Lateral Systems – Moment Frame vs. Braced Frame

The first thing we look at during preconstruction is the lateral system—the type, extent, location, etc. And the first big question is whether the design calls for braced frames or moment frames to resist lateral forces (or both, but that is rarer—to this author’s knowledge, only Covenant House in our portfolio used both extensively).

Pound for pound, braced frames are far stronger than moment frames. Because of this, we see far less raw steel in braced framed buildings (although likely more total pieces which can influence erection costs).

This is borne out by the projects we have seen. In the designs we have built or priced, braced frames have been 7.50 – 8.50 lb/sf in raw steel weight (including the weight of connections). The corresponding range for moment frame buildings is 14.50 – 17.50 lb/sf.

Besides added raw weight, the higher forces in moment frames mean that the column splice connections within those frames may grow very large (when bolted). Sometimes so large that to accommodate them in the floorplans, the connection type for the column splice must become a full penetration weld in the field. Full penetrations welds are time consuming and costly. They require additional certified and competent welders (along with inspections). Since we cannot monopolize the crane to complete these welds, it also means we must detail and buy temporary connections to take the welding out of the critical path for steel erection.

So why build moment frame buildings at all? Mostly because the braced frames can be obstructive and inconvenient. On the interior they can be inconvenient for architectural floor plans. On the exterior they can obstruct potential window locations. As we will cover later, the braces also create thorny conditions for detailing.

Besides added raw weight, the higher forces in moment frames mean that the column splice connections within those frames may grow very large (when bolted). Sometimes so large that to accommodate them in the floorplans, the connection type for the column splice must become a full penetration weld in the field. Full penetrations welds are time consuming and costly. They require additional certified and competent welders (along with inspections). Since we cannot monopolize the crane to complete these welds, it also means we must detail and buy temporary connections to take the welding out of the critical path for steel erection.

So why build moment frame buildings at all? Mostly because the braced frames can be obstructive and inconvenient. On the interior, they can be inconvenient for architectural floor plans. On the exterior, they can obstruct potential window locations. As we will cover later, the braces also create thorny conditions for detailing.   

Eccentrically Loaded Beams

Sometimes we need a heavier beam or heavier connection not just for the lateral system of the building but simply to allow for eccentric loading on the top flange. This can also occur at interior beams (not just the spandrel) due to permanent conditions—for example, the plank changing direction (blue)—or due to temporary conditions created during the erection sequence. The latter might not necessarily be known in advance to the engineer of record so can be even more problematic and sometimes dangerous. 

Shear Lugs

Moment frames also have significantly higher forces on the anchor bolts at the base of the column. For this reason, we often request (if the detail is not shown already) that the base plates for moment frame columns be detailed with shear lugs (which will be embedded and grouted into a pocket made by the foundation contractor). Since the shear lug does a lot of the structural work that the anchor bolts would otherwise do, they reduce the importance of the placement of those bolts. This way, when some are inevitably in the wrong location by a small amount, the necessary modifications are far less costly.

Added Tie Beams

When erecting a building, the steel contractor has to drop back and plumb columns and square the building. We have found it sometimes pays (in added speed and fewer issues) to add tie beams to projects in areas with too few beams for easy plumbing (for example, where the original design calls for less than a beam every three bays).

Sometimes these beams are temporary and will be removed. Sometimes (when concealed in a drop ceiling) we simply leave them in place.

Column Sizes

Another steel preference we have relating to ease of erection is regarding column size. Engineers will often show two-story columns and there are erectors out there using four-story columns. However, all else equal, our typical preference is for three-story columns. This length balances the positive of fewer overall connections (especially important for full penetration welded moment frame column splices) with total weight and difficulty of the column. This can also be influenced by crane logistics and size, as the columns can become especially heavy.

One qualification to this point is that it is often worthwhile to use a four-story column where necessary to avoid the inefficiency of having a 1-story remainder column at the top of the building.

Upset Beams

Without getting into Girder Slab, the most common way to handle the design concern of the depth of the steel beams is to upset the beams. This could mean, in certain areas, designing the plank to bear on the bottom flange. But more commonly it will mean adding angles (and stiffeners to pick them up) so that top of plank and top of steel can be close to the same elevation. If and when we see this condition, we usually question it because it can generate significant additional costs. The detail can also create temporarily unstable conditions due to eccentricity.

