Footing with cold joint 1

[ottles]

Let's say you have concrete placement done one half of it (see the red lines) and the other top half (or rest of portion) of it placed next day. Can it affect the punching shear strength? How about one way shear strength?

 

[ottles]

More than what? I'm not sure what we're comparing pad footings to.

Smaller spread (or pad) footings should slide more compared to larger combined footings.. larger footings have more surface and hence more coefficient of friction.

I've never heard of such a report. Again though, this should be a moot point as the shorter building should be designed not to slide.

But in usual buildings.. the pad or spread footings are not doweled or keyed to the soil or rock underneath.. without these.. most spread footing buildings should slide.. don't they? how do you design them not to slide without using dowel or key to soil/rock?

Also for adjacent structures. of course no problems if both don't have basements. But in practice.. you don't know if your adjacent lot has basement parking.. so I'm asking what are the in field seismic performance where the adjacent building has 2 basement parking and one beside it doesn't have any basement and the designer doesn't know.. so most should slide (especially spread footings without dowel or key).

 

[KootK]

larger footings have more surface and hence more coefficient of friction.

I disagree. The coefficient of friction has nothing to do with footing size. The frictional force developed is a function of the total weight on the footing of which the footing self weight is only part.

most spread footing buildings should slide.. don't they? how do you design them not to slide without using dowel or key to soil/rock?

I design my buildings not to slide. Sometimes that requires only friction, sometimes it requires the foundations to be keyed into the rock so to speak.

But in practice.. you don't know if your adjacent lot has basement parking..

I do for my buildings. The new foundations need to be coordinated with the positions and other characteristics of the existing foundations.

o I'm asking what are the in field seismic performance where the adjacent building has 2 basement parking and one beside it doesn't have any basement and the designer doesn't know.. so most should slide

I've never seen any reports on that type of seismic performance. And since I don't agree that most buildings should slide, I consider it to be an issue of little relevance to practical design. In my opinion, buildings need to be designed not to slide around unless it's on seismic isolation systems.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ottles]

I do for my buildings. The new foundations need to be coordinated with the positions and other characteristics of the existing foundations.

How do you compute whether a building and foundation can slide or not? In your projects.. how many storeys usually before the weight can make it no longer slide? just estimate? can say 6 storey with 2 bays with say 16 feet span still slide or do only 2 storey (2 bay 16 feet span) building slide?

Does soil also make it slide or only rock underneath?

Please see the image below.

Let's not discuss about building with basements anymore.. let's discuss now building that has no basements and the foundations are level with one another. When let's say someone is building a small house (pink one) beside a large building (on left).. what instance when the larger building footing can move the small house footing and shear it to bend the column? The ground accelerate together but the footing is different sizes and inertia.. so when the ground shakes to the left.. either the small house can slide into the big footing and get bent in the process or if the ground shakes to the right and let's say the big building slide (it can shear the footing of the small pink building), right? what other scenerios can happen?

Thanks a lot!

 

[ottles]

1. If ground accelerates in seismic movement, do all structure foundation (whether it is 50 storey trump tower foundation or a one storey poor man foundation) moves the same amount? If yes. Then if there is no slide, then there is no banging of foundation. Right?

2. The equal displacement principle says that a building lateral system will experience approximately the same amount of displacement under design seismic excitation regardless of the level of ductility built into the system. But let's imagine a solid stone that is 2 storey high. Would it even have displacement between the roof and the ground?? How can the equal displacement principle apply in case of the 2 storey high stone?

 

[KootK]

How do you compute whether a building and foundation can slide or not?

You compare the lateral loads to the available lateral resistance at the foundation level. For a shallow foundation building, the resistance is some combination of friction against the earth, which is a function of reliable dead load, and earth pressures against anything that's keyed into the earth.

how many storeys usually before the weight can make it no longer slide? just estimate? can say 6 storey with 2 bays with say 16 feet span still slide or do only 2 storey (2 bay 16 feet span) building slide?

There's no rule of thumb. It has to be determined on a case by case basis depending on how the lateral system is arranged etc. A very expansive and heavy building may still have sliding problems if all of the lateral load is delivered to a few local shear walls or braces and those systems themselves do not collect enough dead load to resist sliding locally.

Does soil also make it slide or only rock underneath?

Both soil and rock can be used to resist sliding. Rock may offer improved possibilities for resisting a lot of force through keying foundations into the earth.

what instance when the larger building footing can move the small house footing and shear it to bend the column?

