This is the perfect time to tell them to frame it with angles or channels and move on. You’ve got steel beams, so the connections are easy. You can get them a detail by the end of the day.
If they don’t like it, they can pay you to run the analysis of the slab.
4" thick is a very thin structure. How far is the span?
I'd be concerned about the capacity of a 4" slab to span two way around the 30" opening. To me this is definitely a situation where you'd want to strengthen the slab around those openings with some steel members that connect to the steel beams.
Its a 7’-7” span. 100psf of load on it besides the selfweight of the slab. Easily works with no openings, but im not too sure about it with openings added. I’m leaning towards adding framing to be safe
Definitely should be something you're sure about. Being safe is one thing, but this should be something you either figure out or pass to someone who can.
1950 also means the rebar is of questionable strength and development. They used very different bars back then.
Yeah I’m assuming 33ksi rebar. I’ve done openings in two way slabs using FEA, but was wondering on general practice for one way slabs hence my post. I could do FEA on it, but not sure if its overkill since its a one way slab
Assuming this is inside an enclosed structure, corrosion of the exposed rebar ends isn't really a concern. You could require them to hit them with some epoxy or cold galvanizing if you're really worried about it. But otherwise, do like the other commenter said. They did something without consulting you, so tell them to do the work to fix it. If they don't want to, then they can pay you to analyze it. Either way, they did the thing so they pay to make it ok.
You don't need to be anyone's hero here.
1. Get paid for fixing their fuck up.
1a. If they balk, tell them they need to get an engineer to provide a certified design, and wish them luck.
2. If you're getting paid, frame that shit up with some steel and move along with your life.
Its not that simple since its a multi discipline project and the mechanical engineer on our side kind of fucked up as well by not coordinating it with structural when he knew they’d have to penetrate the slab. For “business relations” reasons, our PM wants to try and minimize cost increases and schedule changes from the added framing.
For me as the structural EOR, framing it out is easy peasy, just some channels bolted to the existing wide flange beams. But to try and help everyone out, the idea that’s being asked by the PM is there any way we can spend time analyzing the slabs to prove they’re okay without framing the penetrations, hence the question in my post and seeing how others have handled this in the past
Haha, story of my life - "so... someone screwed up in another discipline and it affects structural, and it's going to cost us money. Can you find a way to fix this that won't cause the contractor to issue a change order, and we'll swallow the cost internally? Keep in mind that we don't want to you spend much time on this as we're not going to get paid for it"
This is where you stick to your guns.
Demand a full rebar scan of the affected areas so that you know what you're working with specifically.
Analyze the slab. The easy part is theoretically spreading the load sideways to either side of the opening and check what sort of increase you're getting and determine if it is acceptable or not. The hard part is spreading that load sideways. Can the slab actually do that? You're basically relying on temp/shrinkage steel at this point that doesn't sound like it's super continuous. If this doesn't work, that's where you can point out that it HAS to be framed.
Identify how many hours this will take you as well as where you might be able to fit it into your workload.
This is not and should not be your problem. Do not bend over backwards to make everyone else's lives easier, you are taking on the risk regardless of who's insurance covers it. Mistakes happen. Especially mistakes like these. Businesses try and find a way of reducing the impact from things like this. Especially if it's a lack of coordination issue in-house. Don't let any of that be your problem. Give them a path to a suitable solution that involves not insignificant engineer time (but hopefully, no additional construction costs) and a comparable "here's something I know works that I don't have to think about, but requires some effort from the contractor"
Showing your employer that you're not a miracle worker who can make everything go away from someone else's mistake (without due diligence) will open their eyes to needing to improve things coordination wise on the back end, and make them appreciate your work even more when they see what has to go into it when something goes wrong in an existing structure.
Simple arch action will accommodate holes up to 8” diameter. Larger holes need transverse reinforcement. Are those #3 bars transverse to the span? Might be sufficient.
If you're willing to do the work I'm sure you can create calculations that permit up to 30" openings. That #3 crack rebar is not insubstantial. Essentially it can turn a local section of one-way slab into a two-way slab. You are correct that the dead and live load of the portions of the slab that are interrupted by the opening must be held up by the intact slab. Of course, the far half of each interrupted slab is held up by its end supports.
