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Design Load Combinations For Sub Structure & SuperStruct
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sharath kn
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PostPosted: Fri Jun 10, 2011 11:06 am    Post subject: Dear Sefian, Reply with quote

As per IS 1893 part 1, For underground structures and foundations at depths of 30 m or below, the design horizontal acceleration spectrum value shall be taken as half the value design horizontal seismic coefficient Ah. For structures and foundations placed between the ground level and 30m depth, the design horizontal acceleration spectrum value shall be linearly interpolated between Ah and 0.5 Ah. when we designing the foundation if we take the factored critical load of the structure then the SBC can be taken with factor of 1.5, since the SBCis already factored. and in load combination the live load with DL and Eq, if LL is upto 3KN/M2 it should 25% and above 3KN/m2it should 50%
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thirumalaichettiar
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PostPosted: Fri Jun 10, 2011 10:53 pm    Post subject: Reply with quote

Is it right to post this article in response to this posting?

T.RangaRajan.



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binaya shahi
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PostPosted: Wed Sep 21, 2016 10:56 am    Post subject: Reply with quote

6.3.1.2 Partial safety factors for limit state design
of reinforced concrete and prestressed concrete
structures
In the limit state design of reinforced and prestressed
concrete structures, the following load combinations
shall be accounted for:
1) 1.5( DL+lL)
2) 1.2( DL+ZL+EL)
3) 1.5( DL+-EL)
4) 0.9DL+- 1.5EL

6.3.5.2 Increase in allowable pressure in soils
When earthquake forces are included, the allowable
bearing pressure in soils shall be increased as per
Table 1, depending upon type of foundation of the
structure and the type of soil.


according to table 1 the increase allowed in allowable soil bearing capacity for type II or medium soil for isolated and combined-isolated footing is 25%. so above equations become

1) 1.5( DL+IL)
2) 0.96( DL+IL+EL)
3) 1.2( DL+EL)
4) 0.72DL+- 1.2EL


My question is why isn't everyone mentioning the load combination 1.5(DL+IL) for design of footing. Won't using just DL+LL and DL+0.8LL+EL result in significantly weaker design than that required by code.

I have seen many people calculating the area of footing from just DL+IL load combination and them applying 1.5(DL+IL) for calculating reinforcement. why is that?

Please make me clear about the use of these load combination. Right now I am designing footing area and reinforcement just with one load combination DL+IL+LL.
Am I doing something wrong?

I have attached Table1. Please also have a look at it.



Table 1- % increase allowed in allowable Pressure.jpg
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rahul.leslie
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PostPosted: Wed Sep 21, 2016 2:47 pm    Post subject: Reply with quote

Design:
1.5(DL+LL)
1.2(DL+LL+EL)
1.5(DL+EL)
0.9DL+1.5EL

Footing sizes/pile no:
1.0(DL+LL)
1.0DL+0.8(LL+EL)  :w/SBC enhancement
1.0(DL+EL)             :w/SBC enhancement



Note:
1)    All EL's represent +/-EL, and ELx/ELy
2)    LL in Design cases with EL included
          represents 0.25LL(<3kN>3kN/m2)

Rahul Leslie
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mallesh_ng
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PostPosted: Thu Jan 12, 2017 8:14 pm    Post subject: LOAD COMBINATION GENERATOR-STAAD PRO Reply with quote

HI

USE THE ATTACHED SHEET FOR GENERATING LOAD COMBINATIONS, WHICH CAN BE DIRECTLY PASTED INTO STAAD EDITOR AND TO GENERATE LOAD COMBINATION DETAILS REQUIRED FOR PREPARING REPORT


ANY FEEDBACK/CLARIFICATION PLEASE LET ME KNOW.

TURN ON MACRO!!!

REGARDS
MALLESH N G
malleshnenkat@gmail.com
+91-9148765688



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LOAD CASE & LOAD CASE REPORT GENERATOR-STAAD PRO.zip
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sakumar79
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Joined: 18 Apr 2008
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PostPosted: Fri Jan 13, 2017 12:29 am    Post subject: Reply with quote

Dear Er Binaya Shahi,

     The calculation of SBC and pile capacity are under working stress conditions. Hence, when we need to decide size of footing or number of piles, we work with working load combinations 1(DL+IL), etc. Once size is fixed, actual footing design and pile design are based on Limit State and hence we use 1.5 (DL+IL) for the same. With DL+IL+EL, when sizing footing, we can increase allowable bearing pressure to increase 25% which is equivalently calculated as reduced load combination with factor 1/1.25 or 0.8(DL+IL+EL) for the actual allowable bearing pressure.

