Reinforced Concrete Mat Foundation

[bendit]

100 year old barn resides in Coastal High Hazard V Zone surrounded by old homes in coastal Connecticut town. Site is not on beach having direct access to open ocean wave action (scouring). Structure is timber framed and measures 24' x 34' with upper hayloft floor. Desire to rehab hayloft to accomodate an apartment. Ground floor to remain as garage space (FEMA).

To meet 85 mph design wind forces, propose to insert welded steel moment frame between ground floor elevation and hayloft floor incorporating FEMA "breakaway" closure below the 5' "design flood elevation". Structure supported on what seems to be dry laid stone foundation incorporating a 2" wood plank floor over a 12/18" crawlapace. FEMA doesn't like crawlspaces that can fill with water.

Propose to jack building, remove wood plank floor, fill crawlspace with gravel (etc.), replace existing stone foundation with reinforced concrete (floating,raft) mat heavy enough to resist all gravity,lateral, uplift and overturn loads that may be imposed by the 85 mph wind. Because barn is sited witn very little (12" on one side)setback from property lines it is necessary to perform all concrete work from inside the foorprint!

The point of all this - need design data covering concrete mat foundations. Much on spread column footings, reinforced concrete floors in multifloor buildings, residential slabs on ground, etc. Any suggestions? Alternate footing ideas welcome.

Comments: This barn is timber framed vertically while the hayloft floor and roof are framed with undressed joists and rafters. There are no diagonal knee braces anywhere between the vertical and horizontal timber framing components. Horizontal strucural members are joined to vertical members with pegged mortise/tenon joints. There is no wall sheathing; Weather siding is 1 x 8 shiplap board nailed directly to the structural frame. The building shows no signs of racking or other distress other than rotting of the foundation plate. More astonishing - the structure withstood the infamous hurricane of 1938 and many nor'easters with 70/80mph winds over its long history. Why didn't it just fall over ?!?!

Thanks, Bendit

 

[Focht3]

Bendit -

Considering the size of the proposed structure and the (relatively) light loads that it will carry, you can really treat the mat as a large spread footing. Aside from differential settlement issues, it is unlikely that a "reasonably" designed "mat" will experience any significant problems (unless the soil underneath is washed away or settles unevenly.)

But why a mat? (I love mat foundations - this isn't a bias against mats.) It seems to me that the use of a series of spread footings would be much more practical - you could shore up an area of the barn, dig and cast the new footing, then install the new column. With a mat, you'd almost have to "wish" it into place. How would you sequence it without leaving the old footings and columns in place until after the mat had been poured and cured? Yuck!

Drilled piers or drilled and underreamed footings ("belled piers&quot will also work for this case. You would need a short-masted truck-mounted rig, though. And the right combination of local expertise, equipment and favorable subsurface conditions.

What has your geotechnical consultant told you?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[CSEllc]

Mat foundations sounds like overkill to me especially since it is a barn with a hayloft being converted to residential loads.

Have borings been done on the site? I agree with Focht3, piers and footings might be a better approach, if the soil will allow them.

Also, if you are in a FEMA "V" zone, the area below the structure is to be open and the lowest structural member, typically dropped girder, will need to be above the flood elevation. Some localities require "freeboard", which is added to the height requirement. Example: FEMA V Zone elevation 20. 1 foot freeboard. Therefore bottom of structure must be at elevation 21 (minimum).

I don't know the extent of renovation work being done, but most communities follw FEMA guidelines about renovation work and complying with current FEMA standards. Typically, if the renovation costs exceed 50% the value of the structure, the locality will require that the structure be brought into FEMA compliance.

 

[jheidt2543]

A couple of comments:

1. I agree with Focht3 and CSEllc that shoring and jacking the barn up to install a mat foundation is overkill.

2. One other alternative would be to install the perimeter foundations by underpinning with concrete strip footings in 4'-6' sections. It takes a little longer, but the amount of heavy shoring is reduced significantly.

 

[Horseman]

What if it is within the Wetland Buffer Zone ?

 

[Focht3]

Then a mat would be even more difficult to justify, in my experience - too much damage to adjacent areas.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[bendit]

Thanks for comments thus far. To clear up some issues:

Wetlands are not an issue. The subject barn resides in a high density residential area consisting of upscale old (1800's)houses. Very difficult to work outside the existing barn footprint !!

We are familar with FEMA regs and DFE. In this case, the DFE at the site is 5'.

Replacing the existing old stone foundation with a conventional footing and wall by shoring the structure in short sections of 4/5' was considered too labor intensive. Would require excavation to 42".

Barn already rests over an 18" crawlspace - the excavation has already been accomplished for a mat!! Only cleanup required around perimter required. Lay perimeter forms, provide for frost protection (ocean enviroment - no deep frost), install rebar, place concrete all at one time. The mat seemsed to us to be the most cost effective.

Existing barn hasn't settled in 100 years. Site is old slag dumping ground for local 1850's foundry. Subsurface well compacted. No environmental problems !

Still requesting source for mat foundation design info.

Bendit

 

[Hookem]

Dead Load: Calculate weight of barn structure to weight of concrete mat and divide by perimeter.

Live Load: Estimate weight of contents and divide by area; consider single points loads such as heavy equipment (tractors, etc.).

Wind Load: Calculate overturning moment. Use 25 - 30 psf over the largest area projected into the wind to determine force. Multiply by moment arm (center of area down to level of bottom of mat). Divide product by section modulus (short dimension).

Multiply dead load by factor of 1.4. Multiply wind load by factor of 1.7. Combine loads. Compare to allowable soil pressure.

If loads are less than allowables-all is okay. If loads are close to or exceed allowables, you will need to consult geotech. Options are to overexcavate and recompact with select fill; add piles; etc.

For references, consult any number of good college grade soil books. This subject is not for beginners.

 

[AlohaBob]

This little structure would probably be fun to get to work like you wish. Rather crafty. Why not hire a structural engineer and work with him to explore these suggested alternatives?