Hold Downs to Multi Storey Residential Building

[Supun93]

Hi. I'm designing a two storey timber house. There are several questions i need to ask.
1.Do we need to have holdowns in first floor similar to the holdowns in the ground floor or will the WSPs will resist the lateral loads on the 1st Floor?
2.If we need to go for straps of HDUs, how do I know it?
3.Where do we need strong walls to be installed?
I'm still new to designs. So, please be kind enough to explain me with the basics.
Thank you

 

[phamENG]

I'm assuming you mean ground floor as the lowest level and the 1st floor as the first elevated floor - correct? (In my part of the world that's just first and second floor, or ground floor and second floor in some places, so I just want to make sure I understand you.

1. You've got it flipped around. Do the upper floor first, and then follow the load path. You'll have hold downs at each end, and that tension has to find its way to the foundation.

2. Do you mean straps OR HDUs? It's all based on the demand and the arrangement. Straps are generally weaker than hold downs. Straps are hard to install at interior locations. So you pick it based on the capacity and constructibility.

3. That question is a bit too broad to answer. By strong walls, are you referring to the proprietary product manufactured by Simpson, or using it as a generic name for shear walls?

You mentioned your new to designing. You do have a senior engineer working over you on this, correct? These are the sort of questions that are best asked of a supervisor so you can learn the design process. It's entirely too detailed to go through it step by step here. Several entire books have been written on the subject.

 

[Supun93]

Dear PhamEng
No, I don't have a senior engineer working over me for timber designs. I'm studying this myself because i'm very interest in timber designs. As strong walls i'm referring to simpson manufactured. The question is how we know that this shear line needs a strong wall, this location need a strong wall? I was searching all over the internet for good lessons, but i was unable to find a good course yet to study.

 

[phamENG]

Got it. From your initial post it sounded like you were actually designing one, not just studying the concepts.

Shear walls have aspect ratio limits. In the US, we use the National Design Standard (NDS) for Wood Construction, and the accompanying document the Special Design Provisions for Wind and Seismic (SDPWS). If you don't have a copy, it's free to view online. It has a lot of very valuable information.

When these site built options don't work - architect put in too many windows, building is too narrow, etc. - the Strong Wall is a proprietary product that has been tested to work beyond the limits that restrict the site built shear walls. So while a 2' wide by 9' tall wall segment can't be built on site to be a shear wall, a manufactured strong wall may be capable of performing as a shear wall there if the demand is less than the capacity of the strong wall.

 

[ChorasDen]

Hi Supun93,

phamENG has great points, but to add to this topic, the shear walls are used to resist lateral loads on the structure due wind and/or seismic loads, and their capacity requirements, sizes, locations, and detailing need to determined based upon the structural shape and loads to be resisted. The requirements will also change based upon if you assume rigid or flexible diaphragm analysis if you are resisting seismic forces.

Generally speaking, hold downs will be required at each shear wall, unless the factored dead load from the ASCE 7-16 load combination (0.6DL + 0.7E) is large enough to resist the overturning force at the top of the wall due to shear loads. Because wood framed buildings tend to be light weight, without a significant amount of dead load, in my experience, it's rare to have a situation where the dead load overtop is sufficiently large to resist the overturning force, but that may be different in other parts of the US where wind design controls. Therefore, hold downs are usually specified at shear wall ends in my region.

There are different methods to determine shear walls, you can use the perforated or segmented methods, both of which are details in SDPWS as phamENG indicated. If you want to use Simpson Strong Walls, you'll need to perform a stiffness comparison between the strong wall and the other shear walls on the wall line to ensure that you are distributing lateral shear loads appropriately to each shear wall segment. Commentary to SDPWS provides an example of how to do this for different wall types, but additional analysis and derivation will be required for proprietary walls like those that Simpsons make.

 

[dold]

unless the factored dead load from the ASCE 7-16 load combination (0.6DL + 0.7E) is large enough to resist the overturning force

dont forget to include vertical seismic effects when looking at shearwall overturning for seismic loads:
(1.0 + 0.14Sds)D + 0.7E, or, (1.0 + 0.10Sds)D + 0.525E + 0.75L + 0.75(Lr or S) for compression (the force that you design the studpacks/posts/'chords' at the end of the shearwalls),
and (0.6 - 0.14Sds)D + 0.7E for tension (the force that you will design the holddown for).

OP, to your question #3: Generally I only specify simpson strongwalls if I have a wall with a lot of windows or doors, or a garage door that results in wall piers (segments of your shearwall beside an opening) that do not meet the aspect ratios (3.5:1 for example, or a min of 24" in other cases). Dont use strongwalls if a standard shearwall will work.


I have this book: Seismic Principles ASCE7
It is a great reference for the basics of lateral force resisting systems (wood, steel, concrete, masonry). It sounds like you might benefit from this in your studies. There are tons of other references also. Just google "basics of lateral force resisting systems" or something to that effect. Plenty of youtube videos as well.