Hope that clarifies it for you.
Yours sincerely
Arunkumar

binaya shahi wrote:
6.3.1.2 Partial safety factors for limit state design
of reinforced concrete and prestressed concrete
structures
In the limit state design of reinforced and prestressed
concrete structures, the following load combinations
shall be accounted for:
1) 1.5( DL+lL)
2) 1.2( DL+ZL+EL)
3) 1.5( DL+-EL)
4) 0.9DL+- 1.5EL

6.3.5.2 Increase in allowable pressure in soils
When earthquake forces are included, the allowable
bearing pressure in soils shall be increased as per
Table 1, depending upon type of foundation of the
structure and the type of soil.


according to table 1 the increase allowed in allowable soil bearing capacity for type II or medium soil for isolated and combined-isolated footing is 25%. so above equations become

1) 1.5( DL+IL)
2) 0.96( DL+IL+EL)
3) 1.2( DL+EL)
4) 0.72DL+- 1.2EL


My question is why isn't everyone mentioning the load combination 1.5(DL+IL) for design of footing. Won't using just DL+LL and DL+0.8LL+EL result in significantly weaker design than that required by code.

I have seen many people calculating the area of footing from just DL+IL load combination and them applying 1.5(DL+IL) for calculating reinforcement. why is that?

Please make me clear about the use of these load combination. Right now I am designing footing area and reinforcement just with one load combination DL+IL+LL.
Am I doing something wrong?

I have attached Table1. Please also have a look at it.
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vijay_d
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Joined: 20 Jun 2012
Posts: 4

PostPosted: Sat Jan 14, 2017 6:50 am    Post subject: Reply with quote

Please go through IS 456:2000 for
Limit State of Serviceability &
Limit State of Collapse.

For Sizing, Stability checks use Serviceability load combinations
and for Design of Footing Elements(Raft, Pile cap, Pedestal) use Collapse load combinations.

In any design you have to study all possible load combinations and design for the worst case. This approach of studying forces under all possible  load combinations helps us grow. Also our understanding of structure behavior & resultant forces transferred can be correlated.

Regards,
Vijay
binaya shahi wrote:
6.3.1.2 Partial safety factors for limit state design
of reinforced concrete and prestressed concrete
structures
In the limit state design of reinforced and prestressed
concrete structures, the following load combinations
shall be accounted for:
1) 1.5( DL+lL)
2) 1.2( DL+ZL+EL)
3) 1.5( DL+-EL)
4) 0.9DL+- 1.5EL

6.3.5.2 Increase in allowable pressure in soils
When earthquake forces are included, the allowable
bearing pressure in soils shall be increased as per
Table 1, depending upon type of foundation of the
structure and the type of soil.


according to table 1 the increase allowed in allowable soil bearing capacity for type II or medium soil for isolated and combined-isolated footing is 25%. so above equations become

1) 1.5( DL+IL)
2) 0.96( DL+IL+EL)
3) 1.2( DL+EL)
4) 0.72DL+- 1.2EL


My question is why isn't everyone mentioning the load combination 1.5(DL+IL) for design of footing. Won't using just DL+LL and DL+0.8LL+EL result in significantly weaker design than that required by code.

I have seen many people calculating the area of footing from just DL+IL load combination and them applying 1.5(DL+IL) for calculating reinforcement. why is that?

Please make me clear about the use of these load combination. Right now I am designing footing area and reinforcement just with one load combination DL+IL+LL.
Am I doing something wrong?

I have attached Table1. Please also have a look at it.
[/quote]
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bijoyav
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Joined: 13 Dec 2008
Posts: 114

PostPosted: Sat Jan 14, 2017 1:47 pm    Post subject: Reply with quote

Dear Mr Binaya,
Er Arunkumar said it rightly. Foundation is designed in WSM. So FoS is applied on the bearing capacity.... Say 3-4.5.
But the concrete part is designed in LSM. Here you are considering Characteristic load and Characteristic strength. So you are applying PSF. Being the Characteristic load & strength, PSFs are probabilistic values, there are different combinations in LSM.
I request you to refer explanatory code to IS456...

BijoyAV
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