So I'm designing a simple two story classroom with CMU Corridor walls and CMU exterior walls (see attached pdf) The walls are typically spaced at 36-8.67-36 spacing. So over the classrooms I could easily use a 28LH770/300 bar joist to support a DL =94 psf and a LL = 60 psf. OK so a fairly conservative design. Using a composite slab 5" thick with 1 1/2" deck. And if I go into the Floor Vibe software and use WALLS for my girder spans and say set by girder span to 25 ft. And the overall floor length of say 120 feet or any other largely large value everything works fine. I'm assuming an church type assembly with .02 dampening. And I do have the ability to put in a few full height partition walls. So with typically that adds .02 to .05 additional dampening. If I use just .05 of additional dampening this joist size works fine. So I'm going to use full height partitions at the storage rooms between the classrooms. And with just .025 dampening it works. And even without any partition wall dampening it is within the bounds of engineering judgement. So with any additional damping it is OK.
HOWEVER the corridor walls aren't exactly straight. At the end of each classroom they jog outward a bit. And this reduces the joist span to 30 feet for about 15 feet or so. Storage rooms are located between the classrooms and this job is associated with that and allows students to enter the classrooms from an alcove that is created instead of directly from the classroom. Now if I take the same exact set up as above and reduce the Floor length to 25 feet (now matching my girder length which remember are walls with infinite stiffness) the floor vibration analysis fails badly. And since there is no direct dampening in the 25 foot width classroom the only solution is to increase the joist loadings. Which is what I did on two other schools with this exact design. And they've been in service now for a few years no complaints.
But I can't help but feel this was an overly conservative interpretation of the intent and costly. The software and associated guides have language that states that the maximum floor length is the length of the building however intervening columns or non-typical bays can reduce the floor length. If I limit my floor length to 25 feet it fails. So from the example layout the corner bay would not work unless I assume some of the adjacent spaces will vibrate with it.
Also as an experiment I change the joist spacing from 5 ft on center to 6 ft on center. The deck design is good up to 6 feet so why not. It helps a little bit in all cases since it more closely matches the overall floor joist loading with the loading the stiffness requirements for floor vibrations would want. But ultimately the floor vibration design is still driven by the assumed floor length. If I'm conservative and assume a floor length between the wall undulations it is much shorter and needs heavier joists. If I assume that 6 foot inward undulation will not restrict the vibration from moving outward and beyond then it is fine.
I suppose this is always the issue with empirical design methodologies. Because the testing is limited but the real world situations can vary vastly.
I'd just like to hear other various engineer's opinions on how they have handled situations similar to this. And I get these are opinions. I remember when designing the original buildings I actually called up Dr. Murray the author of the methodology and the software. He was very kind. Sent me extra books with the software when I upgraded my license. But even he was hesitant to give guidance and I understand that. So I just want opinions. I know that is all they are.
John Southard, M.S., P.E.
HOWEVER the corridor walls aren't exactly straight. At the end of each classroom they jog outward a bit. And this reduces the joist span to 30 feet for about 15 feet or so. Storage rooms are located between the classrooms and this job is associated with that and allows students to enter the classrooms from an alcove that is created instead of directly from the classroom. Now if I take the same exact set up as above and reduce the Floor length to 25 feet (now matching my girder length which remember are walls with infinite stiffness) the floor vibration analysis fails badly. And since there is no direct dampening in the 25 foot width classroom the only solution is to increase the joist loadings. Which is what I did on two other schools with this exact design. And they've been in service now for a few years no complaints.
But I can't help but feel this was an overly conservative interpretation of the intent and costly. The software and associated guides have language that states that the maximum floor length is the length of the building however intervening columns or non-typical bays can reduce the floor length. If I limit my floor length to 25 feet it fails. So from the example layout the corner bay would not work unless I assume some of the adjacent spaces will vibrate with it.
Also as an experiment I change the joist spacing from 5 ft on center to 6 ft on center. The deck design is good up to 6 feet so why not. It helps a little bit in all cases since it more closely matches the overall floor joist loading with the loading the stiffness requirements for floor vibrations would want. But ultimately the floor vibration design is still driven by the assumed floor length. If I'm conservative and assume a floor length between the wall undulations it is much shorter and needs heavier joists. If I assume that 6 foot inward undulation will not restrict the vibration from moving outward and beyond then it is fine.
I suppose this is always the issue with empirical design methodologies. Because the testing is limited but the real world situations can vary vastly.
I'd just like to hear other various engineer's opinions on how they have handled situations similar to this. And I get these are opinions. I remember when designing the original buildings I actually called up Dr. Murray the author of the methodology and the software. He was very kind. Sent me extra books with the software when I upgraded my license. But even he was hesitant to give guidance and I understand that. So I just want opinions. I know that is all they are.
John Southard, M.S., P.E.
