• Building Design
  • David Schonacher Jr., PE, President, Avanti Group
  • Pavel Gonzalez, PE, Partner, DDA Engineering
• Joist Manufacturing
  • Andrew Holton, PE, Engineer, Canam Steel Corporation
  • Ben Pitchford, PE, Engineering Manager, New Millennium

Why it’s important to consider wind and uplift forces

Uplift pressures present a reversal of the usual directionality of forces that joist design engineers deal with. Andrew says, “Essentially, instead of downward forces and gravity loads, these are upward, so you get compression and tension reversal on your joists and girders that you have to take into account.”

This has implications for joist bracing, especially as wind loads approach the same value as gravity loads. Ben says, “When my joist or girder goes into uplift, all my webs and chords that were in tension are now in compression. As wind load increases, so does bracing of the joist bottom chord. It’s not uncommon for us to see joists that need more rows of bridging than usual to properly brace the bottom chord.”

The trend toward bigger structures that don’t necessarily fit the calculations database also puts pressure on the engineer of record (EOR). David says, “We’re seeing diaphragm systems be challenged upward of 600 to 900 pounds per linear foot. We’ve had to pay special attention to these bigger tilt-wall buildings and consider site-specific conditions to recalculate some of our standard conditions.”

Best practices for communicating wind and uplift requirements to joist manufacturers

It’s critical for EORs to specify uplift loads on plans so joist manufacturers understand the actual net or gross load case to design to. Andrew says, “Typically, the EOR will include a key plan showing all the uplift zones, including any perimeter zone links, corner zone links, dimensions and so on. Along with that, we typically see an uplift chart, which corresponds to tributary areas with the uplift pressures the EOR has determined as appropriate and applicable.”

Ben adds, “If the loads are given to us, we can design the product. The chord sizes get bigger. Webs that are now in compression get bigger. We may have increased amounts of bridging on the product, but I wouldn’t necessarily call that a challenge.”

Specifying wind uplift values by zone is another key to helping the manufacturer understand what’s needed during the design process. Pavel says, “We do a key plan indicating the area corner zone, intermediate zone and interior zones, and we include a table with a net wind uplift for the joist for a corresponding zone. We deduct the weight of the system, which is about 8 pounds per square foot. That’s what we use when considering the metal decking, joist, girders, insulation and roofing.”

It can also help to invest time in communicating well with your manufacturing partner as David and Ben do. David says, “We went through the ASCE 7 calculation so that we could understand each other as an EOR and a delegate engineer. Every year or so, we review our processes and procedures and adapt the zone diagrams to communicate exactly what ASCE 7 tells us. We do the exact math so that if Ben and I are in separate offices and doing the same calculation, we come up with the same exact solution.”

Design process considerations in vulnerable coastal regions

Designers and manufacturers based in hurricane alley know that most structural components are governed by wind, from the joists to the walls to the foundation. These considerations add to the cost of the structures. Pavel says, “There is a lot of pressure from developers to reduce cost, but we have to stick to the structural requirements to meet the wind load criteria. We don’t design the joists, but we provide loads for them and control the connections, columns, bracings and overall integrity of the building.”

Adding to the challenge are the larger sizes of today’s tilt-wall industrial buildings. David says, “We used to average 24- to 26-foot clear, and now most of our buildings are 30-foot minimum clear. That increases the loads on the individual components and connections. When diaphragm values are high, I’m looking to reduce joist spacing toward the end of the building to make the same 22-gauge deck work.” He adds that it’s important to avoid overdesigning the stiffness of the deck because that can create forces that are difficult to manage. “We try to reduce the spacing and keep the connections reasonable, and we try to avoid doing things that interfere with fabrication processes.”

Temperature swings that wouldn’t be a problem elsewhere can also wreak havoc in South Florida. Pavel says, “Joists get installed in 100-plus degrees, when materials are elongated. So when a cold front of even 50 degrees comes, buildings experience a lot of contraction and movement.” In extreme situations, girder brackets pull out from columns, bolts tear off from brackets, or metal decking rips away from supports.

