AREMA Steel Structures - Load Combination 3

[mungchucks]

Does anyone know if AREMA has a formal load combination table for steel stuctures? They have one for Concrete for both Allowable Stress and Load Factor Design. They do not have one for Allowable Stress Design for Steel Structures. I'm wondering there are any strict rules on what loads should be combined. Or should (Dead + Live + Impact) Load be combined individually with Seismic, Wind, and Braking.

 

[purdue86]

No AREMA does not have a formal load combination table for steel structures. Until seismic forces entered the picture all loads were combined together as appropriate. What I would recommend is:

DL + LL + Imp + Wind on Train + Wind on Bridge + CF + Nosing + Braking

DL + LL + Imp + Wind on Train + Wind on Bridge + CF + Nosing + Traction

DL + Wind on Bridge (only on tall and light structures)

DL + Seismic (See chapter 9 section 1.4.6)

Note most members do not receive all of the loads in the combinations. Girders will generally be designed for DL + LL + Imp with a redistribution of LL due to Wind on the Train, CF, Super Elevation and Eccentricity. The bracing will be designed to resist Wind, CF, Super Elevation ane Eccentricity, Nosing and Flange Stress. Bearings and foundations will be designed to resist all of the forces in the appropriate directions.

Howard Swanson

 

[purdue86]

And another thing...

As a Bridge Engineer that has designed bridges for a class 1 railroad for over 15 years, there are a few items that I would like to pontificate on (which is appropriate for a bridge engineer based on the Latin root).

Before you design a railroad structure, make sure you have a current copy of the AREMA manual and use it. You wouldn't design a highway structure without the AASHTO manual, would you? Also, AREMA manual is a set of recommended practices, not a set of specifications. Make sure that you have the railroad's guidelines in addition to the AREMA manual. The railroad's guidelines may place greater restrictions on your design than the AREMA manual.

Remember, the majority of the AREMA manual has been around for a long time. It was developed when everyone used slide rules. You should be able to design most railroad structures using a steel manual, an engineering pad and a calculator. In general, you should not need a structural analysis program. I have seen too many instances when plans are submitted based on what the computer tells the data entry clerk than what an engineer has designed.

The impact factor equation seems to trip up most engineers. There are a few tricks to getting through it. 1st remember that the units are percent of live load. 2nd the load for the rocking effect is applied as a couple at the rails. Although track gauge is 4'-8 1/2", center to center of rail is about 4'-11 1/2". When calculating rocking effect this is generally simplified to 5'. Therefore, the couple solves as [20% Live Load x 5']/[Center to Center girders(in feet)] (assuming single ply construction).

Howard Swanson

 

[mungchucks]

Thanks for the input. It really helped.

Mungchucks

 

[KuJayHawk]

I wonder how AREMA specifies the rocking effect exactly. Chapter 15-1.3.5 says all the impact effects should be applied on top of each rail. But the rocking effect is actually calculated for individual girders. For example, if I have four girders spaced at 2.5ft, the gage is 5ft, then RE is 24% for the two exterior girders and 8% for the two interior girders. Assume 30% due to vertical impact effect. When I calculate the impact, should I use

(a) the combination of girder REs under one rail
1+IM = 1+32%+30% = 1.62 for one rail (average 0.81 for one girder)

or (b) the largest RE
1+IM = 1+24%+30% = 1.54 for one rail (average 0.77 for one girder)

I read Part 9 Commentary but still could not figure out. I feel that (a) makes sense as eventually the moment/shear calculations are for one rail (or half track, or in this case involves two girders), but other folks in the office said (b) is usually the way how RE was handled. I wonder if anyone can help point to the right solution, or if there are any references on this subject that I can go look for the answer?

Thanks!

Yuan Zhao