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olliewilson87

Equivalent Horizontal Forces

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Hi all,

 

I have recently been looking at the load combinations for my first RC frame to Eurocode and one thing that I don't fully understand is what factor (Ψ) to apply to the EHF in ULS combinations. Can anyone help please? I realise there is no partial factor to apply but not sure on the Ψ factor.

 

There seems to be a bit of ambiguity on whether EHF’s are considered a permanent, variable or accidental load case. One thing I read said it could be treated as a dead load because it predominately comes from dead loads in which case there would be no Ψ factor applied. But then looking at something in an IStructe Article, it applies a factor of 0.2 to the EHF so I thought I’d best check if anyone knows or has any useful links to anything on this subject?

Thank you.

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In publication "Eurocode load combinations for Steel structures" in Chapter 4.1.2 state "Frame imperfections may be incorporated directly into the structural analysis by defining an initial sway for the frame. However, the more general approach is to replace this geometric imperfection with a system of equivalent horizontal forces (EHF), referred to as notional horizontal loads in BS 5950.  Whereas in BS 5950, equivalent horizontal forces were only required in the vertical load case, in the Eurocodes it is deemed that since frame imperfections are inherently present, they should be included in all ULS load combinations. This appears entirely rational. EHF are not required in SLS load combinations. The EHF should be determined separately for each load combination since they depend on the level of design vertical loads. For each storey, the EHF may be calculated as the design vertical load for that storey (not the cumulative vertical load) multiplied by 1/200 (i.e. 0.5%). Depending on the height of the structure and the number of columns in a row, reductions to this basic value of 1/200 are possible, as detailed in Clause 5.3.2(3) of EN 1993-1-1. If horizontal loads (H Ed) exceed 15% of vertical loads (VEd cr) these cr sway imperfections may be disregarded, and EHF ignored – this would more oftern apply to low rise buildings.

 

So in Chapter 4.2.1 Load combination for ULS (based on equation 6.10 from EC1990) is:

1,35G+1,5Q+0,75W+EHF where EHF is "Efective horisontal force"

 

I strongly recommend to read publication.

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In simple words, EHF's represent a percentage of all loads on a particular floor, so they are proportionate to the individual load types on that floor. For examples, on one floor you have a dead load, superimposed load and imposed load. Each of these loads has their own partial factors. When you've derived design dead loads, superimposed loads and imposed loads you just multiply each of these loads on the particular floor but the respective percentage and you arrive at your EHF's, which represent the sum of the previously mentioned components.

 

Hope I"ve made myself clear enough,

Reinis

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