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Design Load Combinations For Sub Structure & SuperStruct
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kiranklk
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PostPosted: Mon May 23, 2011 9:01 am    Post subject: Design Load Combinations For Sub Structure & SuperStruct Reply with quote

Hi SEF Users, I Have One Doubt Regarding Design Load Combinations For SuperStructure And Sub Structure. Is There Any Different Types of Load Combinations For Foundation Design In Siesmic Consideration?

Regards
Kiran.
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rajancivildesign@gmail.co
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PostPosted: Mon May 23, 2011 11:21 am    Post subject: Load combination for Foundation in seismic consideraion Reply with quote

Dear Kiran,
During seismic condition, 1(DL+ SLx)  shall be considered for foundation design with increase of 25% in SBC of soil depending upon type of Foundation to be adopted for the structure. Dl- Dead load;SLx -Seismic load in X direction.

V.M.RAJAN.
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suresh_sharma
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PostPosted: Sat May 28, 2011 1:12 am    Post subject: Reply with quote

Dear Mr. Rajan,

Why is it that no portion of LL is proposed by you to be considered for design of foundation? Will you please quote any reasoning, codal reference or a book in this regard? I observe that a uniform practice is not being followed in the industry.
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PostPosted: Sat May 28, 2011 10:03 am    Post subject: Load combination for Foundation in seismic consideraion Reply with quote

For substructure design, in case of multi storeyed building the following load combination is considered with 60% live load i.e. DL+0.60LL+SLx. If you consider increase in SBC by 25% for foundation design, the load combination is considered
is same as DL+LL ; DL+SLX but the allowable soil pressure may increased by 25% of SBC.
V.M.RAJAN.
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suresh_sharma
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PostPosted: Sat May 28, 2011 12:52 pm    Post subject: Reply with quote

Dear Mr. Rajan,

It means that25% increase in SBC will not be considered when we are taking design load as 1(DL+ELx) for designing foundation. Am I correct in interpreting you? What I feel that it should be (DL+ 25% of LL+ELx or ELy) with 25% increase in SBC 0r (DL+50%LL) without increase in SBC. Please examine and advise.
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PostPosted: Fri Jun 03, 2011 9:52 am    Post subject: DESIGN LOAD COMBINATION FOR SUB STRUCTURE Reply with quote

Dear Suresh Sharma,
For multi storey building, reduction of live load up to 50% is allowed. Accordingly there is no live load reduction for roof slab and reduction for subsequent floors from top by  10% of Live Load,20%,30%, 40% and 50% (for more than 10 storeys). The various load combination to be considered for foundation design are
(1) DL+LL(Reduced live load)
(2) DL+SLx or WLx
(3) 0.75DL+0.75LL+0.75SLx or 0.75WLx
Whenever seismic or wind load are considered together with  Live load, 25% reduction of actual load shall be considered  Or (DL+LL+SLx) is considered with 25% increase in SBC of Soil  for foundation design. Your reply also convey the same with little confusion. So I elaborate my views. Kindly correct it if I am wrong.

V.M.RAJAN.
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thirumalaichettiar
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PostPosted: Fri Jun 03, 2011 10:41 am    Post subject: Reply with quote

Dear Er.Rajan
Can you cite the reference code or book from where your suggestion can be read?

T.RangaRajan.
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suresh_sharma
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PostPosted: Fri Jun 03, 2011 12:39 pm    Post subject: Reply with quote

Dear Rajan
I am considering that LL is up to 300 kn/sqm in which case the code permits 75% reduction in live load when the same is combined with EQ load otherwise also in absence of EQ load,  LL may be taken as 50% for the design of column and foundation but not slab and beam in case of high rise building. It has to be noted here that LL of permanent nature is not to be considered for reduction. Floating LL alone is to be considered for reduction.
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gpsarathyy
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PostPosted: Sat Jun 04, 2011 3:48 am    Post subject: Reply with quote

Dear Sefian,

The reduction of live loads for the high rise buildings is appicable but for the industrial structures we will follow the same load combination as the superstructure and if there is any increase or reduction factor to be applied for the seismic / wind cases that we will take care in the foundation design spread sheets.

As already told the load combinations which are used in the industries it is not common to all structures, it varies....

Regards,
G.Parthasarathy
Chennai
GPSARATHYY@GMAIL.COM
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vikram.jeet
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PostPosted: Fri Jun 10, 2011 5:27 am    Post subject: Design Load Combinations For Sub Structure & SuperStruct Reply with quote

Foundations base pressure check

Check under Normal condition
The load comb are  
1.0*DL+ F *LL  
F = 1.0 for single storey
F = 0.9 for Two storey
F = 0.8 for Three storey
F = 0.7 for Four storey
F = 0.6 for Five to Ten storey
F = 0.5 for over Ten storey

Base pressures to be be compared to Net SBC of soil

Check under EQ condition
The load comb are  
1.0*DL+ Y *LL +1.0 EL
Y = 0.25 for LL class upto 300 Kg/m2
Y =0.50 for LL class above 300 Kg/m2
(As per clause 7.3.3 of IS 1893)
The above comb may have respective reduced load on all floors
including roof ,though roof need not be live loaded during EQ

Also
EL needs to be worked on basis of reduced LL
as per IS 1893 i.e  
25% in case of LL class upto 300 Kg/m2
50% in case of LL class above 300 Kg/m2
No LL on roof

Base pressures to be be compared to Net SBC of soil multiplied
with enhancement factor as per IS 1893 table

best regds

vikramjeet


Hi SEF Users, I Have One Doubt Regarding Design Load Combinations For SuperStructure And Sub Structure. Is There Any Different Types of Load Combinations For Foundation Design In Siesmic Consideration?

Regards
Kiran
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