Question on load combinations

[tonghl78]

I am using a structural design program and I was surprised to find out that the program generated load combinations

1. have load cases considering live load = 0. for example, if using UBC97 eq. 12-5, there are load combinatios 1.2D+1.0E

2. When using eq 12-6 in UBC97, have load combinations like 0.7D+Rho*Eh.

Can these be right?

 

[Lion06]

Yes, these are similar to 0.6D+W for ASD - they consider the case of uplift. I am surprised to see 1.2D, however, I would expect to see 0.9D.

 

[UcfSE]

In my UBC '97, combination 12-5 does include live load and snow, the factors f[sub]1[/sub] and f[sub]2[/sub] defined in the lower part of 1612.2.1.

I have a different equation for 12-6 than you list. Perhaps we have different versions or updates. Check the section again and be sure you caught everything.

 

[abed1980]

it is as such : 1.1(1.2DL + (0.5 or 1)LL + E)
but the problem is how to add the "E" in this combination ... there are 16 quadratic combinations to add.
and what is meant by 1.1(0.9DL+-1.0E) the + and - in this combination for what ???

thank you

 

[Lion06]

The + or - signifies that you are to take the combination that produces the worst effect. The seismic event can occur in any direction. So, if you consider the event in the positive X-direction and that produces a compressive force in a column, you need to also check that column-footing assembly for uplift when the seismic event occurs in the negative X-direction.

 

[abed1980]

StructuralEIT
the E term in the combination " 1.2DL+ 0.5LL + E " IS FOUND TO be added linearly ??
Is it a linear combination , if yes this would cause a zero result in some cases for example moment on columns
-40-10+50= 0
plz answer

 

[abed1980]

suppose we need to obtain results on a column Fx, My, Mx ...
take for example Moment around y-axis "My" @ top of the column . we observe that the dead load gives a positive moment and the seismic gives a -ve moment thus when w add the combination 1.2DL+0.55LL+E it won't give the worst case infact it will reduce the moment and all other forces.

the ubc load combinations are not purely linear they are conditionally linear ...
Am I right ?????
thanks

 

[Lion06]

abdeb-
This is where the + or - comes in. If you use -E instead of +E, you will get -40-10-50=-100, not 0.

 

[abed1980]

"abdeb-
This is where the + or - comes in. If you use -E instead of +E, you will get -40-10-50=-100, not 0."

but the combination is only +E: 1.2DL +fLL +E
there is no -E in this load combination

????

 

[Lion06]

then reverse the direction of the seismic force and you will get an identical answer to using -E. You have to consider the earthquake to act in either direction, not just one.

 

[abed1980]

Ok , but what is meant by this combination 09DL + or - E
where the code presented the + or - condition and on the otherhand the combination 1.2DL + fLL + E presents only the +E condition???

StructuralEIT i really appreciate your posts
thanks

 

[UcfSE]

Only "+" E is required because E itself can be positive or negative. To put a +/- in front of a quantity that can already be +/- in itself is redundant. You add these quantities algebraically, so you have to consider the direction of the load, which is the +/- part.