Lateral load cases for top heavy equipment platform

[RabitPete]

What load cases/combinations would you recommend for an elevated equipment platform supporting equipment many times the weight of the platform itself? Lateral loads in particular.

Seismic design category A, Sds=0.149, structure is inside so no W/S/R loads, no Live load on the platform itself, though 1 side of the frame will also serve as an interior partition wall. Codes are not very clear about lateral loads on equipment platforms and I've seen suggestions to use as little as 1% of the dead load.

We were going to use 15% of the DL as E + 5psf L applied to the interior partition wall to design lateral bracing.
In this case 1.2D + 1.0E + 0.5L controls lateral bracing and lateral loads on anchors. Is it too conservative?

 

[human909]

I would have thought you just apply your relevant seismic approach. For me for small structures that is pretty much just 10% of the load as lateral load. So that is what I use. For what it is worth for light maintenance type platforms where the load is general workers, I use 10% of the live load in designing bracing. Sure that is probably on the high side, but we are generally talking some pretty light bracing so cost isn't an issue. A wobbly platform for workers is an issue.

 

[RabitPete]

Just to clarify, there is no live load on the platform itself - 5 large air compressors, tanks and air drying equipment. Design intention was to have no workers on the platform, due to space/height constraints. All occasional service to be performed using portable ladders and/or personnel lifts.

 

[HTURKAK]

You may look to PIP ( Process Industry Practices PIP STC01015 Structural Design Criteria )..
If there are 5 large air compressors , the vibration design of the platform probably will be the most critical case.

IMO, DL of eq's and platform , LL of tank , erection loads, maintenance , thermal, vibrating eq. ... possible loads..

 

[bones206]

I believe this would be the section of ASCE 7 governing your minimum lateral load requirement:

Section 1.4 covers the baseline requirements of general structural integrity, including a 1% notional lateral load. So I think you have room to skinny the 15% down and still comply with the letter of the code if you wanted to. If you wanted to be a little more conservative, you could design as SDC B and follow Chapter 15 for Non-Building Structures Similar to Buildings.

 

[RabitPete]

@HTURKAK: Why would you count air receiver tanks as LL?
I always considered them as DL, even liquid storage tanks in cases like this I would still consider DL, taking the full weight of the tank and liquid into consideration. Only if uplift is an issue I would split it into DL+LL, so 0.9D+W gives us conservative result even if tanks loose their content.

 

[structdtl]

I would really look into possible vibrations from your air compressors and resonance with the structure.

 

[HTURKAK]

There is a misunderstanding ..you may consider the tank itself as DL. The liquid content also DL as per ASCE 7 but this is not a worldwide common practice..

 

[RabitPete]

I would really look into possible vibrations from your air compressors and resonance with the structure.

Good point. Those are screw type rotary compressors, and they will be installed on housed spring vibration isolators.

 

[human909]

Just to clarify, there is no live load on the platform itself - 5 large air compressors, tanks and air drying equipment. Design intention was to have no workers on the platform, due to space/height constraints. All occasional service to be performed using portable ladders and/or personnel lifts.

Regardless of what the type of load is the reality is there needs to be SOME lateral restraint. For small equipment support platforms I'm just followed the 10% of DEAD+LIVE load rule. This is overly conservative but like I said the required bracing ends up being pocket change for small structures anyway. Easier to do this than waste your time thinking about it too much. This is my approach, not necessarily a one size fits all. For higher seismic areas or higher vibration loads you might need more. For larger structure in a very low seismic region you can likely get away with less.

(I'm talking about platforms where DEAD+LIVE loads might be 10-50kN. So you end up with lateral loads of 1-5kN which are pretty easy to brace away.)

 

[RabitPete]

For small equipment support platforms I'm just followed the 10% of DEAD+LIVE load rule

What load case do you usually add these lateral 10% to?

 

[human909]

What load case do you usually add these lateral 10% to?

Often none. It is its own load case and bracing sized as needed. Unless this is a particularly tall structure the difference this will make on the gravity load members will be neglible. Like I said I use it to size up my bracing.

Perhaps I've misinterpreted things on what you want here or perhaps you are overthinking things. Like I said I personally use 10% and move on with my work. That said if the steel platform is 20m tall and 5m wide well I'd be giving it more thought both because seismic might be higher than 10% or because 10% might be overly conservative in a low seismic region.

Why 10%? Because it works and also that is what my seismic codes says for structure <12m. Aka "Fi = 0.1Wi" Where Wi is the seismic weight.

How tall is you platform and what is its base length and width?

 

[bones206]

If you were to follow the code path that I laid out previously, these would be the applicable load combos per ASCE 7: