H1 Tie vs. A35 Clips 2

[medeek]

For a couple years now I've been in the practice of specifying A35 clips at the top of all my shearwalls either connecting the top plate to the rim board of the floor above or the roof truss above (usually a gable end truss). I've also always shown a detail with an A35 clip connected to the "vertical" bird blocking where the trusses bear on the exterior walls. See detail (4) at this link:

http://design.medeek.com/cad/autocad/INTERIOR_SHEARWALLS.pdf

I've had some builders complain that all this hardware is redundant. You will note that in the detail (4) I'm also calling out H1 ties at each rafter. I've given this some thought and went back to my Simpson Catalog to see if I could eliminate the A35 clips entirely and just use the F1 value of the H1 clip to take the shear load. If I do this then Simpson suggests a unity equation on page 198 of their Wood Construction Connectors Catalog (2015-2016). In other words you need to factor in the uplift and max. horz. reaction(s) of the truss along with the shear load to accurately size the connector.

u/U + f1/F1 + f2/F2 < 0

Their alternative method is to use 75% of the allowable published load to check against when utilizing the connector in simultaneous loading.

This seems like a lot of extra work in order to eliminate some clips, but in certain cases is will probably work since the H1 clips have a fairly good F1 allowable load. Don't try this with their SDWC truss screw though since its F1 allowable is surprisingly small.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Another construction issue that is driving this is most of the local contractors don't install their blocking vertical, it is usually perpendicular to the roof slope. This creates issues with using the A35 clips in the first place. How does one install an A35 clip when the angle between the top plate and bird block is greater than 90 degrees. I also might at that most of the bird blocks are 2x4s full of 2" holes so structurally I wonder if they are doing much regardless.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

I wouldn't be thrilled with using the H1 clip to transfer diaphragm shear. The Simpson catalog actually mentions it's not intended to replace diaphragm boundary members. The load path with the H1 would be a little convoluted: EN to blocks, blocks bearing in compression on truss, truss bending in weak axis across plate, load taken out by H1 clips. Seems keeping the load within the block is a more direct load path.

We commonly specify H2.5A clips @ 48" oc and A35 clips at each rafter bay. We have relatively low uplift forces though (110mph wind ASCE 7-10). Contractors in our area expect the A35 clips and frequently install them even when not specified.

With the slanted block we've done 2 things: either install a double block and have the outer one be decorative and not used for lateral transfer or use a LS clip instead of the A35. Those clips can be field bent.

I agree the bird holes can be problematic if they are too large or there are too many of them. We usually limit size and spacing to try and keep block in tact.

 

[jdgengineer]

While I don't do this, if your loads are small you may be able to get by with just toe nailing the blocking to the top plates in lieu of A35 clips.

 

[JedClampett]

Medeek, I do the exact same thing.
Go to your local Menards or Home Depot. An A35 (there's a big box of them) costs about 56 cents each. Yeah, there's labor involved and nails, but this has got to be the cheapest building material since dirt.

 

[medeek]

I guess I'm back to the A35 and LS50 clips. Talking to a framer today, he just takes his hammer and flattens out the A35 clips until they more or less fit.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Kind of makes you wonder what we're doing as engineers especially when it comes to residential structures. 15 years ago or so I don't think I ever saw a single A35 clip back when I was working on a construction / framing crew as a college student. None of those homes have ever failed to my knowledge. I remember when the holdown straps became more common and then the HDU seems to have taken its place more recently. I'm finding with residential work there is a lot to do, a lot of effort expended but the monetary return is somewhat lacking, sure wish I could figure out how to change that balance even slightly more in my favor. The aerospace industry was more pay for a lot less effort but then it was boring so I guess you can't win them all.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JAE]

I'm finding with residential work there is a lot to do, a lot of effort expended but the monetary return is somewhat lacking...

...probably the reason a lot of structural engineers stay away from most residential. That and a much higher level of risk.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[medeek]

Explain what you mean by the higher risk.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

On the subject of A35 clips again, let's assume you have a short shear wall panel in a wall that is mostly Windows so the unit shear is quite high. Your calcs show that you need a clip every 12 inches to connect to the roof diaphragm. Wouldn't it make more sense to calculate the number of clips you need and then space them out 24" o/c assuming the top plate is continuous and can handle the load. This way the roof diaphragm is not so locally stressed at the location of this shear wall.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Risk = lawsuit prone. Usually envelope related but we get sucked in anyhow.

