Formula for concrete from bags is:
1 part cement
1 part gravel
2 parts sand
Use a bucket to measure.
This formula makes a really strong mix. (C35P)
The amount of material per step in metric units is:
25 lbs cement
50 lbs sand
100 lbs gravel
mixed with 2 gal water
A single bag of cement (92 lbs) should produce enough mix for 4 steps.
For each set of four steps, concrete supplies needed are:
1 bag of cement,
200 lbs sand
400 lbs gravel.
Tuesday, October 27, 2009
Calculating the required amount of concrete.
Here's a way of estimating the amount of required concrete for stairs.
The formula for calculating the needed concrete is:
Yards = 324 square feet /thickness in inches.
I yard will cover 324 square feet to a depth of 1 inch.
For stairs, the depth of concrete is tread plus 6 inches. Depth ~ 12.5".
324/12.5 = 26, so one yard of concrete builds 26 sq ft of tread.
If average tread length is 3', then you get 6.6 to 7 treads per yard.
The formula for calculating the needed concrete is:
Yards = 324 square feet /thickness in inches.
I yard will cover 324 square feet to a depth of 1 inch.
For stairs, the depth of concrete is tread plus 6 inches. Depth ~ 12.5".
324/12.5 = 26, so one yard of concrete builds 26 sq ft of tread.
If average tread length is 3', then you get 6.6 to 7 treads per yard.
Concrete -- Delivered
Short Load Concrete is available from multiple vendors, including:
Salmon Bay Sand and Gravel http://www.sbsg.com/
Has Used concrete which will be about two hours
New will be about $350 and used (not fresh) will be about $200. 8" slump and get a finish. May take a couple of days.
Burien Sand and Gravel: http://www.bsgsales.com/pg/conc.html
Quick and Easy in Woodinville: http://www.qeconcrete.com/
Salmon Bay Sand and Gravel http://www.sbsg.com/
Has Used concrete which will be about two hours
New will be about $350 and used (not fresh) will be about $200. 8" slump and get a finish. May take a couple of days.
Burien Sand and Gravel: http://www.bsgsales.com/pg/conc.html
Quick and Easy in Woodinville: http://www.qeconcrete.com/
Tread to riser ratios for outdoor steps.
Typical tread and riser dimensions for outdoor steps
Two considerations: Basic comfort and disability recommendations.
Based solely on comfort and ease of general use, optimum step size appears to be 7.5" risers with 9.5 to 10" treads.
ADA site: http://access-board.gov/ada-aba/comparison/chapter5.htm
ADA requirements are for risers not to exceed 7" and for treats to be at least 11". This makes sense. Here's the language from the ADA building requirements.
504.2 Treads and Risers. All steps on a flight of stairs shall have uniform riser heights and uniform tread depths. Risers shall be 4 inches (100 mm) high minimum and 7 inches (180 mm) high maximum. Treads shall be 11 inches (280 mm) deep minimum.
The 7" riser is a good mark. It could be a little thinner. And the 13" tread recommendation for flagstones is a valid mark.
The 7"-11" ratio gives the right "rule of 77" result. But this may mean more steps. 7" by 13" may do better. See the note that says on twisting stairs the tread depth is minimum 11"@ one foot out from the tight end.
Also see the note that says the riser high can vary by no more than 3/8".
A 3 foot wide step with base is roughly a 1.7 cu ft volume of concrete.
http://teachers.henrico.k12.va.us/math/HCPSAlgebra1/Documents/6-1/SlopeStairsLab.pdf
Notes on code for stairs. Interesting ratio formulas.
Shows twi formulas for stairs.
r+t ~ 17.5
r*t ~ 77
That says for a nominal stairs, t = 17- r
-r^2 + 17r - 75 = 0
Riser = 8.5" max = 6.5" min. Nominal riser = 7.5" (Except ADA says 7" max)
At 7.5", optimum tread depth is about 10 inches.
At 6.5" optimum tread depth is 11.5"
This also fits another formula from the source below:
2R+t = 24.
6.5" x 2 + 11.5 = 24.5
http://books.google.com/books?id=SplcE_LjuFMC&pg=PA57&lpg=PA57&dq=steps+tread+riser+ratios+OR+slope+-patent&source=bl&ots=e2-GmxW2yA&sig=MGBhGQOh19ExDJSe1nmQvN4Nm8w&hl=en&ei=dzXnSsHTDcbU8AaZ1JyIBw&sa=X&oi=book_result&ct=result&resnum=10&ved=0CCcQ6AEwCQ#v=onepage&q=steps%20tread%20riser%20ratios%20OR%20slope%20-patent&f=false
Two considerations: Basic comfort and disability recommendations.
Based solely on comfort and ease of general use, optimum step size appears to be 7.5" risers with 9.5 to 10" treads.
ADA site: http://access-board.gov/ada-aba/comparison/chapter5.htm
ADA requirements are for risers not to exceed 7" and for treats to be at least 11". This makes sense. Here's the language from the ADA building requirements.
504.2 Treads and Risers. All steps on a flight of stairs shall have uniform riser heights and uniform tread depths. Risers shall be 4 inches (100 mm) high minimum and 7 inches (180 mm) high maximum. Treads shall be 11 inches (280 mm) deep minimum.
The 7" riser is a good mark. It could be a little thinner. And the 13" tread recommendation for flagstones is a valid mark.
The 7"-11" ratio gives the right "rule of 77" result. But this may mean more steps. 7" by 13" may do better. See the note that says on twisting stairs the tread depth is minimum 11"@ one foot out from the tight end.
Also see the note that says the riser high can vary by no more than 3/8".
A 3 foot wide step with base is roughly a 1.7 cu ft volume of concrete.
http://teachers.henrico.k12.va.us/math/HCPSAlgebra1/Documents/6-1/SlopeStairsLab.pdf
Notes on code for stairs. Interesting ratio formulas.
Shows twi formulas for stairs.
r+t ~ 17.5
r*t ~ 77
That says for a nominal stairs, t = 17- r
-r^2 + 17r - 75 = 0
Riser = 8.5" max = 6.5" min. Nominal riser = 7.5" (Except ADA says 7" max)
At 7.5", optimum tread depth is about 10 inches.
At 6.5" optimum tread depth is 11.5"
This also fits another formula from the source below:
2R+t = 24.
6.5" x 2 + 11.5 = 24.5
http://books.google.com/books?id=SplcE_LjuFMC&pg=PA57&lpg=PA57&dq=steps+tread+riser+ratios+OR+slope+-patent&source=bl&ots=e2-GmxW2yA&sig=MGBhGQOh19ExDJSe1nmQvN4Nm8w&hl=en&ei=dzXnSsHTDcbU8AaZ1JyIBw&sa=X&oi=book_result&ct=result&resnum=10&ved=0CCcQ6AEwCQ#v=onepage&q=steps%20tread%20riser%20ratios%20OR%20slope%20-patent&f=false
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