evg0101

Enough rebar in a column in a high rise (photo)?

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We are planning to buy an apartment in a 25-floor high rise building. The building is said to be in compliance to withstand a magnitude 8 earthquake (on the Richter scale). The building is currently under construction and the picture posted is taken from the 15thfloor. I have some doubts if there is enough rebar material in load bearing columns (rebar size is #8 or #9 imperial, I think reasonably thick). As you see, there are only 10 bars in a column measuring 2 by 3 feet. I have seen other constructions with a lot more bars in a column in a similarly high rise building. Is this sound enough engineering of a column for a high rise in a seismic zone or I am missing something and there are many other factors to consider?

The columns at the ground level are much bigger with more bars inside.

Thanks!

 

Edit: I got too impatient and obtained a plan from the engineer, which I attach below. I also updated the rebar picture. Can I know with greater certainty if the rebar in the column is adequate now that we can see the overall building design? Many thanks for your views on this again.

 

New Picture2.jpg

Plan.jpg

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of course someone would have to see the plans to make that judgement but its kind of like the lower part of a retaining wall usually is a much thicker block and more rebar than the upper level, maybe in this case it's just not enough weight or lateral loads to require more rebar.. Call the engineer of record from the building permit and ash them to make you feel better about it. 

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And if I suggested that there should be twice as much bar, how beneficial would that be for you?  Perhaps you need to speak to the design firm that did the structural engineering.

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18 hours ago, DRAWZILLA said:

of course someone would have to see the plans to make that judgement but its kind of like the lower part of a retaining wall usually is a much thicker block and more rebar than the upper level, maybe in this case it's just not enough weight or lateral loads to require more rebar.. Call the engineer of record from the building permit and ash them to make you feel better about it. 

Thank you for your quick reply. I've done key due diligence and got all the assurances that the construction is sound. I have no engineering background but I figure its not possible to determine based on the limited information provided. At lower levels, there are massive retaining walls as well, the so-called diaphragms.

 

18 hours ago, CJSpud said:

And if I suggested that there should be twice as much bar, how beneficial would that be for you?  Perhaps you need to speak to the design firm that did the structural engineering.

Understood. Thanks! :)

17 hours ago, Chief16Designer said:

where is this California?

Correct. Its SF bay area. It's a small apartment 1000 sq.feet though still costs quite a bit. But I expect the price of this purchase to double by 2020. Bank foreclosed property is drying up, there will be a supply crunch and developers have been underbuilding despite growing population and increasing population densities in inner cities. Plus banks are sitting on ungodly piles of hoarded cash, they won't let inflation eat up trillions of dollars, sooner or later they will start lending, the Fed is printing money like crazy. Real estate is a great inflation hedge so I figured it's a good time to buy as an investment while prices are still low, but wanted to make sure its a sound engineering design..

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35 minutes ago, Chief16Designer said:

I would not be worry about the rebar 

Screen Shot 2017-11-26 at 8.07.16 AM.png

Thanks for sharing! hehe It appears this building has a raft/mat foundation and settling faster than expected. Its better to have bored piles into bedrock that way uneven settling can be avoided.

Edited by evg0101
edit:

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it appears this building does have friction piles (instead of end-bearing piles) as bedrock is too deep:

 

The foundation of the structure is a concrete slab built on 60–90-foot-deep (18–27 m) concrete friction piles through the fill and Young Bay Mud, and embedded into dense Colma sand. A number of other buildings in this part of San Francisco required use of end-bearing piles, which load directly onto bedrock, rather than friction piles; however, the subsurface conditions in San Francisco vary greatly and there are a number of other buildings supported on a deep foundation system bottomed in dense Colma sand, similar to the Millennium Tower. If end-bearing piles into bedrock were used for the Millennium Tower, they would have needed to be approximately 200 feet (61 m) deep to bear onto bedrock, up to three times longer than the existing friction piling solution used. An examination in 2016 showed the building had sunk 16 inches (41 cm) with a two-inch (5.1 cm) tilt at the base and an approximate six-inch (15 cm) tilt at the top of the tower. The building is leaning toward the northwest.

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6 hours ago, evg0101 said:

Plus banks are sitting on ungodly piles of hoarded cash, they won't let inflation eat up trillions of dollars, sooner or later they will start lending, the Fed is printing money like crazy. Real estate is a great inflation hedge so I figured it's a good time to buy as an investment while prices are still low, but wanted to make sure its a sound engineering design..

And don't forget about the next crash which is coming, no matter how you look at it. Buy low sell high has its merits. IMO ,right now is not the time to buy in California..

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32 minutes ago, DRAWZILLA said:

And don't forget about the next crash which is coming, no matter how you look at it. Buy low sell high has its merits. IMO ,right now is not the time to buy in California..

Even when there is a stock market crash property prices never really plunge unless there was a bubble building, prices just dont grow as fast, the rate of growth slows. When real estate is bought and sold for investment, prices tend to go up faster, the only way for a stock market crash to bring the housing market down is if banks start foreclosing and excess supply develops. This is still a great time to buy using leverage, interest rates are at 50-yr lows after the 2008 bubble there was a correction, 2008 was a horrible time to buy, during 2005-2008 developers were aggressively building, now is not the same. Unemployment and interest rates are irrelevant (e.g. 1980 both in double digits and home prices kept growing) they only matter when it's a buyer's market. I will buy in CA now sell when the price doubles in the early 20th and retire in FL:)

 

1 hour ago, Chief16Designer said:

I would not be worry about the rebar 

Screen Shot 2017-11-26 at 8.07.16 AM.png

Reading on theMillennium Tower, I guess doing bored piles till 200 feet depth would be very costly for 50 or however many columns. But I would at least construct 4 corner piles as end-bearing piles rather than friction piles that way swaying could be prevented.

