accidental explosion pressure

[cmrdata]

Section 911.2, item 1, of the International Fire Code indicated that "Walls, ceilings and roofs exposing surrounding areas shall be designed to resist a minimum internal pressure of 100 psf..." Could anyone share his or her thoughts on if such explosion pressure should be treated as load at strength level, or at service level?
If it is to be treated as load at service level, what load factor should be applied when performing load combinations?

 

[CrabbyT]

In the book "Design of Blast-Resistant Buildings in Petrochemical Facilities" the ASCE recommends the following load combination.

1.0(DL) + 1.0(LL) + 1.0(BL)

They also state that blast loads shouldn't be combined with earthquake and wind loads, although I'm not sure I agree with that. It seems there would be instances where it would be more likely for an explosion to occur during an earthquake.

 

[ThomasH]

I am not familiar with the given reference, but are you sure that the 100 psf represents an explosion? THe value seems low, also what is the duration of the given impulse?

In my experience, two accidental loads are usually not combined.

Thomas

 

[GC_Hopi]

I also wouldn't combine accidental loads based on the probability and duration of the loads. It's unlikely that an explosion would occur in the limited time that the earthquake happens and highest magnitude of the events is unlikely to coincide.

If there is a serious concern for a events coinciding then maybe the 1.0 factor on one and 0.5 on the other.

 

[MotorCity]

I would assume that the average user of the International Fire Code is not a structural engineer and therefore not familiar w/ ultimate (strength) level design.

As such, the average user would consider 100 psf to be what us structurals understand as a service load.

In addition, if one did assume it was ultimate, that would convert to roughly 100 psf/1.67 = 60 psf service level which is way too low.

For load combinations, do not include wind or seismic. The odds of having a blast load at the precise second (since blasts lasts milliseconds) of the design wind or seismic event is zero.

 

[ThomasH]

Thinking a bit more about the question.

Like I said before, I am not familiar with this code. But I think that the question regarding how to combine should be answered in the code. I would be surprised otherwise since it is fairly fundamental. And where does the assumption that it is an explosien come from?

I notice that OP has not returned. Perhaps the issue is solved.

Thomas

 

[cmrdata]

Thank you all for your feedback.

I have also sent my question to ICC for interpretation, and here is their response:

Thank you for query below regarding blast loading in the 2018 IFC and the 2018 IBC.
The interaction of a real world blast with a structure can result in very complex inelastic dynamic behavior.

For buildings that house hazardous processes, building codes require special safety considerations. The 100 psf noted in section 911.2 of the 2018 IFC is intended to be the design load minimum internal pressure. This design requirement dates back the legacy codes: For example, the Uniform Building Code stated that “walls, floors and roofs separating a use from an explosion exposure shall be designed to resist a minimum internal pressure of 100 pounds per square foot in addition to other conventional loads.”

Naturally, such structures need to be designed and detailed to resist progressive collapse.

Unfortunately, both the 2018 IBC and ASCE/SEI 7-16 do not explicitly define how to handle the defined blast loading.

ASCE/SEI 7-16 commentary section C1.3.1.3 states, “The alternative procedures of Section 1.3.1.3 may be used to demonstrate adequacy for one or more design loads, while the standard procedures of Sections 1.3.1.1 and 1.3.1.2 are used to demonstrate adequacy for other design loads. For example, it is relatively common to use the alternative procedures to demonstrate adequate earthquake, fire, or blast resistance, while the standard prescriptive procedures of Sections 1.3.1.1 and 1.3.1.2 are used for all other loading considerations.”; hence, ASCE/SEI 7-16 recommends using Performance-Based Design Procedures of ASCE/SEI 7 section 1.3.1.3.

While blast loads are dynamic, in some circumstances it makes sense to use equivalent static blast loads for design. Blast loads are typically combined with gravity loads and load factors are set equal to 1.0 and one can use a more realistic guess at day-today live load. Hence, in addition to the typical design load combinations of IBC and ASCE/SEI 7; ASCE 59-11 (Blast Protection of Buildings) recommends the following additional two combination where B = Blast loading:

• 1.0 B + 1.0 D + 0.5 L
• 1.0 B + 1.0 D + 0.2 W[/color]

 

[JLNJ]

I'm not familiar with this particular code provision, but I have participated in the design of blast-resistant structures with incident pressures in excess of 7 psi.

I take the 100 psf to mean that even thought your analysis might indicate that your pressure is always one direction (say inward) that you still need a minimum resistance in ANY direction (i.e. outward). As ThomesH said, 100 psf is quite low in the world of blast so it shouldn't be too difficult to accommodate.

 

[ThomasH]

Based on the data you currently have you can't do the dynamic analysis mentioned because the description of the load is incomplete. The load varies with time, and you don't know the time (the duration). You can probably assume that impule is triangular with a very short rise time but you need the total duration. The negative part of the pulse can often be ignored.

It may be so that the 100 psf is intended to be used as an equivalent static load. The problem is that for two different structures subjected to the same blast load (peak pressure and duration), the static equivalent load will differ. But I know of cases where the static equivalent method is used without considering this. Is is often assumed as a better approach than doing nothing.

One possibility is that this load is not expected to be governing, it is just a "minimum" requirement for blast. I would not say that it is a very small load, just not a significant load in a blast context.

Thomas

 

[Trenno]

Critical elements designed to Eurocode are designed to withstand 34 kPa (680psf). This pressure load is combined with service level loads, as it it's deemed to be an accidental load case.

This all stems from the gas cylinder explosion at Ronan Point Apartments.

 

[haynewp]

I think the 100 psf has to do with deflagration design. I would look into NFPA 68 for how to treat the load.