Drag Struts and diaphragm chords on Hotel exterior, or absence of them

[jeffhed]

First the question. I am designing a 4 story hotel where no longitudinal exterior walls line up and creating drag struts the length of the building will be difficult. I am trying to keep the longitudinal exterior walls tied together so they will work together. The architect gave me the structural drawings from the previous project for reference on how they wanted to build this project. The previous drawings don't show a single coil strap or drag strut anywhere on any floor or in the details. How was this accomplished? Where the longitudinal exterior walls jog back and forth how are the longitudinal exterior shear walls expected to work together? How are the diaphragm sections between the transverse shear walls intended to work without continuous chords at the exterior walls?

Now my possible solution. I want to see what others think about it. Right now, I have transverse shear walls between every other unit. Because I will have edge nailing over these transverse shear wall lines, I was thinking I could use them as diaphragm chords in the long direction. Each two unit segment will have its own diaphragm to transfer loads to the exterior longitudinal shear walls and the interior hallway longitudinal shear walls. Because my diaphragm spans are 29'-0" and less and the minimum depth of my longitudinal diaphragm is 24'-0" the forces are not very high at all. The chord forces got a little high at the bottom floor, but overall there wont be anything too crazy. Now my final question is will this work as far as getting equal deformation on all the exterior shear walls. The diaphragm forces for each two unit bay are distributed based on a ratio of the length of the two units versus the length of the entire building. It seems to me like this should work, but since I haven't done this before I am not 100% sure. How will this affect the building when I have multiple little diaphragms trying to transfer load to the exterior of the building that are different aspect ratios? Any other suggestions? If I have to use collectors then I have to use them. However, this particular client has built hotels all over the country with similar plans. So I am trying to conform with their previous construction practices if possible. I have uploaded the roof framing plan for reference. The exterior walls stack all the way down so the floor framing plans will be similar.

 

[AELLC]

This looks like a typical apartment building, where we would make each transverse wall where the wall jogs out to be shear wall, so there is no concern for drag struts or collectors.
If you add any construction cost at all to this compared to the structures already built, then of course the Owner is going to object.

 

[jeffhed]

AELLC,
How do you analyze the diaphragm in the longitudinal direction without collectors? How do you ensure the shear walls on the exterior longitudinal walls work together and have equal deformations? Are you saying that the transverse shear walls top plates also function as diaphragm chords for a group of diaphragms in the longitudinal direction?

 

[AELLC]

jeff,

I am saying the standard of care for designing the structure of this type of structure is much more relaxed than you envision it. I worked on quite a few apartment buildings, and the Owner always expected the member sizes, details, and shear wall components to be similar to those already built.

The forces and stresses of the roof and floor diaphragms in the longitudinal direction were always quite low, and we never strapped the roof or floor decks. If this is being built in a high-seismic region where straps are routinely specified, then you would have precedence favoring you.

As far as equal deformation of shear walls, that was never a concern. The floor deck drifts uniformly, so that forces the shear walls to deflect equally. We were of the opinion that if the total of all shear walls were calculated to resist the entire shear, then one shear wall could not fail by itself. Or, if one shear wall was somewhat overloaded, the excess shear force would be taken up by the other shear walls. The finished floor, roof, and shear walls form an assembly that is not that all flexible.

 

[jeffhed]

AELLC,
I am in a higher seismic region, so I am not used to seeing things without collectors, drag struts, etc. The original structure was done in a completely different state that I believe is a low seismic area, so I believe that may be part of the difference. It makes sense to me that the diaphragm would be fairly rigid considering the length to width ratio. I guess my biggest hurdle in this situation is we have pretty stringent plan check in this area. If I don't have numbers to prove something, they will ask for numbers for what we are doing. Typically they do not accept professional opinions for things with the absence of numbers. How could you justify this sort of thing with calculations? I guess I could show that the diaphragm deflection is so low that the diaphragm would essentially behave pretty rigidly. How do you resolve the tension forces that develop in a seismic event when the building wants to reverse direction? Tension capacity of the floor sheathing and the nailing shear resistance?

 

[AELLC]

I would calculate the drag loads at the exterior longitudinal shear wall line - since they are offset there is no continuous double top plate and a steel strap would carry drag load in excess of the shear capacity of the deck.

I am very rusty regarding high seismic lateral design, I did a few large custom homes years ago in California, wish someone else would chime in...

See attached excel file -

 

[jeffhed]

AELLC,
That is what I have done on past hotel and apartment buildings I have done in the past. It just threw me for a loop when I saw these other plans and I am always interested in hearing/learning about others methods because I am younger and there are a lot of more experienced engineers out there. Running numbers on this particular project I came up with exterior drag forces of a little over 2k and the drag force on the interior shear line was about 4.5k. So not very much, but enough where I felt they couldn't be ignored. That is what triggered my post in the first place. In reality, the diaphragm will work more like what you said earlier, but I am unsure on how I could provide calculations for it.

 

[AELLC]

I would merely not address those issues in the calcs. I don't know how you present your calculations, but I learned long ago to not present them in a way similar to homework at University, with every step narrated. I only show the "bare bones" to get the project properly designed.

 

[jeffhed]

AELLC,
Up to about 8 or 9 years ago we did do calculations like that. Now days commercial project plan check involves a peer review from another engineer, and they pretty much ask for calculations for everything. So I am stuck showing calculations ahead of time, or getting 25 plan review comments asking for calculations that I did not include. It's much easier to provide them all in the first place.

 

[AELLC]

OK, what I meant is assembling excel worksheets like the one I attached earlier, without any narrative about assumptions, theory, etc.

 

[jeffhed]

Oh. I understand what you are saying now. We have spreadsheets for our lateral analysis that includes shear walls, hold downs, diaphragm, and drag forces. I have attached what our calculations look like, they are similar to yours.

 

[AELLC]

Looks very good.

 

[jeffhed]

Thanks. Essentially we are not showing any of our assumptions for the most part. But if we provide drag force diagrams and we don't show ties we will be asked how we are handling the tension forces. I have always had a hard time deciding hen drag forces are small enough that they can be ignored. If a truss with a drag load ends at the start of a wall I will always provide some sort of tie, even if its just an A35 clip for small forces. But sometimes when the trusses are perpendicular to the shear wall is there a force level that that if the drag force is below that you neglect it? Maybe that is what the engineer for this other hotel project was doing.