Under-house rusting beams and new support piers

[WiltshirePaul]

Hello all. first post for me as I've got issues at home and I'm overwhelmed....

 

Our house is from 1979/80, built into a greensand slope. The double garage sits under the front two bedrooms and bath/cloakroom. Those rooms are held up by three 6m spans of single and doubled up steels sitting in between 9inch cavity walls.  We get a lot of water movement through the hill behind the house and although the garage walls get efflorescence it doesn't get wet. But there's corrosion to the ends of the steels that are underground. They were boxed in but I've cut all this off and removed fireboard, insulation and dead rats to mor or less start again.

 

An old boy surveying engineer came round this week and said you need to stop the wet otherwise anything you do to the steels isn't going to work. His suggestion is to cut the steels and build block piers internally (if the concrete slab is thick enough (it isn't).  I am getting views/price from the steel cutting firm and I need to do/arrange the slab to be cut and excavated so that 300mm of concrete can be laid as a pad for the blocks. 

 

Anyway, I just wanted to ask if there are other options open to me. I can't get to the outside side of the steels as there's a hill etc in the way - but I could cut and dig around the steels inside the garage to see what the damage is - is there any kind of treatment I can do around the steels in the wall itself to stop moisture getting to them?  I should add that I intend to abrade the steels and prime/intumescent paint them afterwards as they were boxed in before. 

 

Thanks, Paul

[WiltshirePaul]

here's some better images as close ups. This is the worse one

Edited Wednesday at 09:28 by WiltshirePaul
adjust image

[Spinny]

When you say 'old boy surveying engineer' do you mean a qualified professional structural engineer with professional insurance providing a paid for written SE report to you as customer ?

 

Why exactly do you believe something has to be done with the steels ? Do you have any movement or cracking anywhere around the house and garage ?

Is it just your personal concern over the appearance of rust and concern to do the right thing for the next 50 plus years ?

 

Rust on steels is common. We had a much smaller pair of steels exposed during our extension work that had been in place since the 1980's and had plenty of rust (see photo). I asked our structural engineer whether they should be replaced or anything. He came and poked them hard with a screw driver looking for anything more than surface type rusting then said they were good for at least another 50 years. So I wire brushed as much rust off as possible by hand and using drill attachment, then used Neutrarust 661 liberally, then 2-3 coats of an iron oxide paint. See Pics.

 

Not saying your situation is the same, but before embarking on costly structural works  - presumably to make sure the house is fit for the next 50 plus years - you really want to base it on proper written professional advice. I am sure your house insurer would expect the same.

 

(PS I am not a builder, engineer, etc, just joe public)

 

 

Edited Wednesday at 11:07 by Spinny

[markc]

Cutting the beams is drastic and not straight forward. First I would be investigating where the water is coming from, looks like the gap around the bean has been filled with mortar - if this is bridging the cavity then you could be pulling water from outer leaf onto the beam which should only be sitting on inner leaf. Maybe the water is coming from above … leaking pipe / drain etc.

more investigation …. Less cutting!

[Ed_]

Rust bulks up much more than the parent steel, around 7 times, so rust always looks very bad.

I'm not saying its not an issue, but don't assume it must be an issue based on how it looks.

 

Concrete inhibits corrosion so it is not a given that there is corrosion under the mortar - is the mortar cracked at all? Rust expands so if it is rusted you might expect to see the mortar was cracked.

 

6m is quite a long span so it is likely that the beam is sized for the bending moment not shear force. BM is maximum at the middle of the span, shear force is maximum at the ends, so SF would be your concern here and it is likely the beam has spare capacity e.g. it is the middle of the span which is most highly stressed not the ends. Also your rust is mainly on the flange and the flange barely contributes to shear strength.

 

I would definitely investigate further before cutting anything. If you scrape the rust off you can measure the thickness and compare to the nominal thickness. If it has lost only a small amount I would clean and preserve as best you can and then monitor. If you are concerned then the belt and braces approach would be to get a technician out to do some ultrasonic thickness measurements and then ask a structural engineer to redo the calculations with the new thickness measurements to check the strength. They could also advise on remaining corrosion margin and when to start worrying about it again.

 

For preservation something like Zinga is ideal, and you can get a spray which might help to get into any tight corners.

[Mr Punter]

Yes, get a proper SE to inspect and propose any remedial work.  BTW intumescent paint goes really manky in damp conditions.

[Gus Potter]

14 hours ago, WiltshirePaul said:

Hello all. first post for me as I've got issues at home and I'm overwhelmed....

Welcome to Build Hub. Your photos are very helpful. 

 

As a word of encouragement, you'll get lots of different opinions, all you need to do is sift out what is most appropriate to your circumstance. Here is my first take on your first good post!

14 hours ago, WiltshirePaul said:

An old boy surveying engineer came round this week and said you need to stop the wet otherwise anything you do to the steels isn't going to work. His suggestion is to cut the steels and build block piers internally (if the concrete slab is thick enough (it isn't).

Ok I'm 62 in September. When I inspect houses like yours you have to try and impart enough info to the Client (You) to allow them to take a view. If you overwhelm them then you are not communicating that well. Sometimes us SE's  omit info to avoid confusion. Some SE's are not good at communicating.

 

So here is some extra info. The idea here is to let you see some of the things I might think about and I hope this helps you a bit. 

 

Take this photo below: Call it photo 01

 

 

And this one, call this photo 02.

 

 

In photo 01 and as you say you have quite a lot of retained soil and mobile ground water. Now in photo 02 the steels are actually at quite close centres. It makes me wonder if the steels are doing two things. Holding up the floor / structure above but also and most importantly providing lateral support to the wall that is up the left side of the external stair case. 

 

This is the key to any remedial solution. I would say the steels are doing more than just holding up the floor / structure above until proven otherwise. 

 

Now at first glance you may think.. what if we prop each steel in turn. Expose the ends and wrap in DPC, after we have checked they are not too badly rusted. Then introduce much more ventilation etc to mitigate the condensation 

 

But if the steels are also acting to restrain the walls horizontally then we could be making a big mistake by introducing a DPC which acts like a slip plane. 

 

I think there may be a solution to this that is not too onerous..but first you need to understand more about how the structure you have is working not just to carry the vertical load but also the horizontal loads from the soil, up the side and at the back of the house.  

 

To expand. It may even be that the beams over the garage are forming port of a stiff floor that is also serving to retain the soil pressure from the garden at the rear. I've designed houses this way before where I take the full floor deck and use it to reduce the cost and mass of the rear retaining wall. 

 

It's going to take you a bit of time to get to the bottom of this.. but persevere. 

 

 

 

 

[saveasteading]

Old boy surveying enhineer here.

Age can help. Grounds how he uses þx terms aurveycand engineer (overused and oftenvabused, I say protectively.)

 

All the advice above is very good. You really do need expert advice, and what you have so far seems sensible. 

 

You may be able to dig new foundation inside the garage slab. Breaker and some very hard work with a spade but a good labourer can dig 1m3/ hour.... if you can find one that accepts hard work.

 

Thar way the wet end if the beam will become redundant.  

Big IF. I'm assuming the beams are not supporting the structure above. 

 

The wetness is another matter and there are cellar solutions.