Elevator Support Steel

One of the most common architectural and engineering coordination issue we see during preconstruction had to do with MRL elevators and machine support. Elevator installers typically install their own machine beams (in blue below) but those beams require something to frame into. Without a ring of beams around the elevator shaft at the elevation between the roof level and the overrun bulkhead roof level (in yellow below), nothing is there to support them. Because the level of the elevator machines does not show up as a floorplan on the architectural drawings when there is no machine room, inexperienced structural engineers will often miss this altogether. The miss can be even worse if the elevator shaft is wrapped with braced frames, which would otherwise interrupt the location of this new ring of beams.

Rebar Couplers

The interface between the steel and grouted plank system often requires nelson studs but sometimes rebar will be required to be tied into the slab or as dowels for CMU walls. In those cases, the rebar will frequently be detailed by the engineer as welded to the steel (either in the field or the shop). Because we do not want to ship the steel with rebar pre-attached and because we almost always want to minimize site welding, we typically ask the engineer to add a detail for rebar couplers (welded at the shop and threaded in the field). The added cost for the couplers (which are not cheap) is often worth the savings in constructability. Where the bar can be hooked into the grouted plank, neither condition should be necessary.

Plank Saddle Headers

Sometimes engineers who are less experienced with plank will add steel framing to facilitate larger openings in the deck. In fact, the adjacent planks can often carry the load by introducing a plank header (not so different from steel or joist framing for this type of condition). In many cases, this can save steel weight and ceiling height.

Column Line, Slab Edge, and Face of Building

One of the most critical relationships in any steel building is the one between the perimeter column/beam line and the face of the building. Too distant and you will be faced with substantial increased costs for the slab edge grout pour stop (since larger dimensions will be too great for a typical, lighter gauge galvanized pour stop). In these conditions, expensive bent plates fabricated by the steel supplier may be required (in yellow below). On the other hand, too close and you will be faced with problematic details where the steel interfaces with the backup wall and where steel fireproofing now extends into the exterior wall cavity.  

It is also worth noting that the introduction of braced frames at the edge of slab may create complicated details (both where the column line is close to the face of building and where it is far). That is because the gusset plates commonly used to attach the braces to columns and beams will interfere with the plank (this interference can occur regardless of the plank span direction in relation to the gusset).

Even without a brace frame, and even when using bent plate pour stops, the condition where the CMU wall “flies by” the spandrel beam can be thorny. This is because of the need to laterally brace the wall. Where it is braced, the bond beam and top courses can become almost impossible to build. This detail that should be addressed early on in preconstruction or construction. One possible solution—developed and implemented by the Monadnock Compass Residences team—is to use bolted bent plate pour stops with a pre-attached u-shaped PTA anchor. This allows the steel and CMU installation sequences to be separated.

Bent plate pour stops can also be advantageous at the edge of the slab that surrounds the elevator shaft because they provide an excellent weld location where we can weld the elevator guiderail brackets. This can otherwise become a difficult or expensive issue on structural steel projects where the web of the steel beam that surrounds the elevator shaft is recessed far from the location of the elevator guiderail.

Relieving Angle Adjustment

Another critical aspect of the slab edge detail for structural steel projects is adjustability of the relieving angles in the field. This must be worked out in advance. It may be surprising, but some structural engineers will provide a detail with a relieving angle welded or bolted to the spandrel beam in a fixed position. Ideally, we will use a detail that provides for adjustability in all dimensions—which typically means adding clip angles and slotted hole connections. We want to be able to ship the beams with the angles pre-installed (with the bolts not torqued) so that the only field work is final precise adjustment and tightening.

Column Flange Orientation at Corridors

Since bolted column splices can take up a lot of space, it is important to check their location and orientation to make sure there is adequate space. One common issue occurs when the connection plate and bolts from splices at the interior column line protrude into the corridor or corridor framing. To avoid this, we like to make sure that columns have their flanges perpendicular to the corridor (with the web parallel). This allows us to conceal the column splice connection within the framing of the corridor demising wall.