Residential Renovation - Structural Questions

[DCStructures]

Hi all,

Working on a residential renovation project that has me coming up with lots of questions. Long story short, it's a 3 story brick, wood framed row house with existing basement, built early 1900's. Floors are deflecting slightly at all levels, likely due to age. There are currently several timber columns and steel shoring posts in basement reinforcing the existing first floor framing. Client wants to install new steel framing below existing first floor framing to replace timber columns and steel shoring posts, help try to prevent further deflection of floors, and create an open basement space. The new steel beams will be cambered. The existing structure will not be jacked up during construction.

My questions, in no particular order:
1. Some of the new steel columns (HSS) are located at the perimeter of the space and their new footings will butt up against the existing bearing wall foundations. How is this situation normally handled? I don't want to add load to the existing wall footings. But trying to size this new footings on its own, with the column positioned eccentrically to get it near the wall, is blowing up the bearing pressure.

2. Would you expect to get steel fabrication drawings for a small residential project like this? Or do I need to detail my drawings so that it can be built off of them?

3. How would a cambered beam be installed in a case like this with an existing floor above and with no jacking? Can they just lift the beam into place and the wood joists will just adjust?

4. Should there be a physical connection between the wood joists and the new steel beams, or just ensure that the beam is well shimmed at every joist?

5. What anchor bolts are typically used for residential? I'm used to F1554 Gr. 36ksi anchor bolts for commercial. Can I use those or will the contractor gawk at it?

6. Similarly, can residential contractors handle some simple fillet welds? I'm assuming they would prefer bolts, given the choice, right? Again, I'm used to commercial.

Thanks.

 

[bookowski]

1. If you can pull the column off the wall and run the beam over the top for a small cantilever.

2. You're better off to detail as much as possible. You should get shop drawings, it's not uncommon for something of this size.

3 & 4. Does it need to be cambered? For connection I show shims at all joists to get bearing. To brace the beam you can add 2x, steel angle, or framing clips nailed to the underside of the existing joists and tight to the new beam top flange.

5. Post installed - slim chance of them casting anchors in the right locn

6. Anyone handling steel should be ok with fillet welds.

 

[DCStructures]

Thanks, bookowski.
1. Pulling column off wall is a really good idea. Hopefully I can convince architect & client. Even if that flies though, I still cannot come up with a reasonable sized footing. Load is only about 20 kips (D+L), but have to assume bearing pressure is only 1500 psf, and eccentricity is killing me. Am I missing something here?
3. I thought cambering the beams was a good idea, but is there a rule of thumb for minimum deflection that you'd bother cambering for?
Thanks again.

 

[hokie66]

1. Your footing doesn't have to be square. Make it a rectangular footing, with the long dimension parallel to the wall.
3. Cambering is going to cause you further problems. The floor is already deflected, and will deflect more when the load comes onto the beam...unless you do some jacking to level it up. What is preventing jacking? No, the floor joists won't just adjust.
4. A connection between the joists and beam is necessary.
6. Site welding is best avoided. You don't want to burn up your good work.

 

[DCStructures]

Thanks, hokie66.

Some follow up-
1. Yes, I plan to use a rectangular ftg. Still, any little bit of eccentricity seems to blow up the bearing pressure.
3. How is cambering going to cause more problems? I could see jacking up the joists locally just under the beam during installation (is this what you meant?), but we want to stay away from any major jacking of the structure.
4. Got it. Going to follow bookowski's advice, "add 2x, steel angle, or framing clips nailed to the underside of the existing joists and tight to the new beam top flange."
6. Some welding can't be avoided, but will try to minimize as much as possible.

What other questions have I not even thought of yet? Any other insights regarding constructability or other issues?

Thanks again.

 

[PUEngineer]

Maybe you can explain your reasoning for cambering, in a little bit more detail.

Usually a beam is cambered during new construction because it can be attached to a column or a girder and then new framing is attached to it, so it's loaded with dead load to eliminate the camber. In your case, you're replacing a timber beam, that's likely got some permanent deflection, with a new steel beam that you want to camber. Yet you say no jacking will done, how is that physically possible?

 

[DCStructures]

Maybe I wan't clear in my original post. Not replacing a timber beam. Replacing 1 timber columns and 2 steel shoring posts under existing (and to remain) wood joists. Perhaps it's not possible to remove deflection already in joists, but would like to minimize further deflection. Seem like good reasoning for cambering the new steel beams? What would you suggest instead?

