Steel beams vs Open web steel joists for Economy...

[KevinChez]

I am designing a 3-story building with steel columns and girders. Initially the MEP wanted to use steel joists to run mechanical through now he is saying it will be better for him to run the ducts under the joists. No need for the open webs now unless there is a cost savings. I will have to run the filler beams in the short direction because they want to reduce the height. I was running the joists in the long direction (longest span 36'). The short direction is approx 18'

If anyone knows a rule of thumb please share...otherwise I will go to RS Means.

 

[BAretired]

Open Web Steel Joists are generally more economical than steel beams. For rectangular bays, joists should be in the long direction, beams in the short direction for best economy. Mechanical ducts can be placed within the joist depth if the openings are large enough to accommodate them.

BA

 

[JedClampett]

Joist prices per pound are pretty close to steel shapes. So if a lighter section can be used, you're going to save money. And sometimes joists are a lot lighter (40% to 50% of rolled shapes).
They have several drawbacks (for one, 20 years from now no one is going to know what load they're good for) and they're a lot more hazardous to work on, but if price is the driving factor, you're going to save money.

 

[Triangled]

I second BA.
I usually estimate steel by weight. You can get the approximate weight of your potential steel joist design from the SJI catalogues available from any manufacturer... I think downloadable from Vulcraft. You can size preliminarily size your wide flange beams and this get their weight. Add up the total floor weight of both systems. Your joist dealer can give you a ballpark $/ton cost and so can your steel supplier. I don't have those numbers at my fingertips. But just the weight comparison will tell you a lot.

 

[jberg]

For conventional steel shapes (AISC W-shapes), I find that the Floor Framing V14 program from AISC (

http://www.steeltools.org/Resources/ViewDocument/?DocumentKey=acd36a98-5236-41cb-9d65-b66731a71fcf)

is very useful for a quick floor framing weight. It handles composite and non-composite framing.

JWB

 

[dcarr82775]

Depending on the use, joists can be problematic with floor vibrations. If vibrations are a concern, the economy of joists on a purely steel weight basis tends to go out the window.

 

[a2mfk]

You can't just look at the weight of steel, think about fabrication costs of a joist versus WF beams. You can space beams much further apart. You also must consider fireproofing ratings and costs. A very popular system in Florida is WF composite beams spaced 8-10' apart with a 5-6" steel deck and slab. It has been a while since I looked but I think at around 5-6" you get a very good deck and slab fire rating which can lead to substantial savings in that department, ie, no fireproofing. Also, much better vibration performance like dcarr said. BUT- you do have to have the floor-floor heights to accommodate the MEP and WF beams.

My former company almost exclusively used this system in "higher end" buildings such as schools, hospitals and office buildings.

But I have also had success with floor joists.