Supporting Elevated House 1

[BR_PE]

Has anyone elevated a house on Masonry blocks greater than 6 feet on piers/pedestal placed on a grid with steel beams to support the load bearing walls? If so what code did you follow?

A friend has ask me to be the EOR for raising his house, after reading thru TMS 402, i think the piers should be design as a column with 4 vertical bars and ties at 10" spacing (#5 vertical bars). My friend's contractor is saying that I have over engineered the supports since they have done this 400 times in wind area with wind speeds greater than 140 mph and when the house height is less than 10 feet above grade they have only used 12"x16" blocks with two vertical bars and ties at 36".

When I review the IRC (which I have never used), I get more question than answers. In section 606.6 it states the grouted piers can be 10 times CMU thickness, and in section 404.1.9.2 it states the max height for an exterior pier supporting a floor girder shall be 4 feet.

 

[phamENG]

Yes. I've done quite a few of these. You'll need the IBC - IRC doesn't have anything to really cover this condition. You'll also need ASCE 24. What are your flood conditions? Are you A, Coastal A, V? If you have wave action, you need an open foundation and will require the piers. You can try to look it at as unreinforced CMU and it may work, though that's rare. I had to fight a similar battle. My old firm turned out some pretty scary designs for home elevations before I took over, and had to fight the "I started building houses with my daddy when 2x4's were 2" thick" mentality to bring the designs into conformance with the code. I will say, adjust your tie spacing to match coursing.

Your friend asked you to be the EOR, ostensibly because he trusts you to keep his and his family's safety paramount. If the contractor wants to push the issue, have him produce calculations proving that what they've always done meets code. You'll need to consider wind action, wave action, debris impacts, etc. If the contractor can't prove it can stand up to all that, then he needs to shut up and build it the way you specified it.

 

[XR250]

Curious - How and why are you putting ties in masonry piers? Never seen or done that before.

 

[BR_PE]

Since the vertical members are resisting lateral and axial loads, I'm treating them as columns. Columns require minimum ties at 16 times longitudinal bar, 48 times lateral tie bar, or least cross-sectional dimension of the member per TMS 402-13 section 5.3.1.4.b.

 

[phamENG]

Where I am, flood elevations get to be 4 or more feet above grade - higher if you have to take waves into consideration. Jurisdiction mandated design flood elevation is 3ft above BFE in most cities around here, so the masonry "pier" is 9+ feet tall if you look at it from top of footing to beam bearing. At that point, it stops being a pier and becomes a column. TMS 402 has design guidance for masonry columns, and they include ties similar to reinforced concrete column design. If your dimensions are large enough or you're using brick, you can put a nice cage in the center and fill with grout. In a 16x16 CMU column with 8x16 blocks, you often have to go to over-sized bed joints and put your ties at 8" on center.

 

[phamENG]

BR_PE - another thought. If you have predictable flow characteristics, you may be able to get away with shear walls parallel to the flow direction. That will help reduce the lateral loading in on direction, at least. I've never been able to use them, but I know it can be done - I believe FEMA has some literature on it. Another another thought - search FEMA's website for documents. They have several case studies on this topic that are quite informative. Sorry I forgot about that on the first go 'round.

 

[JAE]

My friend's contractor is saying that I have over engineered the....fill in the blank

I'd agree with phamENG here in that the IRC is gone - you are engineering a house beyond most all of the prescriptive provisions of the IRC.
That means, in most jurisdictions with competent building authorities, that you are under the IBC and thus need to have an licensed engineer do the design.

It follows then that the contractor is basing his perspective on:
1. His past experience with houses that weren't designed appropriately
2. His view that the limited (very limited) IRC provisions can still apply
3. His probable lack of experience in "engineered" designs.

When IRC houses get kicked up to IBC engineered design, there is typically a big jump and contractors squeal.
I think this is because IRC houses are under designed.

 

[XR250]

Since the vertical members are resisting lateral and axial loads, I'm treating them as columns. Columns require minimum ties at 16 times longitudinal bar, 48 times lateral tie bar, or least cross-sectional dimension of the member per TMS 402-13 section 5.3.1.4.b.

I thought this only applied if you were counting on the rebar for compressive reinforcement.

