2500 psi or 3000psi Slab? (Houston, Katy: home builder, buying, contractor)

[rigas]

Originally Posted by structuralengineer1

punching shear for pedestal to footing is different.. engineer ignore rebar .. it only consider the concrete..

stirrups on beams the one resisting another kind of shear..

i dont think punching shear will be a factor in this case if it is under a beam.. this becomes more like beam with concentrated loads at the top..stirrups will resist that shear.. you may not even need stirrups to resist shear as the beam is deep enough compared to the vertical load..

if it is on flat slab that is 5 inch deep only with slab rebar spacing of 12" then yes probably you should check for punching shear..

i work for oil and gas like kbr, jacobs, fluor

1)if an engineer ignores the rebar and only considers the concrete then he deserves to get fired. only an idiot would ignore the one part of the design that combats tension (the cause of shear)

2)do a little more research and maybe look through your aci book (maybe get an updated one, look through aci-360) stirrups are used in the most critical locations to combat shear.

3)so your GUT FEELING says there is nothing wrong with the design i offered as an example because you assume the loads will be fine. and your an engineer? i thought you engineers only dealt with numbers and hard facts.
how do you know the beam is deep enough to resist shear, you dont know what the loads are.

4)you are the one who suggested a 5" thick slab with #3 spaced 12" ocew and now you are saying it would not stand up to the forces of shear?

5)maybe all your designs work for an oil well but it doesnt cut it on a simple metal building?

[Oildog]

OK to disagree but lets be civil.

[rigas]

to be honest i dont think either of us where not being civil. i think we where both more tongue in cheek then anything but what ever. its all good.

[Proud American777]

I really appreciate you taking the time to address my call for help, it really will be put to good use. My past experiences have shown me that hands on experience always beats book knowledge. But book knowledge is a good place to start when you have no hands on experience. Anyway, I am glad you both took time out of your day. Let me give a more complete description of what I have to work with, so that you can give your best educated guess as to what will WORK for my situation.

The clear beam spans are 89'-2", and wall beam height is 13'-1.75" with 4:12 roof pitch. There will be 3 spans spaced at 25ft. The bottom of the beams measure 8" x 10.5". There will be one 5x8 vertical Red Iron beam placed in the middle of slab, to support a 50'-8" dividing wall.

Please provide the following info.
BOM of rebar sizes and Qty needed, with spacing dimensions, and overlap.
BOM of Post Tension Cables, and spacing.
Dimensions for footer under Endwalls.
Dimensions for footer under Sidewalls.
What size footer should be put under 5x8 beam in the middle of slab.
Any recommended sources for PT Cables, rebar, and concrete.
What PSI concrete is recommended, 3000 or what?

I have attached a drawing of my proposed building and foundation, let me know what needs to be added or changed.

PS. I teach AutoCAD, so I produced these drawings from the actual dimensions of the Red Iron Beams.

Thanks a Million Rigas

[fordlover]

I've been quiet since I didn't want to get into a pissing match.

You mention you want to do as much of the work as you can on your own, so I'd suggest you do a regular slab on grade and avoid the P-T (you can't stress the cables on your own, and you won't know how many to get.) If you go the P-T way, Suncoast will engineer your slab for you, letting you know how many cables required, and stress it for you.

I'd suggest a 2'-0" minimum gradebeam bumpout at your column locations to distribute the point loads from your columns. For the rest of the perimeter beam I'd suggest a 1'-0" wide by 2'-0" minimum. You could also recess your columns with forms which would make holding your embeds easy.

You shouldn't need any grade beams thru the middle of the slab since there will be no load, but I would run a few if it was my slab for piece of mind, since the additional expense would be worth it, and it will tie the perimeter beam together. So if it was mine I'd run a single beam down the middle of the ~50 foot span, and 2 beams evenly spaced to break up the ~80 foot span.

[RCH99]

Originally Posted by fordlover

I've been quiet since I didn't want to get into a pissing match.

You mention you want to do as much of the work as you can on your own, so I'd suggest you do a regular slab on grade and avoid the P-T (you can't stress the cables on your own, and you won't know how many to get.) If you go the P-T way, Suncoast will engineer your slab for you, letting you know how many cables required, and stress it for you.

I'd suggest a 2'-0" minimum gradebeam bumpout at your column locations to distribute the point loads from your columns. For the rest of the perimeter beam I'd suggest a 1'-0" wide by 2'-0" minimum. You could also recess your columns with forms which would make holding your embeds easy.

