NCEES Sample Exam

[mjbikes]

I think they really screwed up the solutions for these problems. It seems they made the loads from the remaining purlins at 1/4 & 3/4 points disappear when the truss and machine loads were added. Shouldn't the condition to be analyzed in problems a) and b be a combination of Figures 804C & D? (i.e. the 2k purlin loads and the 20k machine load). Also they seemed to ignore the reaction from the truss king post in their beam bending analysis in part b. Were we supposed to ignore it for some reason? I got very little bending stress and a large compression stress with the truss in place.

What is your take on this problem?

 

[brosenbe]

I think they really screwed up the solutions for these problems. It seems they made the loads from the remaining purlins at 1/4 & 3/4 points disappear when the truss and machine loads were added. Shouldn't the condition to be analyzed in problems a) and b be a combination of Figures 804C & D? (i.e. the 2k purlin loads and the 20k machine load). Also they seemed to ignore the reaction from the truss king post in their beam bending analysis in part b. Were we supposed to ignore it for some reason? I got very little bending stress and a large compression stress with the truss in place.
What is your take on this problem?

The sequence is explained as so: Demolition occurs (which loads the existing GLB with the purlin loads), then the new truss is installed (so the purlin loads do not go into the new truss because the load is already on the GLB), then the new equipment is installed (so the truss carries the new 20 k machine load only). I think of it as the GLB already having deflected under the purlin load, so it's basically at an "at-rest" state when the truss and new machine loads are applied; thus the truss doesn't see the purlin loads. The GLB as part of the truss does not see a bending load from the 20 k machine load - only an axial compression load.

 

[*Ananda*]

I think they really screwed up the solutions for these problems. It seems they made the loads from the remaining purlins at 1/4 & 3/4 points disappear when the truss and machine loads were added. Shouldn't the condition to be analyzed in problems a) and b be a combination of Figures 804C & D? (i.e. the 2k purlin loads and the 20k machine load). Also they seemed to ignore the reaction from the truss king post in their beam bending analysis in part b. Were we supposed to ignore it for some reason? I got very little bending stress and a large compression stress with the truss in place.
What is your take on this problem?

The described solution for 604b uses the wrong F_bx. Based on the supposed moment, 20k-ft, which is positive, the "tension zone is stressed in tension". In other words, a positive bending moment puts the glulam tension laminations in tension. A positive moment as assumed will not make the compression zone stressed in tension, and therefore the correct allowable is based on Fbx (tension zone in tension) = 2400 psi and not Fbx (compression zone in tension) = 1450 psi.

For the solution Fbx allowable to be correct, the glulam would have had to also be turned upside down so that its "top" stamp was facing down, and then, due to gravity loads the "compression zone would be stressed in tension".

 

[mjbikes]

I think they really screwed up the solutions for these problems. It seems they made the loads from the remaining purlins at 1/4 & 3/4 points disappear when the truss and machine loads were added. Shouldn't the condition to be analyzed in problems a) and b be a combination of Figures 804C & D? (i.e. the 2k purlin loads and the 20k machine load). Also they seemed to ignore the reaction from the truss king post in their beam bending analysis in part b. Were we supposed to ignore it for some reason? I got very little bending stress and a large compression stress with the truss in place.
What is your take on this problem?

The sequence is explained as so: Demolition occurs (which loads the existing GLB with the purlin loads), then the new truss is installed (so the purlin loads do not go into the new truss because the load is already on the GLB), then the new equipment is installed (so the truss carries the new 20 k machine load only). I think of it as the GLB already having deflected under the purlin load, so it's basically at an "at-rest" state when the truss and new machine loads are applied; thus the truss doesn't see the purlin loads. The GLB as part of the truss does not see a bending load from the 20 k machine load - only an axial compression load.

Since the existing GL girder is the top chord of the new truss (line 4 of the problem:"...convert the existing girder into a truss,...", the purlin loads should still be applied to the truss after construction is complete. The caption of Figure 604D says" added loads". I interpreted that to mean: added to the existing condition after demo (figure 604C). If the truss works as it is supposed to, the vertical reaction of the truss rods at the kingpost would support the applied machine load resulting in no deflection. The only load applied transverse to the GL girder is from the purlins. I think the 20ft-kip bending moment on the GL girder is not correct. The only way that is close is if they told us the truss rod(s) failed. Also, their FBD for Case 2 is wrong: it shows the girder in tension and the rods in compression.

 

[mjbikes]

I think they really screwed up the solutions for these problems. It seems they made the loads from the remaining purlins at 1/4 & 3/4 points disappear when the truss and machine loads were added. Shouldn't the condition to be analyzed in problems a) and b be a combination of Figures 804C & D? (i.e. the 2k purlin loads and the 20k machine load). Also they seemed to ignore the reaction from the truss king post in their beam bending analysis in part b. Were we supposed to ignore it for some reason? I got very little bending stress and a large compression stress with the truss in place.
What is your take on this problem?

The described solution for 604b uses the wrong F_bx. Based on the supposed moment, 20k-ft, which is positive, the "tension zone is stressed in tension". In other words, a positive bending moment puts the glulam tension laminations in tension. A positive moment as assumed will not make the compression zone stressed in tension, and therefore the correct allowable is based on Fbx (tension zone in tension) = 2400 psi and not Fbx (compression zone in tension) = 1450 psi.

For the solution Fbx allowable to be correct, the glulam would have had to also be turned upside down so that its "top" stamp was facing down, and then, due to gravity loads the "compression zone would be stressed in tension".

Completely agree.