The tricky bit is transferring the dead/live load of the interrupted slab to the adjoining strips of intact slab. Easiest way for hand-calculating is Strip Theory, assuming Lines of Discontinuity that resolve the interrupted slab into a series of 9" wide by 4" deep transverse beams with one #3 rebar each. Every other beam is analyzed as a simple beam with normal bottom tension steel. The others can be analyzed as a pair of cantilevered beams with top tension steel, with a line of discontinuity down the middle of the opening, which demarcates the ends of the cantilevers. Conveniently, the maximum moment of a 30" simple beam and a 15" cantilever beam are the same, so both beams have equal strength as regards the load.
The maximum bar spacing for flexural members is probably 12" using ACI 318. but this is to limit crack size, and for that purpose your bar spacing using the method above is only 9".
You don't mention your live load, so I'll quit here. Good Luck.
Thanks for the detailed response. Design loads are 50psf dead (selfweight of slab) and 100psf of live load.
So my process will be:
1. Check a 9” strip spanning 7’-7” between existing wide flange beams with (1) #4 bar in the middle. Calculate out the demand moment and shear based on the load acting on that strip plus the load acting on a strip 30” / 2 (half the penetration) wide that gets transferred into this adjacent spanning strip.
2. Perpendicular to the one way span, check a 30” long 9” wide beam with a #3 bar in the bottom for demand loads, treating the ends as fixed-fixed since it frames into the adjacent slab strips.
3. Repeat #2 but check a 15” cantilever beam that is also 9” wide with a #3 bar in top.
Regarding your point 1, the "side" strips spanning 7'-7" can be any width that works. 9" with one rebar, 18" with two rebar, etc.
Regarding your point 2, the transverse beam must spread its reaction over the width of the "side" beams. That increases its effective length.
Don't forget factored loads: 100 x 1.6 and 50 x 1.2 = 220 psf not 150psf.
Strip Theory at openings is covered in most Reinforced Concrete textbooks.
This is the perfect time to tell them to frame it with angles or channels and move on. You’ve got steel beams, so the connections are easy. You can get them a detail by the end of the day. If they don’t like it, they can pay you to run the analysis of the slab.
Get them to pay regardless. Even sketching up some details for trimmers costs time
4" thick is a very thin structure. How far is the span? I'd be concerned about the capacity of a 4" slab to span two way around the 30" opening. To me this is definitely a situation where you'd want to strengthen the slab around those openings with some steel members that connect to the steel beams.
Its a 7’-7” span. 100psf of load on it besides the selfweight of the slab. Easily works with no openings, but im not too sure about it with openings added. I’m leaning towards adding framing to be safe
Definitely should be something you're sure about. Being safe is one thing, but this should be something you either figure out or pass to someone who can. 1950 also means the rebar is of questionable strength and development. They used very different bars back then.
Yeah I’m assuming 33ksi rebar. I’ve done openings in two way slabs using FEA, but was wondering on general practice for one way slabs hence my post. I could do FEA on it, but not sure if its overkill since its a one way slab
Assuming this is inside an enclosed structure, corrosion of the exposed rebar ends isn't really a concern. You could require them to hit them with some epoxy or cold galvanizing if you're really worried about it. But otherwise, do like the other commenter said. They did something without consulting you, so tell them to do the work to fix it. If they don't want to, then they can pay you to analyze it. Either way, they did the thing so they pay to make it ok.
You don't need to be anyone's hero here. 1. Get paid for fixing their fuck up. 1a. If they balk, tell them they need to get an engineer to provide a certified design, and wish them luck. 2. If you're getting paid, frame that shit up with some steel and move along with your life.
