IBC SEISMIC: Eqn 16-28

[SmithJ]

Hello All,

In the IBC 2000 Code, equation 16-28 states that

E = p*Qe + 0.2*Sds*D

I am confused about the 0.2*Sds*D part of the equuation. What is this part trying to represent? If I have a mezzanine that has a high dead load, would this be included in the "D" part of the above equation?

How and when are gravity loads additive to seismic loads?

Any insight you have would be much appreciated.

Thanks,
JS.

 

[doejohn]

Per sect 1602 Notations, D = dead load, E = combined effect of horizontal and vertical earthquake-induced forces. First term in equations 16-28 and 16-29 are horizontal component of E, second term is vertical component. E is to be combined with other load effects per load combinations in sect 1605. If using, say strengh/LRFD combinations of 1605.2, E per eqn 16-28 is used for load combo 16-5 (effects of gravity and seismic ground motion additive per 1617.1.1), and E per eqn 16-29 is used for load combo 16-6 (effects of gravity and seismic ground motion counteract). Note that when these eqns for E are substituted into their respective load combos, load combo eqns may be re-written to combine D terms: for 16-5, (1.2+0.2*Sds)*D; for 16-6, (0.9-0.2*Sds)*D.

Hope this helps

 

[haynewp]

If you have a column in a frame for instance, there would be a force in the column from the couple due to an applied horizontal seismic force.

There is also an axial force 0.2SDSxD (vertical acceleration)to be added to the above mentioned column in compression. (D is the dead load supported by the column, including itself). You still have to add in the normally supported dead load per load comb. (ie 1.0D, 0.9D etc).

 

[JAE]

Please refer to my answer to this question in this previous post:

thread176-98527

 

[SmithJ]

JAE,

Thank you for the link to your previous thread. I am still not sure how this would apply to my current situation. I am designing the sidewall bracing for a building in high seismic region. Using your example, How would I calculate the brace force in each bay? Would the "D" in eqn. 16-28 refer only to the dead load in the particular member I am analyzing (Say an Eave Strut or Diagonal bracing).

Also, Formula 16-18 gives the load combination 0.9D + E/1.4

If I follow your previous thread, the dead load "effect" is already calculated within the "E" part of the equation. Wouldn't this particular load combo result in the dead load being counted twice?

Please help with your insight. I am finding this particular part of the code to be counter-intuitive.

Thanks,
JS.

 

[JAE]

As I said in the previous thread - it helps to keep in mind that the load combination are combinations of EFFECTS - not combinations of directional loads.

So the 0.9D + E/1.4 is a combination of ALL the dead and seismic loads on your structure. The dead load effect that is within E (since E is the lateral force on the structure plus a percentage of the dead load) is an ADDED portion of the dead load.

So you would have the following included in the above combination:

0.9 x D
(0.2 x SDS x D)/1.4
(Lateral Seismic)/1.4

You are never adding vertical loads numerically to lateral loads, rather, in this combination you are INCLUDING vertical and horizontal loads in combination in one analysis with the applicable factors (0.9, 1/1.4, etc) applied.

Hope this helps.

 

[SmithJ]

Thanks again, I think I am beginning to understand this.

Let's say that the Eqn 16-28 is more of a "Load Combination" than an actual equation. So in the case of seismic bracing along the sidewalls (perpendicular to the frame) the brace would only resist the lateral effect of the load since the brace does not carry any dead load.

In the direction paralllel to the frame however, the frame would not only resist the lateral force but it would also have to resist the additional 0.2*SDS*D dead load effect along with the lateral effect. In short, the load combination

0.9D + E/1.4

Could be replaced with

0.9D + ((p*Qe)/1.4) + ((0.2*SDS*D)/1.4)

Which simplifies to

(0.9 + .143*SDS)*D + (Lateral Seismic)/1.4

So the 0.2*SDS*D is actually only affecting the amount of vertical dead load applied to the member under analysis.

This is my understanding of the info in these threads. I hope I have not misinterpreted anything.

Thanks to everyone for helping with this.

JS.

 

[JAE]

I think you've got it.