This shouldn't happen in any instances as both buildings should be designed not to slide. If the buildings were designed such that one was supposed to slide into the other during an earthquake, that would be 1) very complex to design and 2) rather poor practice.

If ground accelerates in seismic movement, do all structure foundation (whether it is 50 storey trump tower foundation or a one storey poor man foundation) moves the same amount?

Yes, and that amount is assumed to be zero. The foundations ride along with the earth and do not move relative to it.

If yes. Then if there is no slide, then there is no banging of foundation. Right?

Right.

Would it even have displacement between the roof and the ground??

Yes. Everything displaces under load. In the case of the two story stone, that displacement would be extremely small.

How can the equal displacement principle apply in case of the 2 storey high stone?

It would apply in exactly the same way so long as:

1) The stone had a yielding mechanism that would kick in prior to reaching the elastic level earthquake load.

2) The stone possessed the ability to strain past yield in a ductile manner.

Of course, neither of those things is likely to be true for a two story rock. It is an interesting thing to consider however.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ottles]

It would apply in exactly the same way so long as:

1) The stone had a yielding mechanism that would kick in prior to reaching the elastic level earthquake load.

2) The stone possessed the ability to strain past yield in a ductile manner.

Of course, neither of those things is likely to be true for a two story rock. It is an interesting thing to consider however.

So the equivalent displacement principle is only valid for similar systems. It doesn't work for stone building block vs column-beam based building. But do you think it is still valid for braced frame vs nonbraced frame? The braced frame is very stiff because it is braced. So it can't have equal displacement vs a special moment frame. So it seems the equivalent displacement principle is only valid for ordinary moment frame (not braced) vs intermediate and special moment frame (not braced)? I tried to google this but I can't find the exact statement that it is not valid for braced frame so wanted confirmation from you. Appreciated your invaluable assistance. Thanks!

 

[KootK]

So the equivalent displacement principle is only valid for similar systems. It doesn't work for stone building block vs column-beam based building.

You misread me. The equal displacement principle does apply to the stone and could be mobilized if the two criterion that I mentioned above were met.

And the principle is very much applicable to properly detailed braced frames. They're stiff and they can be displaced beyond yield without becoming unstable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ottles]

And the principle is very much applicable to properly detailed braced frames. They're stiff and they can be displaced beyond yield without becoming unstable.

The above came from this excellent ref

http://www.iitk.ac.in/nicee/IITK-GSDMA/EBB_001_30May2013.pdf

It's in page 102. There you will see the braced frame has lesser lateral displacement with equivalent lateral energy (say base shear in all cases is 5700 kN). Hence the equivalent displacement principle doesn't seem to work. If you will say when the brace framed building yields.. it has equal displacement as the moment frame on left, but then it will yield at larger seismic energy. Unless you are saying the drawing is not accurate and the braced frame depicted has lesser seismic energy that is why displacement is lesser and if it's equivalent 5700 kN lateral force. The displacement (from roof to ground) is similar??

 

[KootK]

That is a neat reference. Thanks for sharing it.

I believe that you're misunderstanding an aspect of the equal displacement principle. Nothing about it implies that different structures should have the same displacement. It only says that the different possible load-displacement curves of a single structure tend to have similar maximum displacements. Obviously, a skyscraper and a one story shack are not going to undergo the same displacement during a seismic event.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ottles]

That is a neat reference. Thanks for sharing it.

I believe that you're misunderstanding an aspect of the equal displacement principle. Nothing about it implies that different structures should have the same displacement. It only says that the different possible load-displacement curves of a single structure tend to have similar maximum displacements. Obviously, a skyscraper and a one story shack are not going to undergo the same displacement during a seismic event.

In the drawing. Is the first moment frame building and second global braced frame considered a single structure? I think no.

But a building without braced frame and only moment frame. I think ordinary frame and moment moment frame only vary in the ductility and sizes so can they be considered a single structure for purpose of the equivalent displacement principle load displacement curves? Or if you will build a building, would the displacement be the same if its same storey and height and only differs in it being ordinary moment frame or special moment frame?

Of course skyscraper and one storey shack don't have same displacement because of storey shear and storey force but I'm talking about a similar height storey and not braced.

 

[KootK]

Is the first moment frame building and second global braced frame considered a single structure?

Agreed, different structures.

As for the rest, I disagree strongly. A braced skyscraper can be expected to respond very differently from a moment frame skyscraper. There's a lot more to response than just story height.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

ottles,

In the future, when you completely change the subject of a thread, please start a new thread. Keeps things on the site in better order.