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Storeys and Foundation Depths

  1. Mar 15, 2012 #1
    Would anyone happen to know or have any links where these relationships were illustrated? I'd like to have an idea how deep are the foundations for say a 4-storey building versus a 6-storey building. Are they proportional?
  2. jcsd
  3. Mar 18, 2012 #2
    That is a very open ended question. In a general sense, yes a slightly taller building will require a slightly stronger foundation. But there are many considerations. Are you using shallow (bearing on soil) or deep foundations (piles/drill shafts)? Depending on where you are on earth, wind or earthquakes impart most of your lateral loading, which cause overturning moments and depending on how heavy (dead load) you structure is, it might either have selfweight to resist that, or require a foundation heavy enough, or for deep foundations piles with uplift capacity.

    Also, just to be clear, I guess you are talking about an office of residental building when mentioning stories, realize some very large buildings are single story.


    If, you are not concerned about any lateral loading causing overturning and it is a simple question of vertical loads. Then, for a shallow foundation, the depth probably would not vary, the size of the footing would. So you might be told the soil can support 1 tsf (ton per square foot), if your column load is 100 tons, then you'd need 100 sf. There are load factors, and reduction factors that would be applied as well depending on design codes that apply. If you are on a deep foundation, then in its case of how many piles and how deep, both of which will be site specific, and would determined by geotechnical investigation, typically through boring logs. A miniumum depth would be established, and normally capacity in piles increases with depth sometimes very quickly, 100 ton vs 120 ton might be a 50 ft pile vs 55 ft. There is no generalization however, this is entirely site specific, it might be 20 ft someplace and 250 ft somewhere else, soils vary.

    http://en.wikipedia.org/wiki/Foundation_(engineering [Broken])
    Last edited by a moderator: May 5, 2017
  4. Mar 18, 2012 #3
    I'm just interested in 2 to 6 storey buildings. Do they have to use piles? I thought piles are only for those above 10 storey high.

    About beams. Do you know of a site showing the relationship between reinforced concrete beam span and size of RC column. For example is the column for a 12 meter RC beam twice bigger than that supporting a 6 meter RC beam, or is the ratio not proportional but exponential? I'd just like to have an idea.

    Also what is the best forum site that discusses primarily on civil engineering just like physicsforums focusing mostly about physics.

    Thanks a lot.
    Last edited by a moderator: May 5, 2017
  5. Mar 19, 2012 #4
    σσNo, 2 to 6 storeys would probably not have to use piles again it absolutely depends on the local soils and other possible hazards. In coastal areas where scour is possible, then yes they almost certainly would have to be on piles and that still might not carry them through a hurricane. In areas with shallow bedrock from a bearing perspective no, but rock anchors or shear keys might be necessary.

    Column sizes for small loads are often more geometry of rebar, and common form size driven then capacity. Even in larger loads, in an effort to reuse forms, the same size column may be used for many, many floors, and only the reinforcement is changed with load. In reality upper floors columns are much under utilized, but the savings in forms, and easy of construction make it worth the extra concrete used. Bottom line, optiming for most efficent use of material is often not least expensive.

    Now, if you are only interested in the theoretical concept of column size to span ratio, and again depends on what constraints you hold yourself to. Simple span? interior or exterior column?
  6. Mar 20, 2012 #5
    Thanks for the information.

    For RC beam-column and pure steel beam-column and for 2 to 4 storey height. What intensity of earthquakes can they withstand? I understand it has to do with ductability of the connections. Can they be designed to survive say Intensity 10? What intensity can the best design withstand presently?
  7. Mar 21, 2012 #6
    Well, to be honest I'd never heard of intensity 10. Normally we speak of peak ground accerations. And seismic design spectral acceleration values Ss and S1 which are well explained here:

    http://www.structuremag.org/article.aspx?articleID=526 [Broken]

    Anyway, uhmm you can design for nearly any loading you want, it just costs more. Realize also what the design goal or intents are as well. For the most part the goal is design something that does not collapse, but it will almost certainly be useless after the event. All the ductility was used disapating energy, there will be some fractured elements, there will be many buckled elements. Now, there is also the expectation that in more frequent lower energy events only elastic response is expected not full plastic.

    In, some cases, say nuclear reactors, they use a somewhat different code, and they are expected to remain elastic even for the max. design event. That costs a lot more to construct, but the cost of failure is far higher too.

    For a good idea of the difference in energy just look a strain diagram... if you only integrate the area under that short little elastic area, thats your energy capacity if you're chooseing to stay elastic. If you allow plastic you get to count all that area under the curve out to fracture..... well just shy of really but most of the way there. As you can see its probably 20x difference.

    I've also read IC fabs are going for elastic design, as there down time costs simply justify the extra expense.
    Last edited by a moderator: May 5, 2017
  8. Mar 27, 2012 #7
    Hi, what can you say about special moment frames? I read it stated that the beam length must be less than 30 feet for it to be true. Do you believe so? What if the column sizes are increases and it is more than 30 feet, won't it satisfy special moment frames anymore? Thanks.
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