Dowels between new and existing foundation 4

[Algohi]

Hi
I have to design an addition to one story home building. The new foundation is pile type with an 8" foundation wall.
The questions I am having now are:
[ul]
[li]Do I need to specify dowels between the new and existing foundation.[/li]
[li]Does the code specify any guides about that?[/li]
[/ul]

 

[KootK]

There are exceptions but I normally specify dowels to attempt to minimize differential settlement. If you have a geotechnical engineer on the project, avail yourself of their assistance. I know of no code directives on this.

 

[CELinOttawa]

While I agree with Kootk that this isn't strictly limited by code, the Geotech gang have some great guidelines.

I use the Canadian Foundation Engineering Manual (2006) which has a great bit of guidance for dealing with this on P. 178. Table 11.11.

Generally speaking, if you aren't comfortable with the geotechnical end of this work, the anchoring is best.

I recommend treating one half of the addition as unsupported for the base UDL, while the end in contact with the old building is attached for the resultant loads to the existing with the anchorage. Was an old trick I was taught early on, and has served me well.

 

[BAretired]

Is the existing building founded on piles?

BA

 

[Algohi]

@BAritired

No. it is on a strip foundation

 

[jayrod12]

Then you're likely to have some issues. You're mixing foundation types if you're piling the addition. Where I practice this was common, until we had a few dry years and now all of those additions aren't moving, while the existing building on strip footings is literally ripping itself away from the addition.

If they are trying to save money by not putting a basement under the addition I'd recommend belled piers (or end bearing straight shaft) bearing at the same depth as the existing footings to at least mitigate the potential for differential movement.

 

[BAretired]

Differential movement is a valid concern. Obtaining a geotechnical opinion would be a wise precaution.



BA

 

[CELinOttawa]

Gotta admit I missed that entirely: I know lots of places have been pile foundations forever; I made an ASS out of U and ME (assume)...

Jayrod has the comment of the thread; this is a pretty serious issue if you have mixed systems. I was assuming that you were connecting two of the same type of foundation together, one which has been subject to settlement over time, and the new one which will now begin its own settlement journey.

No to mixing. You can't blend your black (existing) and your white (new) to get grey; You're going to get a damned mess.

I actually disagree with Jayrod on the bell footings; the state of stress won't be similar enough. If anything the bell ends is going to be concentrating the load and causing additional movement, or the same relative issues as the traditional piles.

BAretired's got solid advice as well: Get a geotech for this one if you must proceed with different foundation types...

 

[jayrod12]

The billed piers would be sized appropriate for the same bearing pressure as the footings. Or at least that's how we size them here on the prairies on our gross clay.

We've done that successfully for a long time. However, I do agree that you should get geotech involved.

 

[Algohi]

@CELinOttawa
Does the "No to mixing" mean I should use a strip foundation?
If we rely on the fact that the existing foundation has been settled over time and the fact that a shallow foundation(strip foundation of existing building) means more settlement/movement compared to a deep foundation (pile), shouldn't the pile option cause less differential movement compared to strip foundation?

 

[CELinOttawa]

In my opinion, the same type of foundation is always best. So yes, I would use strip foundation on the new addition. As you can see above, there are very good engineers who differ in their advice and approach...

The approach we use is to match foundation types, ensure there is no backfill in the founding materials for the addition, and then to use a wider strip than you normally would for the foundation capacity. I treat the addition as an "end bay" condition, and limit the settlement very severely. Reducing your allowable would effectively be the same solution.

If you can, do involve a geotech. If you know the soils in the area well and are comfortable doing so, then I would suggest further reducing the allowable soil bearing by a 50% reduction, or we can span similarly to what I said above, or otherwise working to rationalize a wider strip width.

The goal is the same, regardless of strategy: Soils that have not yet undergone stress are going to now gain a stress; it is best to keep that new stress lower than the area of existing stress and previous settlement.

If you have bad clays in your area, then the involvement of a geotech becomes that much more important. Don't gamble with sensitive, reactive, or expansive soils.

 

[BAretired]

If the existing building has a basement, shallow footings for the addition may result in more differential movement than piles. Otherwise, it is likely better to use strip footings at the same level as the existing.

