# What lb/ft W8 beam would I need to span 17' supporting 12' floor joists in the basement of a 1 story house?

The floor joists are 2x8's spaced 16" on centre. Also, what size steel column would be needed? Thank You

### 5 Answers

- 1 decade agoFavorite Answer
Without looking at it I would say a 9.5" metal beam and a 4" metal post to support in the middle. You may have to increase depth of concrete at point where post will rest, there should be a local metal I beam contractor that will beable to give a bid on materials, labor, and most important the proper method per code for your span, what you do with the bid is your choice.

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- draco4843 |Lv 41 decade ago
I could have given you the sizes of what you need but on second thought, I decided to show you how this is calculated so that if you need to do some adjustments, then you can later do it on your own. Here we go.

1. First we calculate the loading of the beam

a. Live load- for a residential building the normal design live load is 40 lbs per square foot. This translates to a load on the beam supporting joist with 12 ft span of:

LL = 40 x 12 = 480 lbs per foot

b. For the weight of the flooring, we can assume this to be a 1" thick plywood which weights about 5 lbs per square foot . This translates to a dead load on the bem of:

DL1 = 5 x 12 = 60 lbs per foot

c. For the joists, each 12 feet of 2" x 8" x 12 ft joist weighs about 64 lbs. But since the spacing of the joists is 16" or 1.333 ft, this translates to a load on the beam of:

DL2 = 64/1.333 = 48 lbs per foot

d.. The other dead load on the beam is the weight of the beam itself. Let's take a wid guess on this and assume it to be;

DL3 = 20 lbs per foot.

2. The total load on the beam therefore is:

W = LL + DL1 + DL2 + DL3 = 480 + 60 + 48 + 20 = 608 lbs/ft

4. The maximum bending moment on the beam is:

M = 12(WL^2)/8 = (12 x 608 x 17^2)/8 = 263568 in- lbs

5. The required moment of inertia of the beam section is:

I = Mc/S

Where;

M is the maximum bending moment

c = (1/2) the depth of the beam = 8/2 = 4 inches

S = the allowable stress for the beam material which is 20,000 psi for steel beams.

Therefore the required beam should have a moment of inertia of not less than:

I = 263568 x 4/20,000 = 52.714 in^4

The lightest W8 beam that meets this requirement is a W8 x 18 lbs per foot which has a moment of inertia of 61.9 in^4.

6. For the columns, the load on each column is:

P =WL/2 = (608 x 17)/2 = 5168 lbs

Assuming that the length of the column is 8 ft or 96 inches, the required slenderness ratio is between 60 to 120. Using a slenderness ratio of 80, the radius of gyration of the column section should be about:

r = 96/80 = 1.2

We now try a 3" schedule 40 pipe. The properties of this pipe section are:

I = 3.006 in^4

A = 2.219 in ^2

r = (3.006/2.219)^.5 = 1.164 inches

l/r = 96/1.164 = 82.47

Using the old AISC formula for allowable compressive stress:

Sc = 17000 - .485(l/r)^2 = 17000 - .485(82.47)^2 = 13700psi

P/A = 5168/2.219 = 2329 psi which is less than 13700 psi.

Use a 3" schedule 40 pipe for the columns.

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- Anonymous4 years ago
I love it when people ask you a question and then put stipulations in like yours. " I don't want to lose any headroom!) Well the 2" x 8"s aren't enough to hold the weight (obviously) and you don't want to lose headroom for a 11+7/8 LVL 2 ply beam which is needed - so you must go to a steel beam no taller than 7+1/2" + 3" or about 11". The rule is to never set steel on wood, so you will also need to steel support posts (1 at each end of new steel beam). Myself, I would lose a little headroom and go with the Lvl Header 2 ply. I would be checking the rest of the house if the builder put floor joists on 24" centers. I hope you have at least 3/4" tongue and groove sub flooring? Lvl's are *laminated venner lumber*. They are 1+3/4" thick and your beam requires 2 layers or a 2 ply lvl beam. They cost about $5-$7 per lineal foot per layer. You would need to build a temporary wall on both sides of the header to hold the floor joist while you make the switch. x brace the temporary wall for stability. The lvl layers come in increments of 2' usually. Install them one layer at a time and use heavy duty liquid nail adhesive between them. Use 5/16" carriage bolts to bolt together or pull tight with big C clamps and screw together using heavy deck screws.

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