Loading Conditions

[dabbler101]

I'm currently working on an abutment design for a single span bridge on rural road in western Massachusetts. I am planning on looking at all applicable load combinations for two loading cases. For the first case I would consider a 10 year flood condition and for the second case I would consider an ordinary water level condition. I am curious about the general consensus on whether earth quake loads should be included in the 10 year flood loading condition. It seems to me that because a 10 year flood and an earthquake are major events the probability of them happening simultaneously is extremely low. Is this an incorrect assumption?

 

[SlideRuleEra]

I agree with you that seismic and flood loads don't have to be considered as acting simultaneously. I don't have the appropriate codes to verify that, perhaps someone else can...

however the real reason that I'm posting is to question the 10 flood requirement. All bridges that I have been associated with were designed for at least a 50 year or perhaps a 100 year flood (or more). Those 10 year "events" have the habit of happening all too often.

http://www.SlideRuleEra.net

 

[Bagman2524]

IBC defers flood load design to ASCE 24. I'm not familar with that standard. I don't see anything in IBC where flood loads are combined with earthquake loads.

 

[civilperson]

A ten year flood condition could be exceeded ten times a century. If the road is washed away and your bridge survives, you will miss out on a series of redesign jobs!

 

[DaveVikingPE]

Have you checked ASCE 7-05 and AASHTO Bridge Manual's load combinations? You will save yourself an awful lot of - my opinion - guesswork.

 

[dabbler101]

I have checked all of these codes... and I'm still not sure. I understand that designing for a 10 year flood is on the lower end of the spectrum but the Massachusetts Highway Department was very clear about the design criteria in the scope of work. On top of that, we are holding the low chord of the existing structure so as not to change the hydraulic opening significantly as that would affect all the models for everything downstream. My original question may not have been super clear, forces exerted on the superstructure by the ten year flood waters are transferred to the abutment because the abutments are skewed a component of that force acts as an overturning moment on the abutment. That force is approximately equivalent to the force from the wind load on the superstructure (fairly small 150#/ft). The real kicker is the buoyancy forces on the superstructure, abutment, and backfill over heal as it negates a good portion of the dead weight used to resist the overturning moment. Add the quake load in and this abutment is going to have to be much more massive... Not too mention how the high water table would only further complicate the seismic soil forces acting on the abutment. So I ask again, what do you think? Do the 10 year flood and quake load conditions need to be coupled? Just curious in some more opinions...