Design Ideas

Application Notes

Did You Know?

 

Design Ideas

 

New digital joist design saves the day.

New Millennium’s digital steel joist design capabilities recently prevented a serious erection site problem that all involved say would, probably not have been caught using a traditional 2-D planning approach.

 

The Architect/GC on this large school project required a BIM based 3-D approach to the steel package. So the fabricator, Lyndon Steel Company, called upon Prodraft, Inc. for the digital structural steel planning and New Millennium for the integrated digital steel joist planning.

 

Having imported the joist requirements into our design software component, we emailed the file back to Prodraft for approval. Upon review the next day, Prodraft replied with a question regarding one of the joists in the model, pointing out a possible problem with the bearing and a missing bottom chord extension. By simply locating the joist in the digital model, New Millennium could immediately see a dimensional discrepancy and make the adjustment within seconds.

 

The numerical discrepancy found was in regard to a joist that had different dimensions at its ends, but should have been the same. The correct information was entered into the digital plan and with a click the fix was made. The updated model was emailed back to Prodraft. In retrospect, the wrong joist dimension would probably not have been detected following the traditional 2-D drawing review process.

 

Ed Jumper, steel detailing Project Manager at Prodraft, says that had it not been for the digital plan review, the project would have experienced disruption and cost consequences.

 

“It would have required field modifications of either the supporting steel or the joists coming in,” Jumper says. “For this specific instance, the joist chord member actually interfered with the cap plates on the columns. You’re probably talking thousands of dollars for field labor and field fixes for what is one of the more costly phases on a project.”

 

 

 

 

TOP

Sign up to receive future Roof-Lines e-Bulletins.

 

 

 

 

Digital error detection: A question regarding apparent missing joist information (image 1), was zoomed in on and the section information for that joist was confirmed wrong (image 2). Problem resolution was quickly confirmed (image 3).

 

Application Notes

 

End-moment connections increase design efficiency

In place of traditional girder-to-column moment connections, end-moment connections to columns allow for highly efficient steel allocation. The vast majority of steel joist and joist girder designs call for constant-depth prismatic sections in the joists and girders, with the top and bottom chord designs governed at the center of simple supports. This traditional approach can prove cost inefficient on large projects, when a large amount of steel is structurally underutilized.

 

Adding continuity moment connections at the center columns and developing both negative and positive moment along the length of the joist or girder can more efficiently utilize the steel. This reduces costs for steel material, manufacturing and delivery.

 

 

 

 

Rather than use traditional girder-to-column moment connections, continuity moment connections to columns allow for highly efficient steel utilization.

 

 

 

Bolted moment connections provide the required rigidity to prevent beam end rotation. Related on-site labor is significantly offset by reductions in steel material, manufacturing and delivery costs.

 

 

 

TOP

 

Did You Know?

 

Beam-joist deflection can be a system solution.

Due to the upward‐sloping geometry of a scissor joist, horizontal forces can be applied to the supporting structure. To solve this problem on a recent project, the beam and joist systems were engineered to deflect as cohesive units.

 

 

The project was a 200-room student dormitory and pavilion at Goodfellow Air Force Base in San Angelo, Texas. A key design challenge related to the open‐air, covered basketball pavilion, requiring twenty scissor joists, each weighing a little over one ton when fully assembled.

 

The roof joists for the pavilion were specified as 36LH09, fabricated with a special “scissor” profile to achieve a large covered space for a basketball court. When fully assembled, the joists were approximately 16 feet tall at the ridge and 76 feet long. The joists were also required to be supported at the bottom chord and extend about 5 feet past the exterior wall, creating an eve overhang for roof drainage.

 

Due to the upward‐sloping geometry of a scissor joist, a horizontal reaction can be applied to the supporting structure. The joist deflects under load, so either the ends of the joist must be allowed to displace, or the supporting structure must be able to resist a horizontal thrust force. For the pavilion, the support beams were HSS10x10, and the scissor joists were welded directly to the top of the beams. This welded attachment does not allow relative movement between joist and support. However, as load is applied, the beam and joist deflect together as a cohesive unit. New Millennium determined an allowable deflection of the support beams and calculated the thrust force at the ends of the joists.

 

The fully assembled joists, at 16 feet tall, would have been very difficult to transport as a single piece, so field‐bolted splices were designed to allow the joists to be delivered in pieces measuring 36 inches deep. This allowed a standard delivery, avoiding unnecessary freight charges for over‐height and over‐length permits. The joist halves were matched together at the job site and erected in a panelized method.

 

The special scissor profile created a challenge for the contractor and erector. The joists would be highly unstable if installed individually, so diagonal cross bridging was installed from the ground and the joists were lifted in pairs. A week before the joists were scheduled to be erected, New Millennium provided the erector with detailed information about joist weights, material sizes, and panel layouts. This coordination allowed the erector to develop a pick plan to ensure a smooth, safe process.