What prerequisites do I need for Steel Building Geometry Guide?

You should be familiar with the relevant structural steel design code, have your project loads and geometry defined, and understand the limit states you need to check. The guide walks through each step with code references — have the applicable code edition available for verification.

Can I apply this guide to any steel design project?

The guide covers standard design procedures that apply broadly to steel structures. However, project-specific conditions (unusual geometry, dynamic loading, high-temperature service, fatigue, seismic demand) may require additional checks beyond the scope of this guide. Always consult the full code text for your project.

How do I verify the results from this guide?

Use the free calculators on Steel Calculator to run each check independently with your specific inputs. Cross-check against hand calculations using the referenced code clauses. A licensed Professional Engineer must review and stamp all final designs.

Steel Building Geometry Guide — Bay Spacing, Frame Types, and Clear Height

A practical guide to the geometry decisions that govern steel building design. Covers bay spacing optimisation, portal frame types, clear height vs eave height, roof pitch selection, and column base fixity considerations.

Bay Spacing — The Core Economic Decision

Bay spacing is the distance between adjacent portal frames along the length of the building. It is the single most important geometric decision because it determines the number of primary frames (and therefore the total structural steel tonnage) and the span of the secondary members (purlins and girts).

How Bay Spacing Affects Cost

Primary frame cost. Each portal frame adds a fixed tonnage of steel (columns, rafters, connections, base plates). Wider spacing means fewer frames, reducing primary steel cost. For a 48-metre-long building, the difference between 6-metre spacing (9 frames) and 8-metre spacing (7 frames) is two complete portal frames, or roughly a 22 percent reduction in primary steel.

Secondary member cost. Purlins and girts span between frames, so their required section depth increases with bay spacing. A purlin spanning 6 metres might be a Z150-15 (1.3 kg/m), while one spanning 8 metres requires a Z200-15 (2.0 kg/m) or Z250 (2.6 kg/m). The mass per metre increases faster than linearly because both the moment demand (proportional to span squared) and the deflection demand (proportional to span to the fourth power) grow with bay spacing.

Cladding cost. Wall and roof cladding span horizontally between girts and vertically between purlins. Most profiled metal cladding systems can span 1.5 to 2.0 metres between supports. Cladding cost is relatively insensitive to frame spacing because purlin and girt spacing is typically independent of frame spacing.

Erection cost. Wider frame spacing generally reduces erection cost because there are fewer frames to erect and fewer connections to bolt. However, the heavier individual members at wider spacing may require larger cranes. The net effect is usually favourable for wider spacing up to about 9 metres.

Recommended Bay Spacing by Building Use

Building Type	Recommended Spacing	Rationale
Agricultural shed	5-6 m	Controlled by standard cold-formed purlin spans
Light industrial workshop	6-7 m	Good balance of frame and purlin economy
Warehouse (no crane)	7-8 m	Fewer frames, standard purlin sections available
Warehouse (with crane)	6 m	Crane runway beams govern; tighter spacing reduces beam depth
Retail/supermarket	8-10 m	Fewer columns improve layout flexibility; heavier frames justified
Aircraft hangar	8-12 m	Large clear spans; frame weight dominates, so fewer frames preferred

The Designer Hub S0 geometry page provides real-time feedback: as you adjust bay spacing, the purlin and girt section suggestions update automatically based on the tributary span.

Frame Types — Selecting the Right Configuration

Single-Span Portal Frame

A single-span frame consists of two columns connected by a rafter with moment-resisting connections at both eaves. The ridge may be pitched (gable frame) or flat. This is the simplest and most common configuration for buildings up to about 30 metres span.

Advantages: simple fabrication, no internal columns, straightforward erection, well-understood behaviour. Limitations: rafter depth grows quickly with span (roughly proportional to span squared for a given utilisation ratio), making single spans above 35 metres often impractical with standard rolled sections.

Multi-Span Portal Frame

A multi-span frame has one or more internal columns dividing the total span into two or more bays. For example, a 48-metre-wide building might use two 24-metre spans with a central column line.

The internal column reduces the positive moment in each rafter span by approximately a factor of 2 to 4 compared to a single span of the same total width. For a uniformly distributed load on a continuous beam, the maximum positive moment in a two-equal-span configuration is approximately wL^2/11, compared to wL^2/8 for a single span where L is the individual span length. Since the individual span is half the total width, the effective moment reduction is roughly: (48^2/8) vs (24^2/11) = 288 vs 52.4, or a factor of 5.5. This dramatic reduction translates to substantially lighter rafters.

The trade-off is the cost of the internal columns and their foundations, plus the loss of uninterrupted floor space. For buildings with defined aisles (warehouses, factories), the column line can be placed along the aisle without functional penalty.

Lean-To Frame

A lean-to frame is a single-slope frame attached to an existing building at its high side. The high-side connection to the existing structure may be pinned (transferring only vertical and horizontal reactions) or moment-resisting (depending on the capacity of the existing structure). Lean-to frames are common for building extensions where the existing building provides lateral stability.

The high-side column is typically shorter or omitted entirely if the existing building column can carry the additional reaction. The lean-to rafter is designed as a simply supported member spanning from the existing building to the low-side column.

