Earthquake loading and dead load factors (AS 1170 vs AS 5100)
Earthquake loading and dead load factors (AS 1170 vs AS 5100)
(OP)
This is a topic of confusion in my office because of the way the two codes are written. Hopefully someone can enlighten me...
AS 1170.0
In AS 1170.0, the earthquake load combination is:
1.0 x EQ + 1.0 x G + ψE x Q
To my mind, the above equation makes complete sense. The EQ force is based on the unfactored mass of the structure (allowing for some permanent component of live load).
The 'load factor' for the EQ loading is basically built into probability factor kp in AS 1170.4, and is calculated based on the unfactored mass.
The important thing to note in my opinion is that this 'load factor' scales up the magnitude of the EQ acceleration, but not the mass of the structure.
It then makes sense that the dead load factor should also be 1.0 because we are dealing with acceleration effects on the nominal (unfactored) mass of the structure. It would seem wrong to consider the unfactored mass of the structure for EQ effects, but then to artificially scale the mass up or down for gravity effects.
But --- this is exactly what AS 5100.2 seems to suggest.
AS 5100.2
Let's take a mostly concrete bridge (for simplicity), so the EQ load combination according to AS 5100.2 is:
1.0 x EQ + [0.85 or 1.2] x Concrete Dead Load
At first glance, this would suggest that we have to calculate the EQ force based on the nominal mass of the structure (hence the 1.0 factor), and then scale up or down the gravity effects (by a factor of 0.85 or 1.2). This approach has the obvious problem that only one of the mass-derived loads is scaled while the other remains unchanged, which is inconsistent. Unlike the probability factor kp, which is effectively a ULS factor applied to the EQ acceleration, the 0.85 or 1.2 dead load factors instead scale up the mass of the structure (not the gravitational acceleration).
Or - is it saying that we should first scale the mass of the structure (by a 0.85 or 1.2 factor, say), and then apply both the EQ and gravity effects on this scaled mass? This approach is more consistent in that both of the mass-derived loads are scaled by the same amount, but we are now effectively applying a second load factor onto the EQ effect (i.e. on top of the probability factor kp in AS 1170.4).
My personal opinion is that AS 5100.2 is simply wrong and that this particular load combination has been overlooked. There are plenty of other examples throughout the AS 5100 series where obvious errors have been overlooked and unchanged for several years without revision. The AS 1170.0 approach, in my mind, is consistent and logical.
AS 1170.0
In AS 1170.0, the earthquake load combination is:
1.0 x EQ + 1.0 x G + ψE x Q
To my mind, the above equation makes complete sense. The EQ force is based on the unfactored mass of the structure (allowing for some permanent component of live load).
The 'load factor' for the EQ loading is basically built into probability factor kp in AS 1170.4, and is calculated based on the unfactored mass.
The important thing to note in my opinion is that this 'load factor' scales up the magnitude of the EQ acceleration, but not the mass of the structure.
It then makes sense that the dead load factor should also be 1.0 because we are dealing with acceleration effects on the nominal (unfactored) mass of the structure. It would seem wrong to consider the unfactored mass of the structure for EQ effects, but then to artificially scale the mass up or down for gravity effects.
But --- this is exactly what AS 5100.2 seems to suggest.
AS 5100.2
Let's take a mostly concrete bridge (for simplicity), so the EQ load combination according to AS 5100.2 is:
1.0 x EQ + [0.85 or 1.2] x Concrete Dead Load
At first glance, this would suggest that we have to calculate the EQ force based on the nominal mass of the structure (hence the 1.0 factor), and then scale up or down the gravity effects (by a factor of 0.85 or 1.2). This approach has the obvious problem that only one of the mass-derived loads is scaled while the other remains unchanged, which is inconsistent. Unlike the probability factor kp, which is effectively a ULS factor applied to the EQ acceleration, the 0.85 or 1.2 dead load factors instead scale up the mass of the structure (not the gravitational acceleration).
