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Corroding beam @ stair
4

Corroding beam @ stair

Corroding beam @ stair

(OP)
I wanted to pick smarter brains here for a problem.

Existing stair, built nearly 40 years ago. See attached sketch for a layout. There are four stringers, each TS6x4. One of the stringer has significant rust while another one has some rust on it.

Repair: I figured that adding new plates to the existing HSS is not going to do anything because of the amount of corrosion. So I want to put in new beams to support the stair as shown.

My question is in regards to the existing steel that's already corroded. Would you recommend it to be welded with plates and painted to be left in place, or would you remove the entire portion of the corroded beam and replace it with new?
Also, because of the corrosion issues, I am going to have the new steel galvanized and then painted. Considering that this is an interior application, is that overkill?

Thank you,

RE: Corroding beam @ stair

(OP)
BTW, the reason I am putting a new post in rather then connecting into the existing foundation wall is because the client is reluctant to do that because he indicated that he has had other problems with water infiltration and drilling holes into the existing wall may create other problems.

RE: Corroding beam @ stair

That rust doesn't look that bad to me, can you get someone in there to sandblast it or find how deep it is?

RE: Corroding beam @ stair

It seems odd that the rusting is so localised.  Where is the water coming from?  It would be useful to find the source and seal it up.

I agree with ztenguy that a good first step is sandblasting.  This will give you a better idea of the extent of the rusting.

BA

RE: Corroding beam @ stair

(OP)
BA,

I couldn't figure out why the rust was so localized and at that particular location. To me it appears that there is water infiltration from the basement wall. But why is rust more prominent away from the wall rather then right at the wall is a mystery to me. My guess is that since the water ends up sitting more often in the center of the beam (since the beam is definetly deflecting near mid span), their has been more prolonged exposure to water then the ends.

RE: Corroding beam @ stair

You may have corrosion below the paint, so until you sandblast, you won't know the extent.

BA

RE: Corroding beam @ stair

This looks like a continuous "bent" stringer system where the existing tubes are fully welded at the two kinked locations.  If so, how are you connecting the new tubes into this system?

In effect, you would have a system that looks like the attached idealized sketch:

 

RE: Corroding beam @ stair

(OP)
BA, thanks for taking the time to sketch it. I did not show a moment connection in my sketch but of course I'll have to design a moment connection (possibly 2 welded plates, one each side) between the two HSSs.

I am reluctant to get rid of the existing tubes because they are embedded in conc. I want to enclosed the damaged portions with welded and painted plates and leave the existing in place.

RE: Corroding beam @ stair

heh...so what did BA sketch?  I'm curious....  smile

Also, this is REAL picky on my part, but a bent plate should be called out with the thickness listed first...i.e.

BENT PL 1/4 x 8 x 2 x 0'-6" @ 8" o.c.

 

RE: Corroding beam @ stair

(OP)
Sorry, JAE. Lunch and eng-tips don't mix, do they now :)

Thanks for pointing out the correct designation but can I have a picky question. Since this is a bent plate and not an angle, am I not supposed to show the " (inch) symbol with every dim? I thought with angle you don't have to show " but with bent plate you do. Am I wrong?

RE: Corroding beam @ stair

Actually, the sketch was by JAE, not me.  

It would be desirable to leave the existing steel intact if possible, but the source of water and the extent of rust need to be known before moving forward with a solution.   

BA

RE: Corroding beam @ stair

Joe,

I always use ft and in. symbols on the length only.  I may be wrong from a "picky AISC" perspective though.

I would second BA's recommendation to solve the water source as part of your effort.  Also, I would worry about the slab itself.  Do you know if the landing/stair slab system is deteriorated as well?

 

RE: Corroding beam @ stair

(OP)
JAE,

There are no visual signs of any damage to the slab itself but I am not sure how to verify that.

As far as water source is concerned, like I said before this has to be coming in from the side bearing within concrete. The top of landing and actually bottom of existing HSSs is above grade (see attached pic taken from outside). I can't think of any other water source then the what is creeping up the foundation/basement wall and finding its way into the steel.
 

RE: Corroding beam @ stair

My guts tell me that the water penetration is most likely at the handrail posts.

