Deflagration Loads

[JLNJ]

When using Factory Mutual or NFPA data sheets to determine pressures caused by combustible dusts, are those resulting pressures service or ultimate loads?

It makes sense to me that these would be ultimate loads (which can be used with the ASCE extraordinary even load combinations of 7-16 Section 2.5); however, I don't see in FM or the NFPA where they explicitly indicate one way or the other.

Some of these calculations remain unchanged from many years ago, so they might be borne of a service load tradition.

 

[CrabbyT]

I believe this constitutes fair use. ASCE 59-11 states that member strengths are to be calculated using LRFD, and you should consult ASCE 59 for the strength reduction factors. They also state the following.

3.5.3 Remaining Strength.
The design strength of an element shall be reduced by the effects of the following load combinations using appropriate material-specific interaction equations in order to determine the remaining strength that is available to resist blast loads:
γ_DD + (γ_LL or 0.2S)........... (3-1)
γ_DD + 0.2W...................... (3-2)
where
γD = applicable dead load factor from Table 3-6
γL = applicable live load factor from Table 3-7
D = dead load determined in accordance with ASCE/SEI 7
L = live load determined in accordance with ASCE/SEI 7
S = snow load determined in accordance with ASCE/SEI 7
W = wind load determined in accordance with ASCE/SEI 7

TABLE 3-6. DEAD LOAD FACTORS
Element Description γ_D
Elements in existing buildings for which the actual dead load effect can be established with a high degree of certainty = 1.0
All other elements for which the dead load effect reduces the strength available to resist blast effects = 1.2
All other elements for which the dead load effect increases the strength available to resist blast effects =0.9

TABLE 3-7. LIVE LOAD FACTORS
Element Description γ_L
Elements for which risk assessment indicates that the full live load effect is likely to occur simultaneously with the blast effect = 1.0
All other elements for which the live load effect reduces the strength available to resist blast effects = 0.5
All other elements for which the live load effect increases the strength available to resist blast effects = 0

 

[HTURKAK]

I read this thread a few days ago and curious to hear about from the others.. The title of the thread is ( Deflagration Loads ) and i think the term is correct but not sure regarding the term ( combustion ). IMO, ( dust explosion) better..

I agree that ( dust explosion ) pressure should be considered ultimate load and extraordinary event load combinations of ASCE 7-16 Section 2.5.

But, neither ASCE 7-16 nor NFPA does not address the blast loading of dust explosion for building type structures . NFPA provides some pressure formulas but for grain silos etc.

As far as i understand ;

- ASCE 7/16 requires the robustness ( prevention of progressive collapse ) of the bldg ..
- NFPA addresses the measurements for prevention of fire and dust explosion from combustible solids and there are requirements for ventilation, separation etc.
- Silos, bunkers etc which may subject to the dust explosion shall be designed for that loading,
- Building type structures shall be designed for robustness , some explosion claddings ( weakened roof or wall cladding ) should be provided together with vents and detailed to prevent dust accumulation.
- If the bldg is required to be blast resistant , that is totally different story .. Some of the codes and recommended practices to go with ;ASCE ( DESIGN OF BLAST-RESISTANT BLDGS ), DNV-RP-C204 DESIGN AGAINST ACCIDENTAL LOADS ,API RECOMMENDED PRACTICE 2FB,...

 

[JLNJ]

Thanks for the replies.

I agree that "blast" loading out of ASCE 59 is a little different than the combustible dust type explosions addressed in NFPA 68 or FM 1-44. ASCE is more of an overall building system perspective and the others are more about designing a particular space to allow venting of a limited dust explosion/event. I'm interested in the design of the non-relief walls in a room for a certain overpressure to allow the pressure relief panels to function properly.

On one hand the loads from FM and NFPA seem like absolute maximum loads, and therefore ultimate, but NFPA and FM don't specifically address this. Nor do they address how to accommodate them in load combinations.

In the past, I probably would have treated them as service loads, combined them with dead loads, and used a 1/3 increase in allowable stresses.