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Building steel girder system structure determinate/stable? 1
- Thread starter TedEP
- Start date
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1
- #5
TedEP:
Your arrangement is determinate and can be made to be stable, and can be erected in a stable fashion. But, there may be a slightly more efficient arrangement in terms of beam/girder sizing. Starting at the left reaction, call the reactions R1, R2, R3..., etc., and label the beams B1, B2, B3..., etc. Starting from the left; B1 cantilevers over R2 to a hinge point; then B2 is a short hanger; B3 cantilevers over R3 and R4, in both directions; B4 is a short hanger from R5, out to the hinge on the right end of B3. This will probably result in sightly lighter beams than your arrangement. And, the whole scheme may be partially defeated if you have to consider snow drifting and alternate live loading. In the U.S. we call this cantilever construction, in Canada they call these a Gerber Beam System. Search Gerber Beams on this forum for other discussions and a good paper on this subject.
Be careful placing beams on top of columns, the beams and columns tend to form a hinge laterally, at the col. cap and beam bearing pl. This joint must be strengthened or braced in some way. Also, be careful not to place a beam hinge point at the same location where a stl. jst. or perpendicular beam frames onto or into the girder. You can’t make all the connections work well, or at all.
Your arrangement is determinate and can be made to be stable, and can be erected in a stable fashion. But, there may be a slightly more efficient arrangement in terms of beam/girder sizing. Starting at the left reaction, call the reactions R1, R2, R3..., etc., and label the beams B1, B2, B3..., etc. Starting from the left; B1 cantilevers over R2 to a hinge point; then B2 is a short hanger; B3 cantilevers over R3 and R4, in both directions; B4 is a short hanger from R5, out to the hinge on the right end of B3. This will probably result in sightly lighter beams than your arrangement. And, the whole scheme may be partially defeated if you have to consider snow drifting and alternate live loading. In the U.S. we call this cantilever construction, in Canada they call these a Gerber Beam System. Search Gerber Beams on this forum for other discussions and a good paper on this subject.
Be careful placing beams on top of columns, the beams and columns tend to form a hinge laterally, at the col. cap and beam bearing pl. This joint must be strengthened or braced in some way. Also, be careful not to place a beam hinge point at the same location where a stl. jst. or perpendicular beam frames onto or into the girder. You can’t make all the connections work well, or at all.
MacGruber22
Structural
Definitely stable and determinant. Work from right to left and is it more clear.
Number of independent members, n = 4
Total number of reactions (after breaking each member out), r = 8
Number of equations in co-planer system (excluding x-dir) = 2
2n must equal r for determinate, stable structure (and it does)
r>2n would have made it stable and indeterminate
r<2n would have made it unstable
Is this homework?
"It is imperative Cunth doesn't get his hands on those codes."
Number of independent members, n = 4
Total number of reactions (after breaking each member out), r = 8
Number of equations in co-planer system (excluding x-dir) = 2
2n must equal r for determinate, stable structure (and it does)
r>2n would have made it stable and indeterminate
r<2n would have made it unstable
Is this homework?
"It is imperative Cunth doesn't get his hands on those codes."
- Thread starter
- #9
No, not home work! Its a real building, a tilt up warehouse/office.
I first designed it as true Gerber style, like in the steel book (pg 2-311 in the green book). Then it was explained to me the style on my attached drawing would be easier to erect, since with the drop in links in the steel book you have to go back to erect the links.
My issue with my attached sketch scheme was I couldn't figure out which end to start at to analyze the system. I now see its the left end.
Thanks to all for your great responses and your time.
I first designed it as true Gerber style, like in the steel book (pg 2-311 in the green book). Then it was explained to me the style on my attached drawing would be easier to erect, since with the drop in links in the steel book you have to go back to erect the links.
My issue with my attached sketch scheme was I couldn't figure out which end to start at to analyze the system. I now see its the left end.
Thanks to all for your great responses and your time.
msquared48
Structural
It is stable, but I would move the center pin to the right of the center support if this was a new design. Seems a little floppy without that.
Mike McCann, PE, SE (WA)
Mike McCann, PE, SE (WA)
The structure is determinate and stable but is not recommended because it is susceptible to cascading failure. If either of the columns on the right side were to fail the entire structure would become unstable. The preferred method of constructing a continuous beam line like this would be to have 2 columns under every other beam with the ends of the beam cantilevering beyond the columns and a drop in beam filling the gaps.
MacGruber22
Structural
I sort of agreed with JAE. The problem is that the OP's sketch is not very good.
"It is imperative Cunth doesn't get his hands on those codes."
"It is imperative Cunth doesn't get his hands on those codes."
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