Hold Downs - resistance from architect 6

[JONNH]

Hi All,
I am designing a house in a 110mph, exposure C zone, so we are designing per the WFCM. As always we have specfied hold downs at the ends of shear walls for the architect. The architect has posed a question back to us, asking if we can get rid of the hold downs and increase the sill plate anchorage, they claim other engineers hasve done this for them.... Am I missing something here? We have thousands of lbs of tension at these hold downs and feel that hold downs at the shear wall corners is the right answer. Anyone had this request/pushback in the past?

Thanks in advance.

 

[jayrod12]

I can't comment on your specific case, however shearwalls without holddowns are possible. Here north of the 49 there's factors that are applied to shearwall capacities that don't have dedicated holddowns.

 

[KootK]

I have had that request. And I don't think that you're missing anything. With the exception of perhaps the bolts right next to the wall chords, the additional sill plate anchorage doesn't do anything much to improve hold down. It can be a delicate conversation but, if you explain it well to your client, there may be an opportunity for you to come of the situation looking quite competent relative to your competition. Of course, that assumes that your architect gives a damn about proper engineering.

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.

 

[TehMightyEngineer]

The architect wants you to design the shear walls and sill anchor bolts for combined shear and uplift. There are provisions in the SDPWS for this. Sometimes this does make sense but other times it's clear that hold downs will be a less expensive option.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Of course, that assumes that your architect gives a damn about proper engineering.

If you find one of these, make sure he becomes a repeat customer. They are few and far between.

 

[SLTA]

I've pulled out the "thanks very much for your input but I enjoy having my license too much" gem before, when the architect starts questioning things like that. What is the pushback arising from? Or, why does the architect care?

Please remember: we're not all guys!

 

[JONNH]

Thanks for the responses.

KootK, that is the approach I took, trying to explain to the architect why we need them. Whether they fully believe me is another question.....

 

[JONNH]

This is a large house, 60ft x 32ft x 32ft tall and essentially on the ocean. The hold down forces are quite large, 7000 lbs at some locations.

SLTA: The architect simply responded to my markups showing the hold downs. I suppose it can't hurt for them to ask. He has not questioned me since saying, yes they are required. Just wanted to double check here since he is saying other engineers were able to design similar homes without hold downs.

TehMightyEngineer: I will take a look. Thanks.

 

[BSVBD]

Several years ago, it was a state plan reviewer, who possessed enough structural knowledge to question the lack of strength in the sill plate anchorage, that caused me to investigate Simpson Holdowns. I've been using holdowns ever since. I still specify the sill plate anchor, but, only as an auxiliary not due to lateral wind.

We often have the contractor stating that other firms or engineers "have done this for us".

We will often try to accommodate the request, but, more often than not, we tell them the bottom line is that the particular item, in your case, the holdown is simply necessary. In my experience, with most people, if you give them a reasonable explanation they will accept it.

On many occasions, i wonder if the comment, "other engineers will do this for us", is a bluff. I don't know!

 

[XR250]

FWIW, has anyone ever witnessed a racking failure of a beach house? All the videos that I have seen shows the whole house getting pulled off the foundation as a unit.
Shows what we know.

 

[KootK]

I have a structural engineer colleague who purchased a house on a steep slope. The leeward side faces our majestic mountain range and, consequently, is packed with windows. Initially, the house would vibrate noticeably and creak distressingly in strong winds. The addition of an interior shear wall alleviated the problem. That's all that I've got for real world anectodes.

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.

 

[SLTA]

Racking would be due to a lack of shearwalls, not lack attachment of shearwalls to the foundation.

Please remember: we're not all guys!

 

[JONNH]

Great conversation here, thanks for the responses.

 

[msquared48]

2 to 3 stories with HD8's or the equivalent? I would not rely on the type of anchorage the Architect desires. Maybe on a small rambler with a lot of shear walls where the dead load from the truss package suffices to resist the overturning.

A few HD8's are a small price to pay related to the total cost of the house. This is a ridiculous area to save money. Delete a few ornate Architectural details instead.

You are the structural engineer, not the other engineers, TO WHOM THE ARCHITECT DID NOT RETURN, BY THE WAY. Why?

Just say no.

Mike McCann, PE, SE (WA)

 

[KootK]

Just to be clear, are we agreed that:

1) Additional sill anchorage can help with direct tension uplift.

2) Additional sill anchorage, cannot help with uplift due to shear wall overturning? Except for the bolts right beside the chords of course.

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.

 

[BSVBD]

Kootk:

3) Additional sill anchorage can help with uplift due to shear wall overturning! Technically, what's the difference between direct tension and overturning?

But, when uplift resistance can be accomplished in a single calc with (1) holdown, why would you consider the sill anchors? I suppose if you passed the threshold of allowable tension of the holdown? But, then either increase the holdwon size or add another holdown to the next adjacent stud(s) yielding (2) holdowns each end of shear wall. I've done that many times! (hopefully i haven't gone on too much of a tangent???)

 

[MotorCity]

Maybe I am missing something, but why can't additional sill anchors resist uplift and/or overturning if the anchors, sill, etc are designed accordingly. Not saying that you won't end up with larger, closer spaced anchors, just that I think it is theoretically possible (but unconventional). If shear walls are rigid enough to "span" between hold downs, then they should be able to get anchored anywhere along the span if the sill plate is designed to resist the anchor force

 

[BSVBD]

Motorcity,

I don't think you are missing anything.

I don't think anyone here is suggesting that properly designed, and, if necessary, (perhaps) "hybrid" sill anchors cannot resist uplift.

The OP was simply questioning the contesting of the architect who was wanting to get rid of the holdowns.

As jayrod said, "shearwalls without holddowns are possible". However, with "hybrid" contemporary architectural designs, the contemporary structure generally has to meet higher demands than that of the conventional past. The "typical conventional" sill anchor, at typical/conventional spacing, may not always be adequate.

 

[KootK]

I don't think anyone here is suggesting that properly designed, and, if necessary, (perhaps) "hybrid" sill anchors cannot resist uplift.

I'm pretty much suggesting this. Of course "properly designed" covers a lot of ground. If you take your conventional overturning hold down and distribute that capacity along the length of your wall then you fundamentally alter the distribution of shear in the shear wall panel. And you increase it on a unit shear basis as you reduce the effective depth of the shear wall. Also, rather than having a constant shear demand in the panel, the demand will step at each stud that is located near a tie down. Essentially, from a shear perspective, you are turning one shear wall panel into multiple, interconnected uniform shear panels that are divided at the tie downs. Can such a wall be designed? Sure. I've done something similar when using centrally located point loads to effectively tie down walls. That said, it's very unconventional way to design a shear wall and anyone attempting it needs to be aware of the ramifications. I've never once seen such a design appear in print.

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.

 

[andriver]

Can anyone provide reference material to understand typical home construction/engineering? As a structural guy in heavy industries I deal with heavy concrete and steel structures. I would like to know more about smaller commercial/residential construction out of curiosity but also for my own benefit when buying a home.