IBC Load Combinations

[TTMK]

Can someone inform me of how to do the following using the ASD load combination equations in the IBC.

I was given a structure with reactions so that I can size a footing. In these reactions I am provided a horizontal force (x) and a vertical force . I was under the impression that those equations were regarding strictly vertical loads except the wind and earthquake.

What is the proper way to combine the x and y force so that I can design this footing.

By the way this is my first time using the IBC.

 

[frv]

You need to determine what part of the vertical force is dead, live, and wind (or EQ) and then apply them according to the combinations. For example, if your vertical load is entirely Dead Load and your lateral load is entirely wind load, you would have three combinations: D only; D + W; and 0.6D +/- W.

 

[charliealphabravo]

The SER for the superstructure should provide you with a table of the load combination that were used to generate each reaction. If this is a PEMB then they should also specify the location, diameter, and spec of the anchor rods. You shouldn't have to reverse engineer the loads if I understand what you are asking.

 

[TTMK]

charlie you are on the right track to my problem. I was given the reactions (D, Roof Live Load, S, E, and W) in a chart form. In the chart under each reaction I was given a vertical and horiz reaction. For example, Dead load: Vert = 0.85
Horiz= -0.15

A little more information about what I am doing. I am basically trying to find the bearing pressure that will be put down by the footing that was provided in someone elses design (basically checking them). When I looked at his data I noted that he only accounted for the vertical component when in fact there are horiz and vert components. It seems as though the footing size will not work if I account for any other loads.

I just want to know how you can combined the horiz and vertical components so that my dead load, and any other load, will be larger and correctly model the situation i am in.

 

[jimstructures]

TTMK,

Ask the people who provided the loads if they are service loads (ASD) or factored loads (LRFD). When you get that response then apply the correct factors per IBC for the load combinations you need to use to design the foundations.

Most PEMB Co. will provide service loads when they send out the load cases. some can also provide the load combinations they used to design the frames. You will probably use somewhat different load combinations to design the foundations

Hope this helps.

Jimstructures

 

[SteelPE]

The load combinations in IBC are for loads in any direction (horizontal, vertical etc). You use the equations to combine them together to get your answer. For example:

Load, Vert (Y), Horiz (X)
Dead, 30 kips , 0 kips
Seismic, +/- 20 kips, +/-20 kips

All loads are service loads

Equation 16-15 = 0.6D + 0.7E

Vertical loads = 0.6*30+0.7*(+/-20) = +32 kips or +4 kips
Horizontal load = 0.6*0+0.7*(+/-20) = +14 kips or -14 kips

Basically you place these loads at your column base:

+32 kips Y + 14 kips X
+4 kips Y - 14 kips X

(I hope I did that right)

Depending on the elevation of your footing relative to your column base and the detailing of the foundation at your column location you may end up with moment in your footing due to lateral load.

Basically you have to run the loads through all of the combinations found in IBC to figure out which load combinations are controlling your foundation design. Load combinations that produce the worst loading condition for the structure above may not necessarily be the same combinations that control the design of the foundation system. Once you do enough you will know which combination control and you can probably leave a few out.

 

[JAE]

TTMK - This is basic foundation design with eccentric loads. I think you should find a good foundations book and understand how to do this. The footing you design must resist all the various IBC load combinations when the various vertical and horizontal loads are applied to it.

The design involves checking the soil bearing under various ASD (non-factored) load combinations - assuming you have been given an allowable bearing stress.

With vertical loads, the soil pressure is generally uniform (assuming the load is applied at the footing centroid).
For horizontal loads, there will be lateral shear that is typically resisted by passive soil pressure and/or soil cohesion or friction.
Also with horizontal loads, the load is typically applied above the soil-footing interface and this causes a rotation or moment on the footing, developing a non-uniform soil stress....unless there are intersecting grade beams or foundation walls to resist that.

Once the size of footing is determined based on soil pressure, you then would use factored load combinations to design the actual footing, thickness, rebar, etc.

If you don't know how to do this, then I'd recommend having your supervisor or another experienced engineer walk you through it.

 

[TTMK]

JAE and SteelPE...you guys hit the nail on the head. Just for the record JAE I do understand the basic foundation theory this was just my first project using the ibc (id much rather use AASHTO). I just wanted to make sure i was applying the loads correctly since it is a key part of info.

Thank you all for the help.