives a deflection of L/223.

(3) 44.45 x 6.350 mm square tube, s = 304.8 mm (12.00 inch).

The maximum span length for the above option would be 3163 mm, which would increase the deflection to L/200.

 

[King Tut]

the following steel tube size and spacing currently appears adequate for a span length of 3050 mm, and gives a deflection of L/223.

(3) 44.45 x 6.350 mm square tube, s = 304.8 mm (12.00 inch).

The maximum span length for the above option would be 3163 mm, which would increase the deflection to L/200.

I was checking prices etc - and basically I could not find the 44.45 size - only the 50.8 size and no thicker side walls than the 6.35. So how much less deflection would I get with the

(5) 50.80 x 6.350 mm square tube, s = 304.8 mm square tube. This is the most economical size for me to buy. If it will deflect to where I can feel it walking on it, then what can I do to reduce that.

Will I notice this - ie feel it give a little. I really do not want to feel any defection. How about a 2" wide 3" tall rectangular tube with 1/4 or 3/16 walls?

 

[nvn]

King Tut: The following steel tube option appears to work well, where s = tube spacing, L = span length, and y = deflection.

(4) 50.80 x 4.763 mm square tube, s = 304.8 mm, L = 3050 mm, y = L/290.

 

[King Tut]

King Tut: The following steel tube option appears to work well, where s = tube spacing, L = span length, and y = deflection.

(4) 50.80 x 4.763 mm square tube, s = 304.8 mm, L = 3050 mm, y = L/290.

I just can not seem to understand the defection numbers. Can you give me an idea of what a 2x10 wood joist at 10' would have for a deflection. Those are non detectable in terms of deflection.

Also - I am fine with (5) which was the same 50.80 square but a slightly thicker side wall. the price difference between 4.763 and 6.350 is about $1.50 per foot. What was that deflection - and again give me some feel for defection numbers please.

 

[nvn]

King Tut: 38 x 235 mm lumber joists at a span length of L = 3050 mm is not a fair comparison, because they would be ultra stiff. At L = 3050 mm, you would not get into the detectable deflection range until you go to 38 x 140 mm lumber joists, or maybe widely-spaced 38 x 184 mm lumber joists. E.g., 38 x 140 mm lumber joists, at L = 3050 mm and s = 406.4 mm, would be y = L/333; or at s = 304.8 mm, would be y = L/444. You can compare this to a wooden joist option you listed in post 3. L/360 prevents cracking of plastered ceilings. L/480 is called enhanced stiffness, which can be heavy and expensive.

The following steel tube option appears to work very well. If you want other options listed, let us know.

(5) 50.80 x 6.350 mm square tube, s = 304.8 mm, L = 3050 mm, y = L/352.

 

[nvn]

King Tut: L/333 and L/444 in post 9 are for dry service conditions; but I should have used wet service conditions. This is corrected in option 9, below. All of the following options are for a span length of L = 3050 mm and wet service conditions, where m = total mass of all joists for the entire floor. Some lumber joist options are also listed, for comparative purposes. Are there any steel tube sizes between options 5 and 6 you want listed, such as 50.8 x 63.5 x 4.763 mm rectangular tube, if available?

(3) 44.45 x 6.350 mm square tube, s = 304.8 mm, y = L/223, m = 394 kg.
(4) 50.80 x 4.763 mm square tube, s = 304.8 mm, y = L/290, m = 357 kg.
(5) 50.80 x 6.350 mm square tube, s = 304.8 mm, y = L/352, m = 460 kg.
(6) 50.80 x 76.20 x 3.175 mm rect. tube, s = 304.8 mm, y = L/566, m = 312 kg.
(7) 50.80 x 76.20 x 4.763 mm rect. tube, s = 304.8 mm, y = L/792, m = 455 kg.
(8) 50.80 x 76.20 x 6.350 mm rect. tube, s = 304.8 mm, y = L/983, m = 591 kg.
(9) 38 x 140 mm lumber, s = 304.8 mm, y = L/400, m = 152 kg.
(10) 38 x 184 mm lumber, s = 406.4 mm, y = L/662, m = 152 kg.
(11) 38 x 235 mm lumber, s = 609.6 mm, y = L/1051, m = 135 kg.