Or - is it saying that we should first scale the mass of the structure (by a 0.85 or 1.2 factor, say), and then apply both the EQ and gravity effects on this scaled mass? This approach is more consistent in that both of the mass-derived loads are scaled by the same amount, but we are now effectively applying a second load factor onto the EQ effect (i.e. on top of the probability factor kp in AS 1170.4).
My personal opinion is that AS 5100.2 is simply wrong and that this particular load combination has been overlooked. There are plenty of other examples throughout the AS 5100 series where obvious errors have been overlooked and unchanged for several years without revision. The AS 1170.0 approach, in my mind, is consistent and logical.
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RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
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Why yes, I do in fact have no idea what I'm talking about
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
Are you saying that your approach would be to scale both (i.e. effectively scale the mass and apply both gravity & EQ effects to the scaled mass) or to scale neither (load factors of 1.0, a.k.a. the AS 1170 method)?
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
5100 load combos tend to deviate very heavily from 1170, which is why I wouldn't stick to the "good enough" combination of G+0.3Q+EQ we use for normal structures. I imagine they put the different dead load combination factors for the earthquake for a reason. There's also a very notable absence of live loads in a lot of the ULS combinations, not particularly sure why, but I'd also personally like to capture some of that missing load in the 1.2G case.
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Why yes, I do in fact have no idea what I'm talking about
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
- Inaccuracies in the analysis.
- Differences in load distribution.
- Differences in flexural stiffness to that assumed in the design.
- Load redistribution due to differential settlement or differential creep and shrinkage.
It is therefore quite possible, for example, that the actual axial force due to dead load at a column will be significantly lower than the design value, without a significant variation in the actual mass of the structure.
Applying the standard dead load ULS factors in conjunction with seismic loads is therefore conservative and gives a better estimate of the worst-case load conditions.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
So possibly it's the case the AS 5100.2 deliberately left in the dead load factors for the EQ load combination. I could see that for the design of bearings, for example, this would provide an extra margin of safety against bridges becoming dislodged from their supports or some other catastrophic type failure where vertical loading is important.
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
The seismic mass is worked out based on the actual mass, or at least a realistic estimate allowed for in codes (i.e. G and some proportion of the live load). This determines the lateral loads, this is a fixed value.
The axial loads present at the same time as these seismic loads can vary on an individual element (i.e. as per IDS's explanation), it is not always going to be the average determined based on the overall seismic mass.
It must also be remembered the load combinations are not 'plus' they are 'and', this means you as the designer must determine the worst design action effect. If for example a member is in tension/uplift and the live load part of any load combination were to act to reduce that tension design action effect, then you leave it out and only apply the permanent gravity load portion of the load combination when assessing the critical load case vs strength. You can of course also evaluate the case with the live load, but it will obviously not be the critical case.
https://engineervsheep.com
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
If the Eq load is same sign as the DL then a higher DL factor is applied so 1.2DL + Eq
If the Eq Load is the opposite sign to the DL, then a reducing DL factor is applied so .85DL + Eq.
This has nothing to do with calculating Eq.
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
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Why yes, I do in fact have no idea what I'm talking about
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
Even for a relatively simple single-span bridge, especially where there is a skew, there is some uncertainty as to how the superstructure dead load is distributed to each of the bearings and piles, for example (depending on factors such as torsional stiffness of girders, deck stiffness, settlement, etc.). 0.85~1.2 seems like a reasonable range of certainty.
Why, then (in your opinion), does AS 1170.0 not do the same?
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
I have asked both 1170and 3600 to clarify.
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
It seems that AS 5100.2 begs to differ.
The next draft amendment to AS 5100 has recently been released for public comment, and has no change to the dead load factors to be used in conjunction with earthquake loads.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
thread744-490596: AS1170.4 Section 7 Vertical earthquake actions
RE: Earthquake loading and dead load factors (AS 1170 vs AS 5100)
But I asked the AS3600 Seismic Subcommittee which includes the Chairman of AS1170.4 and they said 1170.4 is correct.
ACI also uses .9 or 1.2 for D so similar to AS5100.
I will ask again!