YANMSE (You Are Not My Structural Engineer)

January 22, 2010 3:09 PM

How Can I Calculate the Load Capacity of Some Existing Storage Lofts?

I need to post load capacities on a couple of storage lofts at one of the sites I am responsible for. I have found this table, and this table, which tell me how to build for a given load, but I not found any tables for determining the capacity of pre-existing structures, when the construction and the species/dimensions of lumber lumber are a given.

I am thinking that this info must exist out there somewhere. I hate the thought of hiring an engineer for this.

posted by Danf to Home & Garden (7 answers total)

You know the species of the wood? There are charts that show the live and dead load for those species based on the size and spacing. Rather than use (and search for) a calculator, search for a chart.

So if it's 40 pounds per square foot, multiply that by the square feet of the platform. Unless you need to be very precise, I would just advertise the live load that you calculate. Dead load is *just* for structure attached to your platform, and does not account for (nearly) any dynamic loads. Even if it is just the plopping down of a box, if you exceed the live load, you can have trouble.

More importantly, however, is the rest of the construction of the loft. That is probably going to be the determining factor. How is the loft supported? Posts and beams? Rails screwed into a wall? Chains hanging from the ceiling?

Whatever the load supporting capacity of those things are are almost surely going to be lower than any competently constructed deck.

Regardless, whichever number is lower is the actual load carrying capacity of your deck.
posted by gjc at 3:23 PM on January 22, 2010

Response by poster: I have been looking for a chart for a week or so and have come up empty. Hence, I turn to AskMe. You fine folks have helped me to do my job on a number of occasions.
posted by Danf at 3:26 PM on January 22, 2010

Best answer: The key term you're missing is "span" table. A quick google search comes up with this table calculator. You can't find them online because they are copyrighted, the tables fill a good size book, and everyone who needs them has access via either their code book or a span book.

Your library should have a span table book.

Here is a small PDF table from Nanaimo.

Here's another PDF for MSR lumber.
posted by Mitheral at 4:19 PM on January 22, 2010 [1 favorite]

Best answer: My local building department has span tables available for use by the public. They're even available online in pdf form. If you want to root around in there some more, the pdf I linked to is in the "Information Bulletins" section. You'll have to do some math to derive the allowable load for your structure from the span table.

What gjc says about the elements that support your deck is also correct.
posted by LionIndex at 4:40 PM on January 22, 2010 [1 favorite]

Does this seem like a terrible idea to anyone else? If I saw a posted load capacity, I would assume it was from an professional engineer and would trust it. And you certainly know who will get sued if someone loads it to the stated capacity and it fails. I think you are better off not posting it, or paying a professional.
posted by smackfu at 6:32 PM on January 22, 2010

IAAL, but IANYL. TINLA. smackfu's post is worth heeding. You can hire a professional engineer to look at the structure and provide an analysis showing how much it can hold. The engineer should have liability insurance. This insurance will be useful when you post the calculated load, and somebody puts their pallet of delicate Ming vases on the floor, and they all fall down.

Without that insurance, you'd better hope the owner of the site you're responsible for has deep pockets, and is kindly disposed toward you.
posted by spacewrench at 8:10 PM on January 22, 2010

Response by poster: This is an OSHA thing. What gives me some confidence is that these spaces will never be over loaded, but mostly have light boxes stored on them. But it's an OSHA reg to have some sort of number posted.

I also feel confidence with reading the tables, once I find them, and matching them up with what I find in the actual construction.
posted by Danf at 7:32 AM on January 23, 2010