Modular Home Foundation Wall Anchorage

[Craig_H]

Hi folks,

I've been asked to design the foundation for a modular home. Let me clarify that this is not a "trailer" that you might be picturing Ricky, Julian and Bubbles growing weed in, but a 2-storey home built in a factory and shipped to site, structurally complete but with finishing materials installed onsite. This offsite manufacturing poses a few unique challenges, which I will get into here.

As I am designing the concrete basement foundation walls to be pinned at the top (wood floor diaphragm) and bottom (concrete footing), I need to provide a connection between the concrete and the wood floor. Nothing revolutionary here, except that the home will arrive on site already assembled, with the subfloor in place. That makes internal fastening of the rim joist to the sill plate challenging to say the least (not much room to get in there from the basement, as the floor joists are 2x10). The home manufacturer provided me the below detail, which I am not stoked on. They seem to be relying on the embedded strength of a flat metal strap.

I do like the adjustibility of the flexible strap, as there is bound to be some misalignment of the foundation relative to the building.

I am struggling to find a connector rated to take the lateral earth loads from the wall and transfer them to the rim joist, while ideally having some degree of adjustibility (like a strap). Simpson's MASAP looked promising until I downloaded the Canadian catalog and found that they don't rate it in the "F2" direction for rim joist installs (but they do in the US, weird). Has anyone run into this situation and developed a good way of anchoring the wall to the pre-fab floor diaphragm?

 

[KootK]

Access issues be damned, it's Simpson a35 -- both directions -- from the inside for me.

 

[BridgeSmith]

The modular home installations I've seen were backfilled before the house was set.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

As KootK said, use one of the Simpson framing angles on the inside to take the out of plane reaction. To transfer in plane forces, the FRFP or UFRP will be your friend.

 

[jayrod12]

As Koot indicated access issues are a problem without backfilling ahead of time. So that means either the top of wall gets temporarily braced, or design it for both conditions, cantilevered and pinned and provide the appropriate reinforcing.

Connection I also agree with Koot and Pham. Simpson angles. Or hurricane clips H4 or similar from each floor joist to the sill. Depending on the loads.

 

[XR250]

How are you going to deal with the inward foundation loads where the joists are parallel to the foundation wall? I have tried unsuccessfully to get the MH manufacturer to install blocking between the joists or switch the joist direction 4 or 5 ft. from the foundation wall. Might have to do cantilevered in those areas as Jayrod suggested.
The last modular I looked at had the bottom covered in 1/2" blackboard for transportation purposes. It was never removed over any of the piers or perimeter.
It seems to be the nature of the modular/manufactured industry to half- ass the installations (and in many cases the product)

 

[Craig_H]

Thanks all for the input! Rod, your point about backfill was very helpful - now that you got me thinking, I have a hankering that they will also want a crane set up as close as possible to the foundation. Thanks for that. KootK, Pham, Jayrod, I totally get the argument for angles, and it's probably what I would be doing if there were no subfloor, and access from above were easy. As XR250 now mentions, I'll have to confirm whether there's a "ceiling" already installed to keep road grime off of the undersides of the joists, and whether that needs to stay in place.

For the walls with parallel joists, the manufacturer has promised to send me their floor layout. I'm hoping that there's appropriate blocking in those locations, and if not, I intended to demand that it be there.

Playing with hypotheticals here, let's assume that I am able to alleviate all the backfill and joist direction concerns. What if the manufacturer's "20 Gauge PWF Strap" was replaced with a STHD hold-down strap? Yes, those are rated in uplift tension, I wouldn't get full nailing into the rim joist, and the catalog values would need to be adjusted for load duration. That said, I think that by adjusting the catalog values, I could create a reasonable connection this way. Is that an offside play?

 

[phamENG]

I don't think straps will solve your out of plane connection problem. Though thinking outside the box about their detail, I wonder if they are 1) assuming friction will be sufficient since you have a tension tie holding it down in firm contact or 2) since the anchor is in the sill plate (but not above it) it will transfer shear. One is more or less plausible, and two only makes sense (though not much) if you're using it as a final security after the house starts to slide. I don't like either of these.

I wonder why they don't want it to go above the sill plate? If you can get them to allow it, what if they do oversized holes in the sill plate to facilitate setting the section down. Then come back and fill the holes with a structural epoxy or similar and use a large plate washer?

 

[Craig_H]

Thanks, Pham. The modular portion is everything above the sill plate, they want the anchor bolts countersunk so that there's a planar surface to set their modules down onto (no chance of an AB clashing with a joist). With their detail, I actually don't see the point of the anchor bolts, as there's nothing connecting the diaphragm & rim joist to the sill plate.

 

[r13]

The project site is a low risk area for wind and seismic, also will not subject to flood, I guess.

 

[phamENG]

Hmm...let's go back to the Simpson framing angle. When the contractor says there's not enough room, tell him to buy one of these:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1581092074/tips/Mini-Pneumatic-hammer-hands-hand-held-air-desktop-small-hand-wood-nail_n50xtw.webp[/url]

 

[BridgeSmith]

If they're setting it with a crane, they may not need to backfill beforehand, but for the installations I saw, they 'slid' the house directly off the truck onto the foundation and didn't use a crane. For that approach, they needed to backfill up to within about 3' of the top of the wall so that they could back the trailer up to the foundation.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

Sliding would also explain the prohibition on bolts extending above a sill plate. Sounds like the sill plate is there to give them a surface to slide across - a concrete or CMU wall would likely snag and damage the floor framing.

 

[Dvdt18]

Craig H,

The modular home company we continuously do work for every few years comes up with new ideas on how to attach their modular homes to the foundation, of course trying to be as cost effective as possible...the most recent idea they've come up with are doubling up the sill plate and countersinking the anchor bolt within the upper sill plate thus not compromising the lower sill plate.

A few other ideas they've tossed around in the past are the use of a Simpson Framing Angles, reducing the spacing of anchor such that if one is cut off at time of construction it is considered negligible, installing concrete wedge anchors if one of the anchor bolts has to be removed (access is not a problem for their homes), and even leaving the anchor bolt in place and changing the floor framing on site to account for the anchor bolt locations. It's even been proposed by others to install light weight fill around the entire home/garages, which was shut down right away by the manufacture...

As modular home companies in our area tend to be quite cost sensitive, they hate the idea of anything out of the ordinary...They've proposed the use of Simpson Straps similar to your proposed application but I ran into a similar issue with the lateral loads. Other ideas were tossed around with manufacturing steel T plates, but that turned out to be not cost effective enough..

Hope this helps, thanks.

 

[Craig_H]

Thanks all for the input here. I'm still waiting on some documentation from the home fabricator, so I will see how it all comes together, how the building will be set (crane vs jack-and-slide), etc.

I like thinking that having some sympathy for the contractor makes me a good engineer, but clearly, this is a situation where that can only go so far. I'm in agreement that there aren't really any good, solid solutions apart from angle brackets. That said, if this modular fabrication business keeps getting more popular, perhaps there's a market here to be exploited for adjustable, bend-up connectors that can be fastened from the exterior.

 

[phamENG]

Here's another idea - trying to think a little outside of the box.

 

[Craig_H]

Thanks, Pham! That's a great out-of-the box idea. I feel like it would be a much easier ask for a detail like that, even if it involved more fasteners, as it's so much for ergonomic (and easily inspected!)

 

[phamENG]

You're most welcome. Glad I could help.