Basic Question- about Load combination 1

[STEVE LIU]

HI everyone....
i have a basic question confuse me long time.
U=1.4(D+F)...............(1)
U=1.2(D+F+T)+1.6(L+H)+0.5(Lr or S or R)....(2)
U=1.2D+1.6(Lr or s or R)+(1.0L or 0.8 W)...(3)
U=1.2D+1.6W+1.0L+0.5(Lr or S or R).........(4)
U=1.2D+1.2F+1.0E+1.6H+1.0L+0.2S............(5)
U=0.9D+1.2F+1.6W+1.6H......................(6)
U=0.9D+1.2F+1.0E+1.6H......................(7)

First, according to what loading i have and fill in load combination to calcualte
if i dont have live load thal L=0 fill in.
my point is if i dont have wind load and earthquake why i must use load combination 4~7 ?
In my opinion the load factor in 4~7 were calculated cause effected by short term load (wind ,earthquake)
if we dont have W and E , use W=0 , E=0 fill in 4~7 reasonable?

thanks for you help

 

[JAE]

Yes you are correct that if a particular load case (say Live Load) equals 0, then that load combination would be used with L=0 and it may not control.
In fact, if L=0, using your engineering judgement, you may not even need to use it.

There are times when wind load = 0 (interior structure) but in most all cases there is wind and in most areas there is some level of seismic loading.


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[STEVE LIU]

hi~~ＪＡＥ

thank for your help and let me explain my situation

I design a pit now, and i dont need to consider wind load and earthquake at this area
if i consider combination 6 &7 and have not water inside pit
U = 0.9D + 1.2F + 1.6W + 1.6H ......................（6）
U = 0.9D + 1.2F + 1.0E + 1.6H.......................（7）

F＝0, w＝0, E=0

0.9D+1.6H
H including groudwater, this combination make uplift load too large (1.6 multi)
let my analysis model error (compression only spring have tension)

 

[JAE]

We have used a document from the US military or corps of engineers in the past that provides various levels of safety against bouyant uplift.
I can't seem to find it here on Eng-Tips (I think i once posted it here). But I'll look at it when I'm back in my office and see if I can find it.

Basically it used different safety factors for different levels of water up the side of the tank (or pit) - with the highest level of water (i.e. perhaps at the ground level) having the lowest safety factor and the lower water level (such as half-way up the tank/pit) as the highest safety factor - i.e. I think it was 1.5.

I'll see if I can find that document.

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[JAE]

Steve - here is a link to the document I referred to:

http://www.dtic.mil/dtic/tr/fulltext/u2/a403467.pdf

Also - a copy of the safety factor table within that publication:

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[STEVE LIU]

Hi JAE~~

Very thank for your help and i check the safety against bouyant uplift.
i think ok .

But i think my problem is the loading combination to analysis stress and design rebar
as below

U = 0.9D + 1.2F + 1.6W + 1.6H ......................（6）
U = 0.9D + 1.2F + 1.0E + 1.6H.......................（7）
F＝0, w＝0, E=0
0.9D+1.6H ( disccount dead load, and increasu uplift load)

i would like to do not use these two load combinaiton
but i dont have any foundamental to support my idea , mabye i am wrong.
I dont understand why i must to use these two load combination
even i dont need to consider wind load and earthquake

 

[klaus.]

You should use your 'engineering' sense
if you have no wind you DO NOT need to consider wind

These load combination are a basic guideline on how to combine different loads


best regards
Klaus