Brace for the Future

Introducing Brace for the Future, a smart configurator tool that allows architects and engineers to explore and compare the use of three different lateral load resisting systems (Special Moment Frame, Buckling-Restrained Braced Frame, and Eccentrically Braced Frame) for a steel frame structure. It currently supports automatic framing of a cuboid structure.

Brace for the Future has a built-in algorithm that automatically selects the best system given the configuration of the spans, which is extremely helpful during conceptual design. Users can also manually select a desired system, as well as control the number of bays that will have braces. It will output the gross weight of the steel framing, as well as an adjusted weight and cost using the baseline cost of a moment frame that assumes the same type of columns and beam members.

Inputs

Revit Box Mass selector or length, width, and height as dimensions of the structure
Number of spans in each direction or maximum span length
Story Height
Intelligent Mode (Auto Selection of Bracing System) toggle
Bracing system selector
Number of bays to have braces
Steel shape selector for column, beam, and brace

Outputs

Gross Weight
Adjusted Weight
Adjusted Cost

Quick Start Guide

Define the geometry of the cuboid structure by either selecting a Revit Box Mass element or entering the Bldg Width, Bldg Height, and Bldg Length. Don’t forget to toggle the “Use Mass Form” selector.
Select the number of divisions in each direction or input the maximum spacing. Don’t forget to toggle the “Use Number of Divisions (vs Spacing)” selectors.
Input Floor to Floor Height.
Select whether to use intelligent mode. If not, select the desired building type: 1 for SMF, 2 for EBF, 3 for Chevron BRBF, 4 for Single Diagonal BRBF.
Select the number of bays of braces in each direction.
Select the shapes for beams, columns, and braces.
Run Script.

1. Mechanism of intelligent mode

The logic for automatically selecting the brace type is implemented in a code block and the algorithm is based on the proportions of span length vs. story height, because the efficiency of each type of bracing system is mainly dependent on the angle between the brace and the floor. A larger angle (greater slope) will result in a larger vertical component of the axial force in the brace, which reduces its efficiency and necessitates a bigger brace member. Thus, the hierarchy of braces is as follows:

If the ratio of the floor height and the span length exceeds 3.57, a moment frame is used. In fact, this indicates that the span length is probably too big and should ideally be reduced.
If the ratio is between 2.10 and 3.57, an EBF* is used.
If the ratio is between 1.68 and 2.10, a chevron BRBF is used.
If the ratio is between 0.84 and 1.68, a single diagonal BRBF is used.
If the ratio is less than 0.84, a moment frame** is used.

*: The current setup assumes that the link length of the EBF is 0.2L. This currently cannot be changed.

**: The tool prefers the use of a bracing system to a moment frame; thus, the hierarchy uses a moment frame when the span is too short for any type of braces to properly work. The user can force the use of a moment frame by switching to manual mode.

This hierarchy is based on the following logic:

An EBF is the most efficient when its diagonals are inclined at 40° to 50° and its link has a length of 0.15 to 0.20L. A non-symmetric EBF is avoided as there are no prequalified connections for this system, which increases structural cost and complexity.
A BRBF is the most efficient when diagonals more than 50° are avoided.

The tool uses the above hierarchy to automatically select the most suitable bracing system in each direction (u and v) using the ratio of floor height and span length in that direction.

2. Adjusted weight and cost computation

Assuming that the user inputs the same member shapes originally intended for a moment frame, the tool can output the adjusted weight that takes into account the much higher structural efficiency of the BRBF and EBF systems. In this case, although both BRBF and EBF have higher gross weights due to the additional braces, in theory the total weight of the lateral load resisting structure can be more than 50% less, because smaller shapes can be used in the beams and columns of the frame (compared to a moment frame). This functionality is intended to provide a rough estimate of the reduced weight without going through a full-scale structural analysis.
Furthermore, the cost savings is further exaggerated due to moment frames requiring much more complex connections than BRBF and EBF systems. Moment frame connections required sophisticated welding and other details such as the RBS detailing, which significantly increases design and construction costs. EBF will be cheaper than BRBF as buckling-restrained braces are proprietary, which may generate slightly higher costs.
Weight adjustment algorithm, considering the framing system in two directions separately:

A moment frame has a weight multiplier of 1.0
A BRBF has a weight multiplier of 0.9
An EBF has a weight multiplier of 0.8
(The multipliers used here are more conservative, considering the fact that the tool also creates some gravity beams and columns that are not in the lateral force resisting system, i.e., the moment frame is only part of the entire frame)

Cost adjustment algorithm (gross cost = weight * unit price of steel)

Cost of fabricated structural steel is $0.90 per pound
A moment frame has a cost multiplier of 1.2
A BRBF has a cost multiplier of 1.0
An EBF has a cost multiplier of 0.9

3. AutoBracePlacement Node

3. Dynamo implementation of brace placement

The brace placement relies on a custom node called “AutoBracePlacement.” It takes in the type of brace, number of bays, and two lists of points representing the upper and lower beam-column joints on a gridline.

The node automatically locates the points on the upper level (representing the corresponding bays) that braces will be placed within. It then draws a line between each two points and use a set of parameters determined from the type of brace ([0.4,0.6] for EBF, [0.5,0.5] for chevron BRBF, [1,1] for single diagonal BRBF, and [] for SMF) to create two new points on that line with Curve.PointAtParameter.

The two new points are then connected with the corresponding beam-column joint on each side on the lower level to form the line of the brace. The duplicated line for the single diagonal BRBF case is removed.

4. Dynamo Graph

Brace Placement Portion replacing Beam Systems Portion

4. Overall Dynamo logic

The overall structure builds upon Glenn’s example 8.3.A.2 but the beam systems portion is removed and replaced with nodes that implement intelligent mode and automatic placement of braces, as explained in Sections 1 and 3.

First, two surfaces are created that follow the desired floor plan using the input geometry. The surfaces are then used to create a grid of points (beam-column joints) using the desired span lengths, which are further joined to create framing lines. Section 3 explains how the grid of points is further used to create braces. Finally, the framing lines which only exist in one level are copied to the levels above to form the frame of the entire structure.

After the framing lines are generated, they are used to place elements in Revit using the Dynamo curves and desired member shapes. With the Revit elements created, the weight, adjusted weight, and adjusted cost are computed as explained in Section 2.

Ethan Sun