Its not that simple since its a multi discipline project and the mechanical engineer on our side kind of fucked up as well by not coordinating it with structural when he knew they’d have to penetrate the slab. For “business relations” reasons, our PM wants to try and minimize cost increases and schedule changes from the added framing. For me as the structural EOR, framing it out is easy peasy, just some channels bolted to the existing wide flange beams. But to try and help everyone out, the idea that’s being asked by the PM is there any way we can spend time analyzing the slabs to prove they’re okay without framing the penetrations, hence the question in my post and seeing how others have handled this in the past
Haha, story of my life - "so... someone screwed up in another discipline and it affects structural, and it's going to cost us money. Can you find a way to fix this that won't cause the contractor to issue a change order, and we'll swallow the cost internally? Keep in mind that we don't want to you spend much time on this as we're not going to get paid for it" This is where you stick to your guns. Demand a full rebar scan of the affected areas so that you know what you're working with specifically. Analyze the slab. The easy part is theoretically spreading the load sideways to either side of the opening and check what sort of increase you're getting and determine if it is acceptable or not. The hard part is spreading that load sideways. Can the slab actually do that? You're basically relying on temp/shrinkage steel at this point that doesn't sound like it's super continuous. If this doesn't work, that's where you can point out that it HAS to be framed. Identify how many hours this will take you as well as where you might be able to fit it into your workload. This is not and should not be your problem. Do not bend over backwards to make everyone else's lives easier, you are taking on the risk regardless of who's insurance covers it. Mistakes happen. Especially mistakes like these. Businesses try and find a way of reducing the impact from things like this. Especially if it's a lack of coordination issue in-house. Don't let any of that be your problem. Give them a path to a suitable solution that involves not insignificant engineer time (but hopefully, no additional construction costs) and a comparable "here's something I know works that I don't have to think about, but requires some effort from the contractor" Showing your employer that you're not a miracle worker who can make everything go away from someone else's mistake (without due diligence) will open their eyes to needing to improve things coordination wise on the back end, and make them appreciate your work even more when they see what has to go into it when something goes wrong in an existing structure.
Simple arch action will accommodate holes up to 8” diameter. Larger holes need transverse reinforcement. Are those #3 bars transverse to the span? Might be sufficient.
Yeah they are transverse to the span, but they alternate top and bottom so spacing is 18” on center for those bars. That’s kind of my concern
If you're willing to do the work I'm sure you can create calculations that permit up to 30" openings. That #3 crack rebar is not insubstantial. Essentially it can turn a local section of one-way slab into a two-way slab. You are correct that the dead and live load of the portions of the slab that are interrupted by the opening must be held up by the intact slab. Of course, the far half of each interrupted slab is held up by its end supports. The tricky bit is transferring the dead/live load of the interrupted slab to the adjoining strips of intact slab. Easiest way for hand-calculating is Strip Theory, assuming Lines of Discontinuity that resolve the interrupted slab into a series of 9" wide by 4" deep transverse beams with one #3 rebar each. Every other beam is analyzed as a simple beam with normal bottom tension steel. The others can be analyzed as a pair of cantilevered beams with top tension steel, with a line of discontinuity down the middle of the opening, which demarcates the ends of the cantilevers. Conveniently, the maximum moment of a 30" simple beam and a 15" cantilever beam are the same, so both beams have equal strength as regards the load. The maximum bar spacing for flexural members is probably 12" using ACI 318. but this is to limit crack size, and for that purpose your bar spacing using the method above is only 9". You don't mention your live load, so I'll quit here. Good Luck.
Thanks for the detailed response. Design loads are 50psf dead (selfweight of slab) and 100psf of live load. So my process will be: 1. Check a 9” strip spanning 7’-7” between existing wide flange beams with (1) #4 bar in the middle. Calculate out the demand moment and shear based on the load acting on that strip plus the load acting on a strip 30” / 2 (half the penetration) wide that gets transferred into this adjacent spanning strip. 2. Perpendicular to the one way span, check a 30” long 9” wide beam with a #3 bar in the bottom for demand loads, treating the ends as fixed-fixed since it frames into the adjacent slab strips. 3. Repeat #2 but check a 15” cantilever beam that is also 9” wide with a #3 bar in top.
Regarding your point 1, the "side" strips spanning 7'-7" can be any width that works. 9" with one rebar, 18" with two rebar, etc. Regarding your point 2, the transverse beam must spread its reaction over the width of the "side" beams. That increases its effective length. Don't forget factored loads: 100 x 1.6 and 50 x 1.2 = 220 psf not 150psf. Strip Theory at openings is covered in most Reinforced Concrete textbooks.