Last post, we discussed how to determine headroom requirements for an indoor lift and used the example of a gantry being used to hoist a boiler. The headroom calculations for that style of lift are straightforward and really only require the knowledge of what is needed and getting the sum of the parts. Your typical lift in an outdoor setting will often employ the use of a crane and that’s where your headroom calculations can become more complex.
For a typical crane lift utilizing spreader beams, the headroom can once again make or break the lift before it even gets started. To find the headroom on these setups, we get to break out one of our favorite mathematical formulas: the Pythagorean Theorem. In a previous blog post, we outlined what Pythagoras’ Theorem is, but this will be a practical application to help set the concept.
For this example, we will figure headroom requirements on a multi-beam pick that was recently used by one of our customers. For this lift, the top beam needed to accommodate 90-tons across a 45’ span with four pick points below the hook. Where we will stray, however, is in the center-of-gravity (CG) where we will assume that the load is perfectly balanced to keep things simple.
Starting from the crane hook down, your first beam selection is the Modulift MOD110 at 45’ long that is rated for 105 tons when the top rigging sits at a 70-degree angle. To find the headroom taken up from that beam, we break out the Pythagorean Theorem:
a2 + b2 = c2 (a = height, b = base, and c = sling length)
For this example, our known values are our base, which is half of the overall beam length, or 22.5’, and the required sling length of 65’. Breaking down and modifying the formula to fit our known values,
a2 = c2 – b2 or a2 = 652 – 22.52
This means that
a2 = 4,225 – 506.25 or a2 = 3,718.75
To find the height, we take the square root of 3,718.75, which equals 60.98’ that we will round up to 61’ of headroom.
The next set of beams only needed to be MOD50 at 12’ long, rated for 50 tons each, with top rigging at a 45-degree angle (requires 8’ slings but we will use a more traditional 10’ sling length). Taking the same steps as above with half the beam length and our modified formula of
a2 = c2 – b2
Our values become
a2 = 102 – 62 or a2 = 100 – 36 which simplifies to a2 = 64
Again, taking the square root of 64 gives us our height or headroom value of 8’ of headroom required.
As equipped, this lift would require a minimum headroom of 69’, but the top and bottom shackles on each spreader is not factored into those figures, so your actual required headroom comes out a bit higher. Provided the calculated headroom is not excruciatingly close to the actual available headroom on site or the maximum boom length of the supplied crane, the lift can proceed as planned. If, however, either of those conditions are encountered, the top beam would need to be swapped out for a higher capacity option that would require less headroom at the expense of increased rigging weight.
As you can see, headroom requirements can add up rather quickly on any lift plan. For that reason, it is important to know how to calculate your headroom prior to proceeding on a lift without first ensuring the space is available. Not factoring, or even factoring incorrectly, any of the above values can stop a lift in its tracks.
For more information and assistance in selecting the best tool for any job, you can always reach out to your local representative at LGH, chat with our support staff at rentlgh.com or give us a call at (800) 878-7305 to speak with one of our rental desk representatives.