Occasional loads B31.3

[giezu]

Hi Guys,
I have some example which bother me. I hope you can help me.
Simple example (description according to B31.3):

Sc= 100MPa
Sh=100MPa
f=1
SL=10MPa- Sustained stress
SE=240MPa- Expansion stress

then Sa= 1*(1,25*100+0,25*100)=150MPa+ (Sh-SL)=150+90= 240MPa (add liberal stress)

So Se=Sa this is OK,
but if the Occasion stress is Socc=10+123=133MPa
Then 133+240=373MPa then it is more then 3xS or 2xRe.

My question is why the occasional loads are not taken into consideration during calculation of the allowable thermal (liberal stress) stress? Since the occasional loads are the primary loads.

Thank for answer in advance

 

[XL83NL]

Because thermal displacement stresses are secondary (forgot the word, displacement controlled?). They are treated (and analysed) different and apart from primary stresse.
There are numerous topics on this subject, both via google (which will lead you e.g. to CAESAR II forum where John Breen posted excellent replies), as well as here on engtips;

http://www.eng-tips.com/viewthread.cfm?qid=246231

 

[giezu]

But the code substract the 1Sl from thermal allowable. Why this substract do not include ocasional stress.

 

[GD2]

Giezu,
Where did you get the (Sh-SL) part in your Sa equation? My understanding is that it’s not part of the code equation. The SE of 240 Mpa is more than Sa of 150 Mpa. Therefore, the design fails.
Occasional loads are primary loads. When combined with Sustained load stress SL, the design will pass when (SL + SOcc)< or = 1.33Sh. When calculating occasional loads, winds and earthquake need not be considered acting concurrently. Hope it helps.

Ganga D. Deka, P. Eng
Canada

 

[GD2]

Hi giezu,
XL83NL had guided you to a beautiful link. Did you go through it? If not, try it first.

Ganga D. Deka, P. Eng
Canada

 

[giezu]

I understand what is secondary and primary stress, stress range, self limiting etc. I am just wonder why occasional loading which are primary loads are not substructed together with sustainded loads due to weight and pressure?
Below is extract from John Breen post:
There is stated thaat code use 1Sh for sustained loads.
Again why in 1Sh are not included loads from occasional loads?


"Spielvogle explains that the B31 rules intend for the piping engineer (analyst) to be able to use the entire range of stress from the material yield point at the operating (hot) temperature to the material yield point at the ambient (or coldest) temperature (less a factor of safety). Since Sh (neglecting the possibility of creep) is set at 2/3 Sy for both the hot and cold conditions, we can calculate the hot yield stress as Sh*1.5 and we can calculate the cold yield stress as Sc*1.5. Taken together the total allowable stress range for the combined SUSTAINED loadings of weight (bending), longitudinal pressure (tension) AND thermal expansion (displacement) would be (1.5*Sc) + (1.5*Sh), or 1.5(Sc + Sh). This range of allowable stress has been reduced slightly to allow for the vagaries of material and for other "real world" inaccuracies. The Code philosophy would then permit the total allowable stress range (after the factor of safety is applied) for all the combined loading described above to be 1.25(Sc + Sh) (if ,in this discussion, we neglect the stress range reduction factor ,"f", for simplicity). However the Code uses 1.0Sh for the sustained loadings of weight and longitudinal pressure (so we then must subtract this from the maximum allowable stress range for thermal displacement alone) and this leaves 1.25*Sc + 0.25*Sh for the allowable thermal expansion (displacement) stress range alone." 

 

[KevinNZ]

"I am just wonder why occasional loading which are primary loads are not substructed together with sustainded loads due to weight and pressure"

There are pressure and weight/sustained terms in the equation for occasional loads.