Wood modular classroom roof structure 1

[Prestressed Guy]

I have been asked to look at the possibility of removing a dividing wall on the centerline ridge of a modular classroom. It is a basic double wide unit on steel road frame with overall dimensions of 26'x66'. The structure is divided into (3) 22' sections with one on the end also divided with a wall on ridge. The rough room sizes are (2) at 26'x22' and (2) at 13'x22'. The school would like to remove the centerline wall between the two smaller room to make a third 26'x22' room. There does not appear to be any type of girder truss at the ridge to span any of the 22' clear-spans.

Does anyone know what these things use for the ridge girder? The only thing that I can see that could support the ridge from each side is a sheet of plywood that is screwed to the ends of each truss. I am thinking that it might be possible that the plywood is a continuous 66' sheet that forms a 48" deep ply-wood beam. I know that this size can be produced as a custom mill-order. The floor of my motor home is a single piece of 3/4" OSB that is 102" wide and 40' long because I watched them unloading the sheets from the truck.

 

[oldestguy]

A sketch might help. However too often in these pages folks forget about the ability of roof sheathing acting as a beam on its side.

 

[amlinerrichard]

The wall in the middle of the 3rd section is only an afterthought. Someone wanted 2 classrooms that are smaller. I would expect the wall was an addon after delivery. Are you getting any movement in the roof/ceiling in the other 2 rooms?

 

[bob33]

School portables are not all made the same way. If no drawings are available, I would open up the ceiling enough to see what was done.

 

[Prestressed Guy]

Here is a quick floor plan of the building. I think that the centerline partition was to give lateral support to the open "ridge" side of the structure while traveling down the road because when the GWB was removed, there was metal strapping on the wall. There was also a (6) 2x4 post in each wall at the in-board ends of the strapping. I have several holes punched up into the ceiling and there is no girder. Just the 3'4" plywood that is screwed to the ends of the trusses. I can see down the entire length of the roof and there is nothing different about the clear span sections and the one with the wall.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1564987836/tips/modular_yyqvmy.pdf[/url]

 

[Warhamer]

Haydenwse,

What it sounds like from your description is a Plywood Ridge Beam. These are not uncommon in Classroom units. APA has a design standard for these in H815. Check your column locations as I have seen very few Ridge Beams span the full length of a unit (unit = single module). The ridge beam is also likely multiple plywood plys. If there are columns along the centerline they are usually supports for the ridge beam. The beams are not usually a single piece but spliced as needed along the length.


If the school system does not have plans, ask if they can find a Data Plate or Modular Label/insignia/decal, depending on age of the unit. The Label will tell you the third party who inspected the unit at construction and the States it was labeled for. The Data Plate should give you a date of Manufacturer, the manufacturer and design loads. If the manufacturer is still in business, they may be willing to help you or the owner out with details from construction. Not all States have a modular program, so it is possible this information does not exist on the unit.

EIT, Modular Building Industry

 

[Prestressed Guy]

Warhamer, thanks for the reply. On my sketch I indicated the 3 locations of multiple stud posts. These locations all have footing piers below them and appear to be the supports for the roof plywood ridge. We would like to relocate the post that was found in the partition wall and put it at the corner of the utility closet. If the plywood ridge is continuous, this would not be a problem, but if there is a splice at that location, I would need to add a beam to support the ridge due to the discontinuity of the plywood. I guess we will need to do some more demo to find out.

Where would the Data Plate be located?

 

[Warhamer]

Haydenwse,

Generally Data Plates and labels are located on or near the Electrical Panel Cover. Other common locations is to have them in a closet, or above a drop ceiling on the hitch end.

Again depending on age of the building, and what State this was manufactured for, the labels and Data Plate may all have been removed, if the electrical panel was updated. Painted over if there was any other updated. If they were stuck to the wall next to the panel box, they could have been removed, by someone not knowing what they were and why they may be useful in the future.

EIT, Modular Building Industry

 

[Prestressed Guy]

Warhamer,
What is the best way to tell if the post in the partition under the ridge between the two units can be moved. I will have them check to see if there are any splices in the plywood. They have a temporary shoring wall installed on one side prior to removal of the partition and once that is removed, the edge of the plywood will be in view. I did notice that the top plate of the partition wall appears to be screwed to the edge of the plywood with star drive screws but am not sure if the top plate is part of the plywood beam design.

 

[Warhamer]

Haydenwse,

Sorry, I got caught by the eng-tips loading error and just now got back in to the website. The best way to tell would be to run capacity checks using the APA H815 equations on the new span condition. The other way I have seen done, is having LVL retrofits done.

EIT, Modular Building Industry