Also, I am not surprised to see the rust in the tube members - there is no ventilation...  For the new galvanized hat sections, I would encourage you to install some vent/drain holes as you will be sealing up the members again if they are not installed, leaving the possibility of future rust.  Galvanize the hat sections after the holes are drilled.   

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

RE: Corroding beam @ stair

(OP)
Mike,

I was definitely going to recommend galvanizing of new steel. One of the reasons I am keeping the new and old steel separate is because existing is not galvanized and as far as I know if galv and non-galv steel are in contact, the galv will try to galvanize the non-galv part. So I may have a problem at the moment connection of the new and old steel i.e welding galvanized plates between a new galv section and existing non-galv section but I don't see a way around it.

As far as vent are concerned, I was planning to place two small holes in the horizontal leg of the bent plate (6" long). For the new HSS I don't want to leave the ends open so I'll use cap plates but would it be advisable to have some small vent holes at the bottom side of the HSS? Do I just put on the drawings that I need vents for the steel guy to do his thing or do I need to spell out size and spacing?
 

RE: Corroding beam @ stair

You have to have vent holes in any closed section that you are going to have galvanized, the galvanizer will insist on it.  And there is hardly any closed section which might not take on some water, however hard you wish otherwise.  So, it is a good idea to put sizable drain holes at the low points.

JAE.... sorry about the mixup on the sketch attribution, even though it wasn't my doing.  Probably happened out of force-of-habit, since most of BA's post usually include the phrase, 'please provide a sketch.' smile

RE: Corroding beam @ stair

BA is a very visual engineer...like all of us.  A sketch is worth a million words.

RE: Corroding beam @ stair

It is possible that the buried ends of the existing HSS are open and water is entering the interior of the tube.  If so, oxidation may be proceeding on the inside.

BA

RE: Corroding beam @ stair

I would say the closed section is definitely corroding from inside.  The second photo looks to me like a void in the wall...but then my eyes are old.  That is an advantage of galvanizing, you get protection both inside and out.

RE: Corroding beam @ stair

Your eyes may be old, but mine are older and I think you are correct.

BA

RE: Corroding beam @ stair

(OP)
This is a follow up question on my original post. I am going to recommend new caulking/sealant for the handrail posts and expansion joints at the exterior, hoping it would stop water infiltration. But I am also going to recommend drilling of some vent holes into the existing steel, hoping that if any water makes its way to steel it would be dried out. And this is where my question is. Is my understanding of ventilation and water drying out correct or my actually providing a source of oxidation by drilling the vent holes for the water that might make it in?

RE: Corroding beam @ stair

JoeBaseplate,
You are planning to enclose the existing hollow section within new steel framing according to the sketch in your first post.  If you drain the old HSS, you will be retaining water on top of the new HSS which you cannot drain.  I wonder if that is wise.

BA

RE: Corroding beam @ stair

BA,
Looking at his first post, those bent plates are not continuous.  Not sure if that is a good idea or not, just saying...  I don't like the proposed scheme of leaving all that corroded steel in place.

RE: Corroding beam @ stair

hokie,

I missed the fact that the bent plates are not continuous.  That makes me wonder how the new 6x4 HSS on the flat can adequately replace the corroded 6x4 HSS members.

I don't like leaving the corroded steel in place either.  I still believe that more investigation is needed to determine the extent and source of the corrosion.

My preference would be to eliminate the source of water and to replace the existing beams, field welding to the sloping portion as required.  I don't think galvanizing the replacement material is appropriate in that case.

BA

RE: Corroding beam @ stair

Me too.  It's difficult from the photos to guess at the source of water penetration.

RE: Corroding beam @ stair

How about cutting a 3"x3" access opening on the underside near the concrete wall?  This would permit inspection of the inside of the existing HSS at the bearing point.  If water has been entering through the open end of the HSS, it could be sealed tight through the access opening.

An opening in that location would preserve the shear strength of the member.  The moment is negligible near the reaction, so the loss in moment capacity should not be critical.

BA

RE: Corroding beam @ stair

After you make the hole, you could use a borescope to inspect further.

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