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well good luck with all that, but I read the opposite everywhere in California, real estate is in a bubble right now and waiting to burst. Hope your right.

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It was in 2008 but for the reasons outlined above I don't think it is now, but thanks:)

 

Btw, how do you like my suggested construction idea for the Millenium Tower, would it work (corner end bearing piles)?

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Looking at 1 column like that isn't going to help anyone assess if there is enough steel.  That said, it does "seem" light - but they obviously have yet to place the horizontal bars.  If I was you, id go down to the City (assumption) and get a copy of the entire plans/engineering and hire an analysis done by an engineer of your choosing.

 

For all we know they could be mixing other elements for structural (lateral and vertical) like steel moment frames etc which could offset the loads for the concrete (alone) drastically.  Looking at one picture though its all guesswork.

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1 hour ago, johnny said:

Looking at 1 column like that isn't going to help anyone assess if there is enough steel.  That said, it does "seem" light - but they obviously have yet to place the horizontal bars.  If I was you, id go down to the City (assumption) and get a copy of the entire plans/engineering and hire an analysis done by an engineer of your choosing.

 

For all we know they could be mixing other elements for structural (lateral and vertical) like steel moment frames etc which could offset the loads for the concrete (alone) drastically.  Looking at one picture though its all guesswork.

It does "seem" light?! ALL columns on the upper floors are like that, they seem evenly spaced and close enough but all like that. Horizontal bars I think are ok as they are dense. It is vertical bars that seem to be of concern. Pls take a look at the attached pic. The top photo also shows lower level columns that have far more bars. There are indeed other structural elements like elevators shafts and staircases fully enclosed in concrete walls, there are also additional cast concrete walls i.e. diaphragms on each floor... 

 

Yet the columns do look like light to me too... You can never know enough, see what has happened to the Millennium Tower...

Untitled2.jpg

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There is no way that anyone on this forum could make an informed opinion on the adequacy of the rebar. Even a licensed structural engineer could not, based on the information given, without knowing the structural loading, the overall system configuration, design parameters, etc. I CAN tell you that columns around the perimeter usually take far lighter vertical loads (sometimes 1/2 the load or less) than the interior columns, so you should not expect to see as much rebar in a column at an exterior wall (which this appears to be) as at the interior columns.

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Remember, concrete takes most of the compression, steel takes most of the tension and sheer.

 

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Structures are always designed in such a way that tensile failure would happen prior to the brittle failure of concrete, if there occurs an action that exceeds the ultimate limit state that makes up the basis of design for the structural members. Tensile failure gives a warning signs in many ways before the structure fully collapses.

In your case the geotechnical investigation and analysis with all the reports of tests will help you to start examining the substructure of the building which is far more complicated to figure out if there is any thing wrong with the design or if any thing was missed during errection of foundations.

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21 hours ago, Richard_Morrison said:

There is no way that anyone on this forum could make an informed opinion on the adequacy of the rebar. Even a licensed structural engineer could not, based on the information given, without knowing the structural loading, the overall system configuration, design parameters, etc. I CAN tell you that columns around the perimeter usually take far lighter vertical loads (sometimes 1/2 the load or less) than the interior columns, so you should not expect to see as much rebar in a column at an exterior wall (which this appears to be) as at the interior columns.

Thanks! I edited the original post to include the plan from the engineer showing the design of the building. Does this help? 
all upper level columns have the same amount of rebar, not just exterior walls..
 

 

17 hours ago, CharlesVolz said:

Remember, concrete takes most of the compression, steel takes most of the tension and sheer.

 

Yes, I know this. Thanks!

 

17 hours ago, yusuf-333 said:

Structures are always designed in such a way that tensile failure would happen prior to the brittle failure of concrete, if there occurs an action that exceeds the ultimate limit state that makes up the basis of design for the structural members. Tensile failure gives a warning signs in many ways before the structure fully collapses.

In your case the geotechnical investigation and analysis with all the reports of tests will help you to start examining the substructure of the building which is far more complicated to figure out if there is any thing wrong with the design or if any thing was missed during errection of foundations.

Thanks. I doubt I will have access to reports of tests for conducting the geotechnical investigation. In addition to vertical loads during normal times, I am also concerned about dynamic lateral loads during seismic activity. Can you take a look at the updated pictures in the original post and see if they can help?

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Look, there is no way that anyone not involved in the structural design of the project can make anything close to an informed opinion based on photographs and a conceptual floor plan. (And frankly, if they try, they are idiots.)  If this is a serious concern for you, you will need to sit down (and better if done in conjunction with your own structural engineer for your own peace of mind), and let the structural engineer who designed the project walk you through the geotech report, and the structural loading assumptions and load-resisting schemas. It is unlikely they will do this for free, however, and may not do it at all. Another approach is that all permit documents, including the geotech report, should be available for viewing at the building department, although they probably won't allow copies to be made. You could hire an engineer to do a cursory evaluation of these filed documents to see if the analysis was reasonably done.    

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