 

[phamENG]

"Foundation Piers" as defined by TMS 402 are empirically designed. These house elevations don't typically fall within the empirical limitations. For instance, A.6.4 Foundation Piers from TMS 402-13/ACI530-13 states "Height shall be equal to or less than 4 times its thickness." So for a 16x16 "pier" you can't go more than 5'4" in height. That means you have to look at the engineered solutions, and if reinforced 5.3 - Columns will govern. It provides for minimum vertical reinforcing and ties. There were a few cases on small houses that I was able to justify using 8.2.4, Unreinforced Masonry, Axial Compression and Flexure. In those cases, I was able to show that the masonry itself was capable of resisting the loads without reinforcing, and we were able to back off. Medium to large houses, or especially tall lifts, though, and you'll have a harder time justifying the unreinforced option.

 

[XR250]

Upon reading NCMA TEK 17-3, it seems like the intent is for ties to be used where you are using the reinforcement to enhance the axial capacity of the pier or if you are in a high seismic area.

 

[struct_eeyore]

I've done a number of houses like this in South Florida. I do believe that the TMS tie requirements apply for all CMU designed as columns (there is a provision for lightly loaded columns, but I believe it only applies to single cell/bar types). I agree it's difficult to justify the ties if you are not in a wind/seismic area (other than citing code), but I have not found anything that relieves their use otherwise.

 

[r13]

My understanding,

1) High compressive stress above design level will lead to shear failure, or the column to split, if without ties. The event may occur during an natural disaster, or some type of over load conditions.

2) Ties are utilized to maintain vertical bars stay in position during construction. A good support for spacers.

 

[XR250]

Generally, my projects involve using flagpoled grouted and 4-bar reinforced 16x16 CMU or 20x20 brick columns under small residential additions or porches to provide gravity and wind load resistance. The axial loads are usually less than 10k.
I have never specified ties and would get laughed at and lose business to the guy down the street if I did .
Can't imagine they are needed for projects of this scale.

 

[dauwerda]

I don't do any masonry design in practice, I have taken a course on it for the SE exam. In that course it was stated that lateral ties are not required as long as the longitudinal reinforcement is ignored in checking allowable stresses (much like XR250 stated). However, reading TMS 402-13 section 5.3 it seems to me the only way to avoid ties is to classify the column as a "lightly loaded column" which requires it support not more than 2000 lbs and may not be taller than 12 ft. I don't see any allowance in section 5.3.1.4 that lateral ties are not required if longitudinal reinforcement is neglected in design. Perhaps this was stated in earlier versions of the code?

 

[phamENG]

Looking back through the code and working out the various references, I'm wondering if maybe "unreinforced columns" aren't really a thing as far as the code is concerned. When you look at the definition of a column, this situation certainly fits. When you look at chapter 5, as dauwerda mentioned, it doesn't leave any wiggle room and refers you to the reinforced masonry sections of the ASD and Strength chapters for their design. That being said, I wonder if there is some room for judgement here. In a reasonably large, commercial building I certainly get it. But for a small to medium sized house - doesn't make much sense. If you design it as unreinforced, limiting your flexural tensile stresses to well within the allowable region, no net uplift, and shear within the unreinforced allowable limits, I have some difficulty seeing the need for quasi-prescriptive ties outside of a seismic region. When wind and flood are the controlling design considerations, it seems like overkill.

Anyone have any case studies or deeper knowledge of the masonry code to shed some more light on it?

 

[r13]

Positive tie downs are absolutely necessary for houses in areas of high wind, and/or prone to flood, in which the lightly built suffers the most.

 

[phamENG]

retired - don't dispute the need for tie downs. I'm talking about masonry ties in columns designed as unreinforced.

 

[r13]

I am just a little puzzled that, if a column satisfies stress limit without reinforcing, then why provide rebars? It only makes sense if the design was done using method similar to reinforced concrete design (that tension steel is required), then usually minimum shear reinforcement is required. I must have missed something here.

Also, it wasn't clear what you intended to say,

When wind and flood are the controlling design considerations, it seems like overkill.

Sorry, If I misunderstood anything.

 

[phamENG]

No worries. I think we agree. It seems that, as dauwarda and I read the code, you are required to design a column as reinforced. Therefore, vertical rebar and ties are required no matter what. This makes sense in a seismic area, but I don't think it makes much sense for wind and flood if the stress levels are low enough.

 

[r13]

I think a possible argument for concrete cored masonry is for relatively low stresses, the bricks will provide adequate level of confining effect similar to shear ties. However, I don't think code any covers this, one have to justify himself.