 

[SLTA]

Quickly, as my kids are coming down the hall...

jacking existing wood floors doesn't work and creates more problems down the line. the deflection is the new normal and quickly forcing them up by jacking can cause cracking, etc, and worse problems than you had by a sloping floor. it took years to get sloping - to force them up in a week is rough on the fibers.

I always feel: if you don't want an old floor, don't get an old building. but I know that's not always reasonable.

we have previously shimmed on top of the existing floor to make it flatter, using feathered plywood. in one case, when the floor definitely had the capacity, we poured leveling concrete on it. worked great.

good luck!

 

[PUEngineer]

So, what are the new steel beams doing? Shortening the existing floor joist spans? If that's the case, then putting in a non-cambered beam and leveling compound per slta might be an applicable approach.

 

[msquared48]

1. Depending on the size of the existing strip footing and what you want to see in the basement, you could either underpin the existing strip footing with a new one for the column, or pour the new footing on top of the existing slab - architecturally a mess.
2. Absolutely. I always require them for any steel project. I am not a detailer.
3. I assume the beam is cambered for 1.5X of the dead load. I doubt that a contractor can do that without leveraging the beam via jacking. As previously mentioned, scrapping the cambering might be advisable without end jacking.
4. Usually there is a 2X or 3X nailer connected to the top flange of the steel beam with Nelson Sill Plate anchors welded to the top. You can use top flange hangers to attach the existing joists to the new wood plate. I would also infill the area between the flanges with a vertical wood nailer (thru-bolted through the web of the steel beam) for extra hanger capacity if needed.
5. 1/2" X 10" A307, but you will need to use drilled and epoxied connectors if anchoring a new sill plate to the top of an existing concrete wall.
6. Some can but most cannot. I would use the golden rule - shop weld and field bolt.

Cheers

Mike McCann
MMC Engineering

 

[bookowski]

You said that you 'have to assume the bearing pressure is only 1500psf' - is that from a geotech report or are you just assuming that value? That is pretty low.

You said that there is not currently a timber beam? If it is a typical row house then you are presumably picking up the first floor framing and the internal bearing wall adj to the stair up the building - usually there would be a timber beam or a brick masonry pier/arch system in the cellar that you are replacing.

Don't camber the beam, it doesn't make sense. Shim the steel beam to get bearing and live with that. If they are doing a complete rehab you can level the floors from above.

 

[msquared48]

Yes, it is low, but is the maximum by code for most soils without a geotech report.

Mike McCann
MMC Engineering

 

[DCStructures]

Bookowski-BYou are correct as far as framing layout above 1st flr. Currently there are various timber and steel posts at the basement level below the interior wall adjacent to the stair. These posts are approximately mid-span of the wood joists. No wood beam here though.

Yes 1500psf is code minimum. Low but no way around it.

 

[DCStructures]

Re-reading all the responses (thank you all so much!), I've got a lot of good advice here. Two items still keeping me up at night:

1. Using code min 1500psf (no geotech), sizing perimeter columns with rectangular footings. New columns need to be as close to existing wall as possible (allowing space for base plates). Pulling columns away from walls is not an option after all. Technically column is not centered on new footing but centered on combined area of new column ftg and existing bearing wall ftg. If I just look at new ftg independently, bearing area is ok, but if I look at new ftg independently with column located eccentrically, then bearing pressure is too high. Is it acceptable common practice in a situation like this to locate column centered on combined area of new column ftg and existing bearing wall ftg, and design new ftg area for concentric loading? I'd prefer to not have to underpin, etc. Would you dowel into existing ftg and roughen surface where pouring new concrete?

3. I'm pretty convinced after all the helpful feedback here that cambering is not appropriate. Calculated deflection of beams is only a half inch total deflection, spans are less than 16ft, and me/architect/client don't want to do jacking. Issue is that another engineer previously recommended cambered beams, so I judt need to explain all this to client.

Thanks again all. Happy thanksgiving!

 

[TDIengineer]

Regarding your footing question. Any new footings placed and then loaded will settle, period. When it is a new structure, us engineers hopefully design the footings consistently based on loads to have equivalent settlement across the structure.
If you're adding new footings to an existing building, the existing footings have already settled. You may want to consider attempting to provide adjustment in the column at the base if the footing does settle. Just a thought. Maybe with residential loads, and your very low 1500 psf soil bearing, differential settlement is not an issue.

 

[bookowski]

Based on typical rowhouse dimensions and loadings I am guessing that your 20k footing load is assuming the new post/beam carries full DL. If you are not jacking anything, but simply placing a new steel beam below existing joists, then I would expect that you will be taking much less than 100% of the DL that is already in place. Taking a look at that might help with your footing issue.