ibc 2003 heliport design

huangyhg

ibc 2003 heliport design
for the design of heliports and helistops the ibc states the following:
landing areas shall be designed for the following loads:
1) dl plus the gross helicopter weight, plus snow load.
2) dl plus two live loads equal to 75% of the toatal helicopter weight spaced 8' apart. (this represents the skids/wheels+impact loads)
3) dl plus 100 psf ll
the helipad will be a 1-way concrete slab on beams spaced at 2' on center supporting a 12,000 lb aircraft.
i am interpreting these requirements as follows.  beams and connections will be designed for either point load according to case 1 or two loads from case 2.  punching shear and 1-way slab design will be based on case 2, as there will not be a single point load based on case 1.  check all conditions against case 3.  is there another condition that warrants consideration?
ive done several heliports in steel framing and it can get tricky when you have several layers of steel and posts because the impact load can only occur on one spot at a time but that spot can be anywhere.
keep in mind that your impact load will total 1.5 times the gross weight. the older codes had it as a single point load equal to 1.5 times the weight.
check to see if a snowmelt system will be required. typically, snowmelt is provided to keep the pad clear so this negates the snow load, although that case rarely if ever controls.
case one confuses me. you would think that the gross weight would be applied as two point loads(the actual condition).
when i do these in steel i take the distributed dead load and add the impact loads at the spots that will be the worst cases. sometimes there are several areas to be checked if you have cantilevers or different spans.
next, take a fully distributed load of dl+ll.
in steel framing you also have to consider the "pass thru" load if the point load occurs directly over a support. any support can have the impact load on it going directly from the helipad to the support and not affecting the bending of the   
thanks jjeng2 for your response. case 1 is confusing to me also, and really the basis of my first post, i'm just not sure that they are trying to tell me.  a single point load of 12,000 lb's, or say the weight of the aircraft spread out over the rotor diameter (10 psf), or the weight of the aircraft over the undercarriage area (30 psf)?  both are less than case 3 of 100 psf.  to design for a point load of 12,000 lbs, or 1.5*12,000 lbs seems like too much.  i think that case 2, compared against case 3, is more realistic of the loads that would be seen.  
faa "advisory circular (ac) 150/5390-2b heliport design" can help shed light on the ibc design loading criteria. it seems to be the basis for ibc 2003. see page 137, paragraph 806 design loads, static loads (your case 1)
"for design purposes, the design static load is equal to the helicopter's maximum takeoff weight applied through the total contact area of the wheels or skids. contact the manufacturer to obtain the contact area for the specific helicopters of interest." ibc adds the snow load requirement.
case 2 is discussed in the next paragraph, "dynamic loads".
case 3 seems to be a conservative assumption by ibc to accommodate the faa "rotor loads".
here is a link to the document
thanks slideruleera
based on the faa ac 150/5390-2b and the ibc, it would seem like the dynamic load (case 2) would exceed the static load (case 1) 100% of the time, if both conditions are treated as live loads.
brhorne,
interesting discussion
and what area is enough for the application of dl + ll(100 psf)
i mean you have a large pad. are you going to aplly the 100 psf all over the area? or some area based on helicopter dimesion over which the helicopter is likely to land?
any relation with rotor diameter?
respects
ijr
there is no live load reduction. i believe the intent is not for the helicopter weight but people and equipment on the pad. we could debate that but it is clear in the code that there is no reduction. even if you were to try to apply one, the area of the pads and areas supported by   
i follow that no live load reduction should be considered.  let's open the debate about the 100 psf load in conjunction with the dl of the helicopter.  in the three load cases mentioned above, the dead load of the helicopter is dh and is only listed in the first load case.  the impact load of the aircraft in the second case.  the third case makes no mention of the helicopter weight, just the dead load d and l of 100 psf.
an assembly of people on the roof to watch fireworks, waiting for rescue from the embasy, etc.