You shouldn't need any grade beams thru the middle of the slab since there will be no load, but I would run a few if it was my slab for piece of mind, since the additional expense would be worth it, and it will tie the perimeter beam together. So if it was mine I'd run a single beam down the middle of the ~50 foot span, and 2 beams evenly spaced to break up the ~80 foot span.

^ this is very good advice, particularly the info about going post tension.

The only thing I will add, is to make sure that all of that fill material that you're soliciting in your other post, gets properly compacted to at least 95%. Coastal Testing and/or others can do this for you at minimal cost. If you aren't doing a soil test or engineering your slab, and you are going to just build a pad and guess at the rest, it will all be a waste if you place it on loose or sloppily compacted fill.

[rigas]

in a case like this, i would not do PT, i think it is going to be more pain then its worth. suncoast will engineer for you but russ price (their head engineer) is not exactly cheap lol.

i would go with a 16x26 beam. 3 #5 top and bottom with 10x20 stirrups spaced 24" addint a few extra under all the columns/corners. a 4" pad with #3 at 18" each way. everywhere the #3 comes to the ext beam turn it down at least 12" to tie it in. with #5 i suggest an over lap of 24" minimum, and 18" over laps on the #3.
for the center column i would put a 3 foot square to the norm beam depth

i agree a soils test is a must, no mater WHO designs your slab it will only be as strong as the ground it is sitting on.
if your soil is not good then build a pad getting it compacted as stated to 95% OR put piers in but IMO the pad is a more realistic and better idea. also more cost effective.

as for the concrete, i would not go less then 3000 psi. 3000 with 1 1/2" aggregate is the norm, this includes about 25% fly ash but depending on when you are going to do this that mix may need to change due to weather conditions.

[fordlover]

Lone star testing will charge ~1,500 for two borings to 20'
Geotech engineering will do the same for a similar price.

R-mac Engineering will engineer your slab using a soils report for ~800-1200 bucks. But without column loads from your building, It's gonna be tough to design a slab for it.

Spending an additional 3 grand may sound outragous, but considering your concrete material and steel costs, I think it'd be well spent insurance so you don't have your slab break up under the movement of the soil or the weight of your building. And you can probably talk some people out of a discount considering most in the construction industry are hungry currently, don't be afraid to ask for a better price.

[rigas]

Originally Posted by fordlover

Lone star testing will charge ~1,500 for two borings to 20'
Geotech engineering will do the same for a similar price.

R-mac Engineering will engineer your slab using a soils report for ~800-1200 bucks. But without column loads from your building, It's gonna be tough to design a slab for it.

Spending an additional 3 grand may sound outragous, but considering your concrete material and steel costs, I think it'd be well spent insurance so you don't have your slab break up under the movement of the soil or the weight of your building. And you can probably talk some people out of a discount considering most in the construction industry are hungry currently, don't be afraid to ask for a better price.

man lonestar has gone way up, when i had them do my property in magnolia about 7-8 yrs ago they came out and did 2 holes 18' for $600.

you are correct most everyone will price things well to get the work except readymix. they still think there is a shortage of cement some days, then fuel prices are $4/gallon the next, and then they thing aggregate is to expensive because they have to truck it in, all of which are fabrications to keep from having to lower their prices. companies like cemex are still jacking rates on cement which is keeping readymix to high but they are doing it so they can try to stay afloat. ever since they bought rinker they have been dieing and not even slowly. they have about 16 b in debt right now and the notes where all due last dec. i would look for 2 things to happen, cemex to work the notes out and the price of cement to come down in the next 6 months to a year OR they cant get it all figured out and cement to turn around and cause readymix to increase about $10 a yard by the summer. i am hoping for the 1st one.

[Proud American777]

Thanks to everyone for the good advice.

I have a couple questions regarding your info.
Rigas, you said (put piers in but IMO the pad is a more realistic and better idea. also more cost effective) What did you mean by IMO, can you explain that please.

Also you mentioned making an improved pad, I think you are saying to bring in improved base material over poor quality soil to support the foundation. My question about that is how deep would this compacted material need to be, and what would be the most cost effective material to use.

Where is a good place to buy cheap rebar, and especially the stirrups, would the stirrups be #3, or #5.
What is the going delivered price per yrd for 3000 psi concrete.

Thanks for the help