LRFD Load Combination

[htennek01]

I have a roof perimeter beam subjects to loads on vertical and laternal directions.

Vertical load: L, D, W1(upward)
Laternal load: E, W2(to the wall)

Considering 1.2 D + 1.6 Lr + 0.8 W and 0.9 D +/- E,
1) how can I fit the vertical W1 and laternal W2 to the first equation?
2) am I supposed to ignore the directions (vertical, laternal) of D and E, and plug into the second equation?

Thanks for help

 

[paddingtongreen]

It is the effects that you combine in the unity equations, not the loads. The vertical gives an Mxx and the horizontal gives an Myy, then you combine the stresses caused by these moments.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[jenDL]

Use all loads that would occur in any specific direction. Then check the beam capacity with an iteraction equation to account for axial loads and bending in more than one direction.

vertical loads: 1.2D + 1.6 Lr + W1
lateral loads: O D + 0 Lr + W2 (don't forget lateral loads may be transferred through bending and axial, depending on the structural layout)

 

[htennek01]

I have stresses in x-dir and y-dir. What will be the method to combine the strsses and make comparision based on the six LRFD equations?

 

[htennek01]

JenDL,

Do you mean I can seperate the analysis in two parts,

Part 1, analysis vertical load only and check the bending, shear..... on vertical-dir
Part 2, analysis laternal load only and check the bending .... on horizontal-dir

and size the members based on both analysis?

 

[jenDL]

yes, use as many iterations as needed in order to account for all directions of the input loads. Then you combine the results together with the use of an "interaction" equation.

The design equations depend on the materials you are using. You'll need to review your material code documents (AISC for steel, NDS for wood). They all have many chapters explaining how to evaluate strength in tension, compression, bending, etc.

 

[htennek01]

I have a rectangular steel building with concrete shear wall core (at the center within steel structure) which supposes to resist all lateral forces. When I do the steel beam and column analysis, do I need to consider earthquake and wind load?

 

[jenDL]

yes, the earthquake and wind forces have to get to the concrete core somehow, which will be through the surrounding structure. I suggest you have a little sit-down with your employer and talk about load transfer.

All the things you have been asking require some critical thinking about a structure as a whole and can't be adequately evaluated with only a beam calc or two. Additionally, the model codes have some stringent requirements for connections which you'll need to become familiar with.

It would be best to find a mentor at your office who can provide you with step-by-step help and check that you are on the right track.

 

[RLC32681]

did i understand it right? W1 is the uplift wind pressure and W2 is a wind pressure acting perpendicular to wall?

If correct, 1.2D + 1.6Lr + 0.8W could become 1.2D + 1.6Lr + 0.8(W1 + W2). uplift wind pressure and horizontal wind pressure act simultaneously. for the second equation, 0.9D +/- E, that could depend on the direction of your seismic. most often if you are working with 3D model, +X, -X, +Z and -Z direction is consider; that is 0.9D+EQ+X, 0.9+EQ-X, 0.9D+EQ-Z AND 0.9D+EQ-Z are consider.

 

[frv]

It's getting toward the end of the semester, and your question sounds very much like the type of question you would get from an end of semester project. Students are not allowed on this forum.

 

[JAE]

The load combinations are exactly as paddingtongreen suggests above. They are really combinations of effects (i.e. shear or moment) but you can combine loads in these equations to get there of course.

Each load combination should include ALL applied wind loads whether they are vertical or horizontal.

The analysis may be done in each direction separately (or all at once if you use a 3d program) and the results are (as jenDL says) combined into an interaction equation.