Floor Diaphragm with Large Opening 1

[Eng_Struct]

Hi Group,

I have an existing two story building (180 ft x 100 ft in plan) and client wants to cut a large opening through the middle of the first floor. The proposed opening size is roughly 30 ft x 30 ft. The existing second floor is 4.5" concrete composite metal deck on joists. As part of the opening we will be removing some joists for their entire length.

In my mind the opening is not going to effect the gravity load path but it will impact the lateral load path due to the discontunity in the floor that is currently acting as a diaphragm. I wanted to get some advice as to how to analyze the floor to capture the effect due to the discountinuty.

I am thinking I will have to provide perimeter beams around the opening and possibly with moment connections to the columns. Depending on the direction of lateral load, the perimeter beams perpendicular to the load will have to deflect around weak axis and take the load to the column and push the column in the direction of the load. The beams parallel to the load between the columns will provide frame action and will resist the load as they now become part of lateral load resisting system. The columns will need to be reinforced and the load path to the beam perpendicular to the load and to new "moment frame" will need to be defined clearly.

Beside relaying on the moment frame, I am not sure how else I can carry the load around the opening through the deck and/or existing joists. Note that the existing lateral load resisting system is masonry wall around the parameter.

My main questions is what is the best way to analyze this condition and what kind of things I should be checking in the analysis model. What I have currently planned is:

1. build a model with floor framing and assign floor deck using plate elements in the analysis program
2. check the forces in the framing members around the opening i.e., axial force, bending, etc.
3. currently i have joists for the framing system and I do not believe the joists will behave as intended if loaded in compression.
4. check story displacement to understand the impact of the opening
5. Do trial and error to come up with what is required however knowing the answer I need to get to before I start on this will help. As of now I do not know what the answer is.

Thanks

 

[KootK]

1) This article describes the general method for dealing with diaphragm openings: Link. Your opening is much larger than a skylight, of course, but similar principles apply. Your diaphragm becomes analogous to a Vierendeel truss near the openings and, to deal with that, you need both a) remaining shear capacity and b) collectors into the body of the surrounding deck. One thing that you have going for you is that your opening should be near the location of minimal deck shear. Still, I'd not design for less than 25% of the max shear anywhere in the diaphragm.

2) Consider what the original load path for diaphragm action in the deck slab areas would have been. If the joists do not have stud on them, it may well have been steel deck fastenings in shear such that the concrete was not assumed to participate. Obviously the concrete will participate, this is just a simplification.

3) Combination roof / floor decks in one diaphragm segment always get my hackles up in high seismic situations. I'd have to think that the stiffness discontinuity that implies forces uncommonly high ductility demands into the diaphragm near the boundaries of the discontinuity. That said, this is not an altogether uncommon situation so we're often required to make a go of it even if it is not entirely comfortable.

4) Introducing new VLRFS elements certainly would help. That said, in many instances, economic considerations lead to us getting pressured to make the diaphragm work without such invasive interventions. It's no fun to pitch and expensive solution and have the guy down the street wind up telling your client that it was unnecessary. Another thing to consider is the relative stiffness of any new VLRFS elements compared to the existing VLRFS elements. If the new stuff is moment frames and the existing stuff is shear walls or braced fames, the new moment frames may not wind up participating all that much.

 

[jerseyshore]

This is the first thing I thought of when seeing this post. You could easily find an engineer who says yeah take it out no problem, a hole in the middle isn't going to matter with a rigid diaphragm. And the difficult part is that they're probably right that it will never matter (in lower seismic regions of course).

If you're in a lower seismic area, this feels like one of those half and half solution situations. I wouldn't dive too deep into the diaphragm design, but I also wouldn't just cut a simple hole and leave it. Add some braces and beef up the connections until you get what feels like a pretty stiff frame around the 4 sides of this opening.

 

[XR250]

Intuitively, based on the scale of things, I can't imagine this being an issue. As KootK mentioned, the shear would low in the area of the discontinuity. How much is the max shear demand?

 

[Eng_Struct]

Thanks all for the usefull tips. The total story shear I have is about 800 kN.

When we rely on metal deck to transfer shear, does this mean no axial forces or very small axial forces are being imparted to the joists/floor framing members?

In my mind, when the deck is fastened to the structural support the fasteners will allow metal deck to develop tension field. Since the metal deck does not seem to be strong candidate for developing compression field (I do not see deck developing compression strut in between the joists/purlins/beams when shear load is perpendicular to the flutes), I would imagine some of the roof framing memebrs have to act as struts (compression members) to provide full diaphgram/truss action. Having said that I personally have not met an engineer who would consider additional axial forces in the roof framing memembers due to diaphragm behaviour. Perhapse I lack the understanding of the system and the deck is capable of both carrying tenssion and compression if adequate fastening is provided. I just like to think of the entire diaphragm as a truss where there will be compression and tension web members for shear.

Perhapse the analogy is not to think of the flow of forces in "truss" but "tearing/shear" of metal sheet in the direction of the load.


Second question I have is I may need to increase the connections to the existing deck around the parameter of the opening to ensure adequate shear strength. Since this is an existing floor, I am thinking if the self-driving screws can be used to provide additional fasteners from below. Drilling through flanges to tie the deck and support members together from below should work but I am not sure if there design/practicality issues i.e., this approach not capable of developing full shear strength of a screw, constructability when installing the screws from below.

 

[XR250]

So that is like 180k total or 90k per edge/100 ft = 900 plf diaphragm shear max. That is pretty substantial! Maybe listen to KootK on this one!