33 % Increase In Bearing Capacity In Transient Load Condition

[Muhammad Hassan]

Can any body give me referrence of clause which is generally used in foundation design that is "33 increase in bearing capacity of soil is allowed under application of wind or seismic loads"

[UmarMakhzumi]

There are 2 things..

First, the 1/3 rd stress increase for soil pressures shouldn't be taken for granted, and shall only be be used after due consultation with the geotechnical engineer. I dont have a code reference for 33% thing but if It isn't mentioned in the code, it is commonly allowed by the Geotech engineer. The following note is from the geotech report for a current project. "The net allowable bearing pressure may be increased by one-third for transient wind or seismic loads."

Second,

Another possible alternative would be to reduce forces and have a economical design ,

The 25% reduction applied to the overall overturning moment at the base of the structure due to lateral seismic forces (talking about the big picture here - not moment on an individual footing). As the structure overturning moment is generally resolved into a force couple in braced frames or individual moments in footings, I read this as allowing a reduction in either the force couple or the individual moments for design, both strength and stability, of the footings.

As basis see the 2009 NEHRP Provisions which basically provide a commentary to ASCE 7-2005 (downloadable - big file - here: http://www.fema.gov/...cord.do?id=4103) which says for this provision (emphasis added):

C12.13.4 Reduction of Foundation Overturning. Since the vertical distribution of forces prescribed for use with the

equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See

Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for

design (strength and stability) of the foundation using this procedure.

Note:This 25% reduction is for seismic only.

Edited June 14, 2012 by Umar Makhzumi
Note added

[Muhammad Hassan]

thanks sir for such explainatory answer.

[UmarMakhzumi]

You're welcome Hasan

cheers!

[Muhammad Hassan]

i was making load combinations for descon new project. it was strange when i found out they are unaware of vertical component of earthquake. Except one project they don't take this. further more they are also unaware of special seismic combinations.

but one thing is encouraging tht they most deal with steel structures and when we use ASD Ev is zero. This clause has saved them till yet

[UmarMakhzumi]

Good Job!

On a lighter note, Life is full of surprises. Specially if you are a structural engineer.

[WR1]

Good Job!
On a lighter note, Life is full of surprises. Specially if you are a structural engineer.

I like it... yes its true

Now see the images in sequence

[UmarMakhzumi]

I just came across this. Here is an article by AISC that addresses that. Its not allowed anymore and reasons are in the article.

13rd stress increase AISC.pdf

[WR1]

I am confused again now..let me do somw work..thanks for the share

[Muhammad Hassan]

Sir umer i had studied this document before starting discussion. This mainly adresses the stress increase in structural materials. This was practice followed before 1990's because at that time factor of 0.75 was not introduced in load combinations. We can increase allowable bearing capacity of soil by 33% under eartquake and wind conditions.

[Waqar Saleem]

soil is the material with most unpredictable behaviour and without any strongly recommended reference it is not reasonable to increase the capacity , yes you are right!!!let the blame going to others typical approach

[Waqas Haider]

Waseem bhai Can you please upload the link/document of ASCE 7 commentry? In my version of ASCE 7-10, Commentry pages are missing. You please share so that we may download.

[Waqas Haider]

What I have concluded after reading above posts and documents, is that 

1) The 1/3d increase in stress by ASD method may be either due to

i) loading rate

ii) loading duration

iii) some other such reason based on engineering judgement

iV) And most importantly due the reason that when two transient loads (loads having tendency to vary in magnitude with respect to time e.g. live, earth quack, wind etc)  act simultaneously on a structure, then one transient load acts as full value load and other structure has less probability of occurrence. So it is not fair to combine these two transient loads at their full value loads. So both of the transient loads are either allowed to reduce upto some extent (As codes specify 25% reduction in transient loads) OR corresponding stress in material is increased by 33%.

 

(Note that 0.75 is reduction factor for transient simultaneously acting loads and reciprocal of 0.75 i.e. 1/0.75 = 1.33 i.e. increment factor of stress. )

 

Basic equation of equilibrium is

 

Loads = Stress 

 

In case of simultaneously acting transient loads,

 

either you can do

 

0.75*loads = stress

 

OR

 

Loads = stress/0.75 = 1.33*stress

 

But keep in mind that since this stress increment is due to the fact that both transient loads can not act simultaneously at their full value so we are reducing loads or increasing stress. This stress increment has nothing to do with DEAD LOADS. So we can not reduce dead load and this factor of 0.75 is applied only to transient loads.

 

You can find in different codes in practice two types of load combinations.

Basic and Alternate.

One type of load combinations include this 0.75 reduction factor in them so using these types of load combinations we can not increase stress. because reducing loads for the same reason and increasing stress for again the same reason SIMULTANEOUSLY is UNSAFE. You can go for one approach at a time.

 

More over, Codes also say that the effect due to load combination carrying two or more transient loads with reduced values must not be less than the effect of only one (most severe) transient load acting at full value.

[Waqas Haider]

Sir umer i had studied this document before starting discussion. This mainly adresses the stress increase in structural materials. This was practice followed before 1990's because at that time factor of 0.75 was not introduced in load combinations. We can increase allowable bearing capacity of soil by 33% under eartquake and wind conditions.

The document presented by Sir Umer clearly says that this stress increment should not be taken even in case of earth quack loads if 0.75 reduction factor has been already used in load combinations.

 

More over, in sizing of footing, we go for ASD approach because bearing capacity given by geotechnical engineer is based on the concept that full factor of safety of 2-3 is applied on soil strength and no loads are increased. This is ASD methodology. So to be compatible, we also go for ASD while sizing of foundation. So in my opinion, if you are using load combinations which are carrying 0.75 reduction factor, you can not increase soil stress by 33% even not for earth quack loads. but if you are using the load combinations which are not having 0.75 reduction, then you can increase this stress as 33% but just for those cases where two or more than two transient loads are acting simultaneously.

[WR1]

yes Waqas you got it right..No increase in 0.75load combinations as in ASCE/ACI

[Waqas Haider]

I am really thankful to members of this forum for their nice and sincere work. This forum is really helping students like me with variety of knowledge, concepts and practicality. 

[UmarMakhzumi]

On 2015-02-12 at 4:29 AM, Waqas Haider said:

The document presented by Sir Umer clearly says that this stress increment should not be taken even in case of earth quack loads if 0.75 reduction factor has been already used in load combinations.

 

More over, in sizing of footing, we go for ASD approach because bearing capacity given by geotechnical engineer is based on the concept that full factor of safety of 2-3 is applied on soil strength and no loads are increased. This is ASD methodology. So to be compatible, we also go for ASD while sizing of foundation. So in my opinion, if you are using load combinations which are carrying 0.75 reduction factor, you can not increase soil stress by 33% even not for earth quack loads. but if you are using the load combinations which are not having 0.75 reduction, then you can increase this stress as 33% but just for those cases where two or more than two transient loads are acting simultaneously.

 

There are two noticeable references mentioned in this thread.

 

One is specific to Load Combinations(ASD stuff) and subsequent 1/3rd reduction, the other is about C12.13.4 of ASCE 7-05: 25% reduction of seismic overturning moment for the case of Equivalent Static Lateral Force Procedure. For some reason, all the replies have been focused on ASD Load Combinations and subsequent 1/3rd reduction. Anyways, ASD is old in a matter of few years would be completely wiped out of the scene for foundations. The current practice in industry is to use LRFD for foundation design. Moreover, make sure you touch base with the Geotechnical Engineer.  Sometimes geotech reports specify what could be done or not with regards to thread subject.

Thanks.