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Doubt in Load Combinations in Steel Design (WSM)

 
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Manoj D. Vaishya
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Joined: 24 Mar 2008
Posts: 7

PostPosted: Tue Feb 03, 2009 6:22 am    Post subject: Doubt in Load Combinations in Steel Design (WSM) Reply with quote

Respected Sirs,

I have some doubt in Load Combinations in Steel Design (Working Stress Method) as per IS 800.

As per revised code IS 800:2007, No increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

Also as per old code IS 800:1984, No increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

IS 800:1984 Clause
3.9.2 Increase in Permissible Stresses in Members Proportioned for Occasional Loadings
3.9.2.1 Wind or earthquake loads
Structural steel and steel castings - When the effect of wind or
Earthquake load is taken into account, the .permissible stresses
Specified may be exceeded by 33.3 percent.
Rivets, bolts and tension rods - When the effect of the wind or
Earthquake load is taken into account, the permissible stresses
Specified may be exceeded by 25 percent.

3.3.4 Where the wind load is the main load acting on the structure, no
increase in the permissible stresses is allowed.

The following Special Publications & Handbooks are based on the code IS 800: 1984 in which 33.3 % increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

SP 47(S&T): 1988, SP: 4O(S&T)-1987 & SP: 38(S&T)-1987
2.2.1 Load Combination
The following load combination have been considered in calculating the design forces for beam and
column in accordance with IS 875: 1964.
a) DL+LL
b) 0.75 (DL + C, X WL,)
c) 0.75 (DL + c, X WL2)
d) 0.75 (DL + C,, X WL,)
Where C, = 0.75 for column forces if the building height is less than or equal to 30 metres, C, = 0.75
for beam forces if the height of frame is less than or equal to 10 metres and C,, = 1.0 for other cases. In the calculation of design forces for dead and wind load combination, the actual forces have been reduced by 25 percent to account for 33.3 percent increase in allowable stresses under this load combination.

It is my request to you Please suggest whether I can increase the stresses by 33.3 % for DL + WL combinations as per new code or not.
If not then how it is taken while using old IS Code where is it mentioned that no increase in stresses is allowed when wind is the major load in load combinations.

Thanking you,

Manoj D. Vaishya
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rahul telang
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Joined: 07 Feb 2009
Posts: 4
Location: Pune

PostPosted: Fri Feb 27, 2009 5:07 am    Post subject: Re: Contribution of snow load at the time of Earthquake Reply with quote

*************************************************************
Dear Sir,
I have an question regarding snow laod at the time of  Earthquake load.
How much contribution of snow load should be consider at the time of Earthquake loading?
(e.g. when snow on top of roof that time if Earthquake occure, how much weight of snow should be consider to evaluate EQ.)
Our IS-1893 doesent specify any factor. They simply told to take approprite snow load.

Regard
Rahul Telang
*****************************************************
manoj.vaishya wrote:
Respected Sirs,

I have some doubt in Load Combinations in Steel Design (Working Stress Method) as per IS 800.

As per revised code IS 800:2007, No increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

Also as per old code IS 800:1984, No increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

IS 800:1984 Clause
3.9.2 Increase in Permissible Stresses in Members Proportioned for Occasional Loadings
3.9.2.1 Wind or earthquake loads
Structural steel and steel castings - When the effect of wind or
Earthquake load is taken into account, the .permissible stresses
Specified may be exceeded by 33.3 percent.
Rivets, bolts and tension rods - When the effect of the wind or
Earthquake load is taken into account, the permissible stresses
Specified may be exceeded by 25 percent.

3.3.4 Where the wind load is the main load acting on the structure, no
increase in the permissible stresses is allowed.

The following Special Publications & Handbooks are based on the code IS 800: 1984 in which 33.3 % increase in stresses are allowed when wind is the major load in the load combination (i.e., DL + WL combinations).

SP 47(S&T): 1988, SP: 4O(S&T)-1987 & SP: 38(S&T)-1987
2.2.1 Load Combination
The following load combination have been considered in calculating the design forces for beam and
column in accordance with IS 875: 1964.
a) DL+LL
b) 0.75 (DL + C, X WL,)
c) 0.75 (DL + c, X WL2)
d) 0.75 (DL + C,, X WL,)
Where C, = 0.75 for column forces if the building height is less than or equal to 30 metres, C, = 0.75
for beam forces if the height of frame is less than or equal to 10 metres and C,, = 1.0 for other cases. In the calculation of design forces for dead and wind load combination, the actual forces have been reduced by 25 percent to account for 33.3 percent increase in allowable stresses under this load combination.

It is my request to you Please suggest whether I can increase the stresses by 33.3 % for DL + WL combinations as per new code or not.
If not then how it is taken while using old IS Code where is it mentioned that no increase in stresses is allowed when wind is the major load in load combinations.

Thanking you,

Manoj D. Vaishya
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Dr. N. Subramanian
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Joined: 21 Feb 2008
Posts: 5302
Location: Gaithersburg, MD, U.S.A.

PostPosted: Fri Feb 27, 2009 3:30 pm    Post subject: Re: Contribution of snow load at the time of Earthquake Reply with quote

Hi Rahul Telang

I am pasting the Abstract from a paper by Prof. O'Rourke who is an authority on Snow loads. Hope it helps.

Subramanian

Roof-Snow Load for Seismic-Design Calculations

by Michael J. O’Rourke, M.ASCE, (Prof. of Civ. Engrg., Rensselaer Polytechnic Inst., Troy, NY 12180) and Robert S. Speck, Jr., A.M.ASCE, (Prin. Associate, Ryan-Biggs Associates P.C., Troy, NY)

Journal of Structural Engineering, Vol. 118, No. 9, September 1992, pp. 2338-2350,


Abstract:      The lateral loads imposed on a structure during an earthquake are functions of the peak ground acceleration and the total mass of the structure among other parameters. For structures located in snowy regions, the question arises as to the appropriate roof-snow load to be used in earthquake calculations. Various codes and load standards suggest values for the snow load, which is to be combined with dead load in earthquake analysis. In this paper statistical information on daily ground-snow loads and statistical information on the conversion factor between roof and ground loads are used to determine appropriate roof-snow loads for earthquake analysis. As defined herein, the appropriate roof-snow load is such that the exceedance probability for the design seismic base shear is identical to the exceedance probability for the design peak ground acceleration. The analysis suggests that the appropriate roof-snow load for seismic calculations is 20% of the 50-year mean recurrence interval ground-snow load.***


Dear Sir,
I have an question regarding snow laod at the time of  Earthquake load.
How much contribution of snow load should be consider at the time of Earthquake loading?
(e.g. when snow on top of roof that time if Earthquake occure, how much weight of snow should be consider to evaluate EQ.)
Our IS-1893 doesent specify any factor. They simply told to take approprite snow load.

Regard
Rahul Telang
*****************************************************
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FARA
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Joined: 31 May 2010
Posts: 17
Location: New Delhi

PostPosted: Fri Sep 21, 2012 3:40 pm    Post subject: Reply with quote

Dear Seniors etc.

i have to design steel truss as per old code IS:800-1984.
Can someone please guide me regarding the load combinations required for Foundation check, permissible increase in SBC, permissible increase in stresses in steel members.<xml><o></o>
The primary load case are Dead load, Live Load n Wind load.<o></o>
Few things I got from code like “permissible increase in stresses in steel members” is 33.3% for Wind+Dead<o></o>
But what about D+L+W, <o></o>
Waiting for the reply.<o></o>
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