OWSJ Configurations

[slipe612]

We are redesigning a community center in Panama that was poorly designed and is currently failing. The structure is approximately 64' x 64' and they want to avoid putting any columns on the interior so as not to obstructing people's view. So we are left designing a roof systems with 64' spans which is causing us problems. We are on a tight budget with limited material availability and weight restrictions so we are trying to use open web steel joists.

When we use a standard configuration like the one on the right side of the attached drawing (sorry, I just drew it up in paint quickly), the loading on the 2 64' girders is too large. One idea we had was to configure the joists diagonally as shown on the left side of the drawing. This would eliminate some of the point loads on the girders by sending them straight to the columns. I want to know if this is possible/acceptable and how we would calculate the tributary areas on the joists.

Any advice would be appreciated. Thanks.

Steve

 

[msquared48]

Use a normal jost layout, just place the girders closer together - use 3 or 4 girders if they are too large using 2.

You go too far outside the norm of construction and framing and you will drive the cost of the girders through the roof.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[paddingtongreen]

I agree with Mike. You could run beams or short trusses from column to column and use those to support trusses in between the ones you have now, so you would have five instead of the two you have now.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

What type decking/roofing will you use? That will determine the spacing of the joists. Don't fiddle around with placing supports at an angle...that will only drive up the cost.

 

[dik]

Don't know your loading, but 3'6" or 4'-0" deep OWSJ @ 7' to 12' centres depending on steel deck over may be the most economical framing...

Dik

 

[dhengr]

Why not just use OWSJ’s spanning 64' from left to right in your drawing? I think this is what Dik is suggesting. The support system at the left and right wall lines would have columns as you show, and either beams or joist girder trusses spanning about 16'. This would probably be the cleanest, simplest, and most economical framing system, with the fewest different pieces and connections. You might use cantilever beam construction (Girber Beams) on the two support lines.

 

[dik]

Pretty much, but I'd use plastic design for the roof beams with a splice at approx 1/7 span near the middle column near the inside... don't have to worry about unequal loading so much with plastic design... might have a light beam on the exterior parallel to the OWSJ and 3 span, also using plastic design... Total of 8 perimeter beams... and use end plates for shear and moment connection (required by plastic design...

Dik

 

[dik]

...forgot to add... I'd use the roof deck for a diaphragm...

Dik

 

[shobroco]

Angles can add costs, but if you run 2 girders from the midpoints of each side, they only span about 45' instead of 64'. Then run smaller joists at right angles to these right across the whole area, so the biggest span is 45'. Yes, there are lots of different sized joists, but you have reduced your max. span by almost 1/3.

 

[dik]

...adding a bunch more sticks...

Dik

 

[SteelPE]

Yes, but we have really good sticks.

 

[dik]

but a bunch of them have to be broken off to the correct length... and then there's the issue with the deck...

Dik

 

[BAretired]

Looks like 64' span OWSJ would be the best solution. With columns spaced at 16' centers, a spacing of 5'-4" or 8'-0" would provide a tie joist at each column.

A good choice in my locale would be a spacing of 5'-4" with 1.5" deep x 22 gauge steel deck.

BA