BA

 

[EireChch]

To be honest a single story extension whether on piles or strips is unlikely to cause much differential settlement to cause any issues, provided you dont have soft CLAY. What are your ground conditions, or are you going to tell us you have no site investigation

No while I am saying that here, on the internet, I would be prudent to a client and recommend that you use similar foundations. I agree that mixing of foundations has the potential to cause more issues than if they were the same. If your site ever experiences any additional external loading such as shrink/swell, frost, seismic, your foundations will perform differently which increases the risk of settlement and cracking etc.

Put them on strips and try to apply the same pressure that the existing footings have applied.

 

[CELinOttawa]

Now this thread has become unexpectedly interesting and useful...

God I love Eng-Tips!

Put them on strips and try to apply the same pressure that the existing footings have applied.

I've heard both this and the "reduce by XX percentage" argument from both Geotechs and Structurals for many, many years...

So: Which is it? Should we combat the accelerated initial settlement with a lower stress, such that the very long term performance match, or accept that there will be some initial accelerated settlement of the new addition and hope to handle this by smearing those stresses over onto the existing as well?

Does anyone have any document that has a definitive answer? Very keen to hear opinions, or see additional references! That said, I get the feeling this is going to go down as engineering judgement and contradictory sources like the "how many threads must be engaged/protruding/past" discussion that comes up perennially.

Also: Is it an Irish Engineer in Christchurch, NZ? I miss that city quite often... I worked there for four years in the mid 2000s.

 

[Brad805]

What was the initial reason to select piles? We have been asked to use piles for single storey areas many times, but our frost cover depth for footings can be 2m or more in many cases, so the request has merit.

 

[Algohi]

@EireChch
No basement but there is a crawling space and it is a one-story house that was built in the 80s.

@Brad805:
The piles were chosen on an assumption that they will limit settlement better compared to strip footings.

 

[Brad805]

Unless this is an atypical application, the spans suggest to me the footing or pile sizes will be based on minimum requirements dictated by your code. This does not seem like a good application to mix foundation types, and would likely add to the cost of construction. I am not sure about your code, but our code does not have any prescriptive designs for piles, so we would need a geotech to provide pile design parameters unless we felt capable to be responsible for the pile design.

 

[BridgeSmith]

Seems to me it's potentially an issue with either of the popular approaches given. With piles under the addition, if the existing foundation experiences additional settlement, it will be a problem. If the new foundation is on a strip footing, it may settle, and you have basically the same problem.

While I think significant settlement is more likely for a new strip footing than one that has been in place 40 years, I'll concur with others who have recommended consulting a geotech. I anticipate the geotech would not recommend a strip footing for the addition. If the geotech indicates settlement of the existing foundation is possible, you may need some underpinning or other mitigation work (soil stabilization with soil mixing or urethane injection?) for the existing foundation to reduce the likelihood of additional settlement. The geotech would also likely be your best source of advice for mitigation strategies.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[EireChch]

CEL - I think the goal should be to make the house/structure perform as one unit. Putting the two structures (old and new) on the same foundation is how I believe you do this. While I agree that there will be settlement of the new structure I think it would be very small and should not really cause any significant issues. (that is assuming you know the ground conditions and are reasonably confident with a settlement calculation).

While settlement of the new addition at the time of construction (or shortly after) is important, I think the issue of future performance should be considered. Different foundations will respond differently when loaded in the future. Again this can be seismic, seasonal moisture change, retrofitting, groundwater lowering, removal of trees.

In my experience (10 years, not a lot but not a little) and when I worked Christchurch, NZ (Irish too, well spotted CEL), we did many rebuilds or partial rebuilds and extensions to houses. Many of this was due to damage caused by the EQ events. I have never seen an extension proposed on a differing foundation type. Of course that in no way suggests that just because I haven't seen it then it cant be true/right. I am only supporting the above as I have done it many many many times and I know it works.

I am sure that many extensions have been built successfully on the basis that piled foundations settle less than strips, and I accept this but I dont believe a differing foundation is the way to go re future response.

I seem to be the only geotech who has commented on this so I would like to hear from other geos!