Steel Joist Sizing

[aokamura]

I need a suggestion or recommendation on how to make a 8" deep steel joist fit in a 9" space.

The existing steel joist is supporting the metal decking and concrete topping on the second floor (e.g. joist supporting second floor), which is supported by a beam spanning in the opposite direction of the joist. It is flush-frame, and the depth of the existing joist is 9" (built in 1960's). However, we want to use an 8" or 10" deep joist... is there a way to fit an 8" deep joist into a 9" space? Or, is there a way to fit a 10" deep joist into a 9" space?

Any help would be appreciated. Thanks!

 

[msquared48]

What would be preventing you from fitting a smaller (8" deep)truss onto a larger (9" deep) space?

Is the problem the accessibility of the bearing points at the ends of the joist? In other wor4ds, is it impossible to maneuver a joist of the same length into the same space?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[apsix]

A 10" joist laid flat will fit in a 9" vertical space.

Clarification is required; as it stands the answer appears trivial.

 

[dik]

Can you use a 6" channel? and what spacing is required? Can you use a 2-1/2" topping on V-rib pan?

Dik

 

[SteelPE]

A sketch or further clarification would definitely help.

Why are you trying use an 8" or 10" deep joist. You could always spec a 9" joist and designate it as a 9 KSP and list the loads on the drawings.

 

[jike]

If you need a joist in an existing space, is the problem physically erecting it? If so, provide a splice at mid-span or leave one bearing seat off and field weld it in place.

 

[Ltwine]

Is the new joist to "replace" the existing 9" joist, or it is to be supported off the existing joist? I don't see problem either way (construction-wise). Need clarification on your question.

 

[aokamura]

Thanks for all the replies.

Yes, I need to replace the 9" joist (severely corroded) with either an 8" or 10" joist, and I have to make it fit in that space. I've thought of using plywood under the 8" joist, but I don't think it'll meet the fire code.

We don't have 9" joists readily available here in Hawaii. It'll cost an arm and a leg to ship that size overseas.

Thanks!

 

[Ltwine]

I don't see how could a 8" joist couldn't fit into 9" space, provides if the 8" is adequate to begin with. You should keep the top flush with the bottom of existing floor, and adjust the end bearing chords using plate, if it is smaller than the original. Now you have extra 1" head room for space below.

If this does not work, you should provide a cross section to explain the space limitation. It is difficult to guess.

 

[aokamura]

Ltwine,

I guess the question was... what do I need to do in order to make the 8" joist fit into the 9" space so that it can be supported by the cross beam underneath? In order to fill the 1" gap, what do I need to do, e.g. use plywood, weld steel plates, etc. And if using 10" joist, do I cut the bottom web to make it fit?

Thanks.

 

[aokamura]

Oops, I meant by cutting the bottom flange, not the web.

 

[msquared48]

If you are worried about the deflections, just use two 8" deep joists in place of each deteriorated 9" deep joist.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ltwine]

You should add 1" steel plate on top flange of the cross beam, and provide identical support mechanism as the original joist (which should have a seat to sit on the cross beam). It is kind strange to have full depth joist sit and accross the support beam, I have never seen it before, can you provide a cross section view? My understanding and comments could be completely wrong.

 

[aokamura]

Here's a couple photos for clarification... I hope I explained it in the correct way...

 

[aokamura]

Here's the second one...

 

[SteelPE]

Are the existing members actually steel joists? When I think of a steel joist I think of as open web bar joist like the ones you see here.

http://www.vulcraft.com/

You stated previously that you live in Hawaii and that getting a 9” deep joist would be to expensive. How is it that you have 8” deep joists available? Usually steel joists are made to order.

I think what you are looking for is an 8” or 10” deep steel beam. Am I correct?

 

[Ltwine]

From the photo, it looks like slender I beam with ends welded to the cross beam. Provides the cross beam is about 1" deeper than the 9" joist, I really don't see problem.

Are you thinking to partially remove and replace the deteriorated segment of joist, and that's reason why you worry about matching joist with different sizes? I don't think partial cut and replace is a good idea. You should replace the full length joist, or you may consider build-up beam segment fit to the cut-out with full pentration weld. Not easy, but can be done.

 

[aokamura]

SteelPE,

I believe you are talking about the truss joists.

What I'm looking at is provided by Dietrich Metal Framing, which has a manufacturing plant out here in Hawaii. In their SSMA catalog, they have joist sections which are called S-sections. They have 8" and 10" deep S-sections available, but not 9".

The existing steel joists (in the pic) are 19" o.c. and are hung off of the cross beams.

Thanks!

 

[aokamura]

Ltwine,

Yes, I will be replacing the full length of the joist.

So, are you saying that I should replace it with another beam or joist and full penetration weld the bottom flange onto the cross beam with a 1" steel plate?

Thank you!

 

[Ltwine]

If you are going to replace full length joist. Then check the original joist-beam connection, and do the similar. You need to make sure the 8" joist is adequate to resist the flexural stresses, and the new connection can take the full load.

Full pen-weld is required only if you are doing cut & fit a small segment of the joist. For full replacement, you may fillet weld the joist web (to the cross beam web) on one side, or both side, depends on the strength level required.

As the original 9" joist is hang off/weld on the larger (looks like 10") cross beam, your 8" replacement joist should fit into it without problem. There is no bearing plate required to fill the gap (2" from bottom of joist to bottom of cross beam).

Before you do anything, why not ask someone to review your cal to make sure everything would work out as planned. Good luck.