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dik said:engibeer: When something may be in a state of impending collapse... changing the load regime at any location may not be a really good idea <G>.
Dik
Close enough.Ingenuity said:For member #11 with 2 each x 1-3/4" PT bars on a 24" x 21" concrete cross section, with PT bars stressed to about 70% of UTS - that is about 540 kips over about 500 in2 of concrete - so about 1,000 psi of axial compressive stress. Self weight support reaction of 950 kips and a member #11 angle of 36o equates to a compressive force of about 1600 kips (3,100 psi axial compressive stress) - add the self weight force to the stress induced by the PT to member #11 and you get a net of about 2,140 kips (or 4,100 psi axial compressive stress). IF (big IF) there was a 10% overstress to PT bars that would only increase the force (and stress) by 54 kips (100 psi) to the cross section, for a total of about 2,200 kips (4,400 psi).
2,200 kips is not "okay" for member #11. Far from it.Ingenuity said:For 8,000+ psi concrete, adequately reinforced with mild steel reinforcing, (including significant confinement reinforcement) and the level of stress to the member should be okay...
PeterDow said:Newbie here.
Peter Dowe said:This tells us that only calculating with a risky safety factor of only 1.2 can we assess that the truss member #11 is just barely strong enough to hold the bridge up with no additional load from post-tensioning bars or from any pedestrians on the bridge. Using anything more cautious for a design safety factor would warn that the bridge is at an unacceptable risk of coming down.
Peter Dowe said:Of course it would be much worse if there were any issues with the concrete not being as specified.
Peter Dowe said:member #11 is just barely strong enough to hold the bridge up with no additional load from post-tensioning bars
I'll recheck about PT rods as something on my working drawings not fit.3DDave (Aerospace) said:old_jim, there is already evidence that the tendons are intact; therefore there was no shock load.
Thanks!bobwhite said:Peter Dow is accurate with the equation with the design axial force.
bobwhite said:Peter Dow is accurate with the equation except that the design axial force using his assumed reinforcement is about 2178 kips, f'c = 8500, 10-#7 fy = 60000 longitudinal bars equivalent to 6 sq in 1.19% of Ag.
Not a slope more like my force vector diagram's 1v:1.4h?bobwhite said:Diagonal #11 has a slope of about 1v:1.6h
If your slope is wrong you will miscalculate the force too.bobwhite said:This results in an unfactored dead load force = 1812 kips based on the truss end reaction of 950 kips.
So "about 1600 kips" is right for the compression force on member #11 from the bridge dead load too, I agree.Ingenuity said:Self weight support reaction of 950 kips and a member #11 angle of 36o equates to a compressive force of about 1600 kips
Agreed.bobwhite said:Diagonal #11 has a slope of about 1v:1.38h based on the prelim drawings.
Agreed.bobwhite said:This results in an unfactored dead load force = 1619 kips based on the truss end reaction of 950 kips.
waross said:I wonder if the upper PT rod in member 11 was removed after it was relieved. Is it visible in any of the pictures?
bobwhite said:I tend to agree with ingenuity that the failure occurred at the top of diagonal #11 and #10 in the canopy slab at least based on what the video shows.