Scaffold wood mudsill design

[MagedPEng]

guys, i have never done any wood design before so i need help in the designing of scaffold wood mudsill under a load of 30kN or 6750Ibs with a soil bearing capacity of 200kPa or 4180psf. I know that the type of lumber and its density is involved but still have no idea how.
Thank you in advance.

 

[gooddammer]

This is my business, and has for over 40 years. We figure soil bearing @ 1000 psf if your foot leaves an imprint in it or 4000 if you can't stomp a 'vee' in it with your heel. Dumb? Not really, if it works. Point is, shoreframe legs @ 5K or 10K don't need that many square feet to spread the joy around without failure. You have modest load = 7K plus very good soil = 4 ksf = 1.5 sq ft req'd. A pair of plain 2 x 12's = 11.5" x 2 ft +/- will do it in any wood but balsa. On good soil, force of concern is compression perpendicular to grain, not really bending. Most books limit 'wood' to low bearing because of obvious crushing over time. For scaffold or shoring loads, the time involved is hours to days. Insufficient to do much to the wood, not even leave a dent from scaffold base. Wood stress allowables published have at least 4/1 FS on ult, and many have 8/1. Dumb, but true, because of age-related deflections and crushing.

 

[gooddammer]

Okay, I could have gotten esoteric and blathered about parabolic load distribution (with a flattened "yield" zone directly under shore post leg force, etc) plus pressure zones extending from the parabolic along/aside those load-stress distribution diagrams, but the question remains: why should a Real Engineer bother with wood sill design under a shoreleg? or a scaffold leg? This function (legally) belongs to the one with the construction contract, and his domain includes those who do it all the time. Let them have it. Besides, a chore performed SAFELY over 10,000 times a week in the USA alone isn't a problem. I have a job in Houston now with about 3,000 legs under it, and another 300 outside the shored area just for scaffolding (another legal subject, by the way: the two are NOT the same!). All of which is moved weekly or monthly. By ordinary workers, guided by supers in The Trade. One must worry more about rain soaking the soil while loads are being supported than niceties of wood shear or other "design" matters.

 

[BAretired]

To begin with, don't use 4180 psf for permissible soil bearing at surface. Use, maybe 1000 psf and select a mudsill which can carry the load easily. We are not dealing with a difficult concept here. You want to have a safe scaffold, so overdesign the mudsill! What is the problem?

BA

 

[MagedPEng]

Thank you guys for your replies but why using a pair of wood planks?. I understand how to distribute the load over the soil but still my question concerns the type of wood and its thickness.