Twelve Bundles of Block and a Light-Duty Rating
A masonry contractor wants to use a 21 ft frame scaffold - three tiers of 5 ft x 7 ft frames, four frames per tier at 7 ft spacing, eight legs total - to stock block for a two-day wall. The top deck is 21 ft long and 5 ft wide, fully planked with nominal 2 in x 10 in scaffold plank on 7 ft spans. The manufacturer rates this frame-and-plank configuration as light duty, 25 psf uniformly distributed. The mason wants the forklift to land 12 bundles of block, 1,100 lb each, three at a time, and break them out during the shift. Each leg sits on a 6 in x 6 in base plate on a 2 in x 10 in mudsill, 24 in long, bearing on undisturbed stiff clay with an allowable bearing of 1,500 psf.
1. Check the proposed loading against the deck rating. Show the arithmetic and state where the deck fails first.
Read the worked answer
Deck area: 21 ft x 5 ft = 105 ft².
Allowable uniform load at light duty: 105 ft² x 25 psf = 2,625 lb.
Proposed at three bundles: 3 x 1,100 = 3,300 lb. That already exceeds the rating by 675 lb - before the plank, guardrails, and two masons (call it another 500 lb). The three bundles alone put the deck at about 126% of rating, and with the crew and plank it is closer to 145%.
Proposed over the shift: 12 x 1,100 = 13,200 lb on 105 ft² = 126 psf, five times the light-duty rating.
Concentrated load is the sharper problem. A bundle footprint of roughly 4 ft x 2.5 ft = 10 ft² delivers 110 psf locally - but more importantly it lands on two or three planks out of a deck of about 30. The uniform rating assumes load spread across the whole deck; it says nothing about a 1,100 lb point load dropped on a 7 ft plank span in the middle of a bay. That is where the deck fails first - a single bundle, not the twelfth. Loading the deck near midspan of a plank is the worst position available.
Verdict: the plan is not permissible at any bundle count the mason proposed. Even 25 psf (medium-duty at 50 psf, heavy at 75 psf) would not license a stack of bundles; the answer is a different method of supplying material, not a different duty rating.
2. The contractor says the ground is good stiff clay, so the foundation is fine and the deck must be fine too. Check the leg load and bearing pressure for the three-bundle case and explain what the numbers actually tell you.
Read the worked answer
Total load, three bundles:
- Bundles: 3,300 lb
- Plank, guardrails, hardware: ~600 lb
- Two masons plus tools: ~500 lb
- Total: ~4,400 lb
Average leg load: 4,400 / 8 = 550 lb per leg.
Bearing area of one mudsill: the 2 in x 10 in lies flat, so contact is 9.25 in x 24 in = 222 in² = 1.54 ft².
Average bearing pressure: 550 / 1.54 = ~357 psf against 1,500 psf allowable. Comfortable.
But the average is the wrong number. The three bundles land over one bay, so two to four legs carry most of that 3,300 lb. Take the worst case where three legs share the stack: (3,300 / 3) + (share of plank and crew, ~370) ≈ 1,470 lb on that leg, giving 1,470 / 1.54 ≈ 955 psf. If the bundles straddle in a way that puts 1,800 lb on one leg, pressure is about 1,170 psf against 1,500 psf allowable - roughly a 22% margin on an allowable value that itself already contains a factor of safety.
What the numbers tell you: the clay is not the governing limit, and the contractor's inference is backwards. The soil has margin; the deck does not. Three bundles put the deck 26% over its uniform rating and put a point load on two or three planks, while the foundation is still inside its allowable bearing. Fixing the foundation would change nothing - the load has to come off the deck or the deck has to change. Note also that the foundation margin evaporates as the stack grows, so a crew that "just adds one more bundle because the ground is holding" is eating the remaining capacity on both the deck and the soil at once.
3. Give the material-handling solution you would actually implement, and state the checks that must be made before any block goes above ground level.
Read the worked answer
Solution - take the stack off the work deck:
- Stage on the ground at the base of the wall and supply the masons in small lifts, keeping the deck at or under its 25 psf rating: roughly no more than one row of block distributed along the deck at a time, placed over the frames rather than at plank midspan.
- Or use a rated materials hoist / gin wheel bracket attached as the manufacturer directs, sized for the bundle weight with the correct rope and a positive brake, and with the landing area controlled so nobody stands under the load.
- Or build a separate loading tower rated for the stock, keeping the work scaffold for people and tools.
- Never land bundles with a forklift by driving the forks over or through the guardrail, and never leave the forks under a load while a worker reaches for it.
Checks before the first bundle goes up:
- Published rating for this exact frame, plank, and span configuration, in writing - not assumed from the frame size.
- Plank condition and span: sound, full-thickness, secured against movement, spans within the manufacturer's limit, no plank overhanging its support by more than the permitted amount.
- Foundation: mudsills sized for the actual worst-case leg load, bearing on compacted material, base plates seated, jacks within their adjustment range.
- Bracing and ties complete for the height and wind exposure before the deck takes stock.
- The load path from the deck down - plank to bearer to frame to base plate to ground - confirmed at each interface rather than assumed.
- A competent person signs off the loaded condition, and the load limit is posted at the access point so the next crew does not repeat the calculation by guess.