The benefits of recent wind uplift zone changes in ASCE

Joist engineers and manufacturers alike see the recent ASCE changes as positive, even though they have required some adjustments. On the manufacturing side, Ben says, “We’ve had to update our tools and retrain some people, so it’s definitely more work for us, but I see the benefit being that we end up with a more accurately designed building.”

On the design side, there’s a similar view of the more stringent codes. Pavel says, “It’s more realistic to the needs we require. For example, in the latest wind load for the interior of the building, the uplift and material of the building were reduced, and it was increased in the corners. I think it’s beneficial for the durability of the building.”

How uplift affects joist connections to joist girders, beams or masonry support walls

Joist to girder connections often need to be welded to withstand wind uplift and conform to code, which can drive costs up. Pavel says, “Bolted connections don’t work here in Florida. Sometimes, developers ask why buildings are more expensive to develop here, and it’s because the connections, the joists and everything is related to the higher wind load.”

Ben says his joist engineers consult with the EOR when considering how to handle connections with higher wind loads. “We’ve had jobs that might have two bolts supporting the girder, but with the tension capacity, those two bolts alone can’t even support the uplift of that girder. In that case, we communicate with the EOR to make sure we’re all on the same page for what that final connection needs to be.”

Joist manufacturer engineers keep a close eye on the important role connections play. Andrew says, “We ensure that our joists and girder seats have the available strength to transfer the uplift in reactions into the structure. You should be checking the ultimate end connections to make sure they’ll be adequate.” He adds that communicating changes is also critical. “Typically, if we catch it on the front end of the project as we’re beginning to detail it, then we will put it on our approval drawings. If we catch it on the back end, we’ll make adjustments and mark up our field-use drawings to make sure they get communicated.”

How anchorage or termination of bridging protects the structure against significant wind uplift

The same principle of wind uplift turning tension into compression applies to anchor terminations, and those forces must also be considered. Ben says, “When a bottom chord goes into compression under a wind event, we use bridging to brace it, and then there has to be a load path for that compressive bracing force.”

Terminating bridging to walls isn’t always possible, for example when two wind frames contain expansion joints and the placement is between them. Pavel says, “In that case, joists are braced to each other, and then we need to rely on diagonal bridging to brace the bottom part of the joists for compression and distribute the load. This creates a three-dimensional truss to handle wind forces and provides redundancy in the design.”

Codes to keep in mind when designing for significant uplift

Pavel favors the ASCE 7 code that rules wind design and adds that the Florida Building Code is based on that.

David stresses the importance of filling the gaps of information between EORs and delegated engineers and requesting any information that can help streamline design processes and keep fees reasonable.

Ben relies on SJI’s Specifications, Code to Standard Practice, as well as the Technical Digest about uplift. He says, “We often turn to that when we’re looking for information, and we’ve shared that with many EORs.”

Last Thoughts

• Andrew: It’s beneficial for the EOR to note what uplift they’re giving us. Is it gross? Is it net? If it is gross, what’s the dead load we can use to calculate the net uplift?

• Ben: 500-square-foot values have become more prevalent in the new code, which helps joist manufacturers a lot. That’s the effective wind area, not the tributary area, so I can do a third of the span times the span to get to that 500 square feet. We can deliver a much more efficient design if we’re given 500-square-foot values.

• David: We’re all on islands right now, designing 90 miles an hour. We’re definitely stressed and overwhelmed and have a lot going on. The only way we adapt and do better is by communicating together, engineer to engineer, about these concepts, and then do the best we can to create synergy.
  
• Pavel: It’s challenging to place rooftop HVAC units during tenant improvement projects in buildings with steel joists. They’re mostly gravity loads, but here in South Florida, we also have overturning forces and wind loads that can cause a reverse moment on joists that were not designed for that specific loading. We try to play with capacity and distribute the load across several joists to make it work.