I agree with your take on spacing out the clips. Spacing should reflect the disposition of shear as it comes in from the roof deck.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

You should space the clips evenly along the plate based on the uniform shear from the diaphragm. Otherwise you are not really providing a full collector along the length of the diaphragm. Ignoring any toenails between blocking and the plate it's essentially equivalent to clustering nails at the diaphragm blocks adjacent to the shear wall and omitting them elsewhere.

 

[medeek]

I was comparing my method of assigning the clips to a couple other engineers and I noticed they just made the assignment of the spacing based on the unit shear of the shear wall. The problem with this method imho is that it assumes that the clips are only spaced out on top of the shear wall and not along the entire wall line or collector. So in reality the spacing will usually be quite large. Would there be any instances were this would not be the case?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JAE]

Explain what you mean by the higher risk.

1. You have (usually) a first time owner/client who usually doesn't know much about construction and can be very paranoid.
2. You have an owner/client who has limited funds and can be very sensitive to costs or cost over-runs even if it is not your fault.
3. Typically smaller architect firms, or one-person architect firms, are involved and many times they do not have professional liability insurance.
4. If you have PL insurance and no-one else does, guess who gets sued?
5. Your fees for the structural in residential are usually (as you pointed out) fairly limited or small - thus the risk-reward is very unbalanced not in your favor.
6. Some residential efforts involve the structural sizing one or two beams or some footings. Once you touch the design of a part of the structure a litigious owner/client assumes you are responsible for it all.
7. Limited opportunity for repeat business from the client - therefore always some limit on personal relationship and developed trust.
8. Smaller contractors who tend to have less experience in structural engineering and details...thus struggle to get your details built correctly - or they ignore them entirely.
9. Residential wood construction has a lot of "traditional" and "standard" ways of framing that aren't always consistent with structural requirements or load paths. In other words, looking at how houses are stick framed is a structural engineer's worst nightmare.
10. There is usually less organizational structure, procedures and processes in place during construction with regards to inspections, requests for information, submittals and their review processes, etc. So things can get built different from design or simply wrong without you knowing about it and problems from that later can come back to haunt you via an angry owner/client (see items 1-2 above).

I'm sure there are other reasons/risks.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[medeek]

Item 10 on your list has hit me recently. Still working on the window wall problem and I'm still waiting on a local contractor to remove the trim so I can examine the structural components. I'm finding a lot of residential contractors tend to skimp on certain things where they can and even at time sloppy work. Even when its not directly related to me I usually get dragged into it as you suggest, at that point it is eating up my time more than anything else.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I'm finding a lot of residential contractors tend to skimp on certain things where they can and even at time sloppy work.

I live in oil country where our economy is wildly cyclical. In boom time, construction is astoundingly amateurish. My brother had his condo built when things were white hot and his condo board has now spent more on repairs than the condo's original purchase price. When they opened things up to investigate some problems, there we entire components of the envelope there were not merely installed poorly but, rather, simply not installed at all. Bathroom fans that just vent into the plenum space etc. Ron would have a field day. It's almost like a gang of elementary school kids decided to get together and build some homes. Lawsuits o'plenty.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I was comparing my method of assigning the clips to a couple other engineers and I noticed they just made the assignment of the spacing based on the unit shear of the shear wall.

Unless I misunderstand the situation, this just sounds wrong. Conservative, but wrong. You attach the diaphragm to the to top plates (chord) uniform-ishly along the diaphragm with the blocking and clips. You attach the top plates to the shear wall panel with diaphgragm boundary nailing for the wall unit shear.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

This is my point. The spacing of the edge nailing of the diaphragm and the A35 clips should be based on the diaphragm unit shear and not the shearwall unit shear which may be much greater depending on the particulars of the geometry. This of course assumes that the top plate (collector) has the capacity to transfer the shear along its length. Assuming the shear load is wind or seismic and the top plate is a double 2x6 DF No. 2 with (12) 16d nails and a 48" splice:

Splice Capacity: 141 x 1.6 x 12 = 2,707 lbs
Cont. 2x6 DF No. 2 Capacity (tension): 1.5 x 5.5 x 575 x 1.6 = 7,590 lbs

Under most circumstances (typical residence) the drag force is probably less than these allowables but it is something to keep in mind.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com