Crane Bracket Frame

When an overhead crane is required, the frame columns are extended above the crane runway level to support the roof structure, and a bracket or stepped column section supports the crane girder. The column is subjected to the combined effects of the roof load (axial), crane vertical load (axial plus bending from bracket eccentricity), and crane surge load (bending about the weak axis).

The bracket depth and stiffening must be designed for the concentrated crane reactions. For cranes over 10-ton capacity, the bracket and the column section below the bracket often govern the design.

Clear Height vs Eave Height

These two terms are frequently confused. Understanding the distinction is essential for accurate geometry input.

Clear height (hc). The vertical distance from the top of the finished floor to the underside of the lowest point of the roof structure. For a portal frame with haunches, this is typically the underside of the haunch tip. Clear height is the dimension that matters for the building user: it determines whether a forklift, racking system, or vehicle can fit inside.

Eave height (he). The vertical distance from the column base (top of footing or base plate) to the intersection point of the column centreline and the rafter centreline. This is the geometric input required by structural analysis software because it defines the frame node coordinates.

The relationship between the two is:

he = hc + haunch_depth_at_tip + rafter_depth + roof_slope_correction

Where the roof slope correction accounts for the fact that the haunch tip is slightly lower than the eave node because of the rafter slope. For typical portal frames with 5 to 15 degree roof pitches, the roof slope correction is small (10 to 40 mm) and can be neglected for preliminary design.

Example. A warehouse requires 6.0 metres clear height for racking. The trial rafter is a W24x76 (depth 607 mm) with a haunch at the eaves. The haunch tapers from 900 mm deep at the column face to 607 mm at the haunch tip. The underside of the haunch tip is approximately at the rafter soffit level. Eave height is therefore approximately 6.0 + 0.607 = 6.61 metres (using the rafter depth at the eave, plus allowance for the haunch geometry). Enter 6.6 metres in S0.

The Designer Hub S0 page provides a diagram showing the relationship between clear height, eave height, and roof pitch, updating in real time as you adjust values.

Roof Pitch Selection

Roof pitch is the angle of the rafter relative to horizontal, expressed in degrees or as a ratio (e.g., 1:10 = 5.7 degrees).

Structural Effects of Roof Pitch

As pitch increases, the frame behaviour transitions from "beam action" (where gravity loads are carried primarily by rafter bending) toward "arch action" (where gravity loads are carried primarily by rafter axial compression). The rafter axial force P introduces a P-Delta amplification of the rafter bending moment. For low-pitch frames (5 to 10 degrees), this effect is negligible. For steeper frames (above 20 degrees), the axial component becomes significant and must be included in the combined loading check.

Wind load also changes with pitch. Steeper roofs develop higher windward-wall pressures (because the roof presents a larger projected area to the wind) and higher uplift on the leeward roof slope. The pipeline handles this automatically through the wind load generation in S2.

Drainage and Snow Considerations

Minimum pitch for drainage: 5 degrees (approximately 1:11). Below this, water can pond in the troughs of profiled metal cladding and cause corrosion or leakage. Some standing-seam roofing systems can go as low as 1 degree, but these are specialty products not typical in portal frame construction.
Snow accumulation: Roofs with pitches below approximately 30 degrees are considered "flat" for snow load purposes in ASCE 7-22 Section 7.4, meaning the full ground snow load is applied without a slope reduction factor. The slope factor Cs = 1.0 for warm roofs (Ct <= 1.0) with slopes less than 30 degrees. Steeper roofs benefit from a reduction: Cs = 0.6 for slopes above 45 degrees if the roof surface is unobstructed and slippery.
Drifted snow: For roofs with changes in height (e.g., a high bay adjacent to a low bay), drifted snow loads per ASCE 7-22 Section 7.7 can substantially increase the snow load on the lower roof near the step. This is particularly relevant for multi-span frames with different eave heights.

Column Base Fixity

The column base connection determines how moment is transferred from the frame into the foundation. The choice of base fixity affects the frame analysis, the member sizes, and the foundation design.

Pinned Base

A pinned base transfers vertical load and horizontal shear but negligible moment (rotational stiffness less than approximately 20 percent of the column stiffness). It is achieved with a thin base plate, 2 or 4 anchor bolts, and no stiffeners.

Advantages: simple fabrication, minimal foundation moment, smaller footing size, no base plate stiffeners. Disadvantages: larger column and rafter sections (because the frame must resist sway without base moment restraint), higher lateral drift.

Pinned bases are standard for lighter single-storey buildings on good soil conditions. For a 20-metre span portal frame with 6-metre eave height, the difference in rafter moment between pinned and fixed base is approximately 10 to 20 percent.

Fixed Base

A fixed base provides full moment restraint (rotational stiffness greater than approximately 80 percent of the column stiffness). It requires a thick stiffened base plate, 4 to 8 anchor bolts, and a footing designed for overturning moment.

Advantages: smaller rafter sections, reduced drift, better frame stability. Disadvantages: larger and more expensive foundation (footing must resist overturning), more complex base plate fabrication, tighter anchor bolt tolerances.

Fixed bases are justified for taller frames (eave height above 8 metres), frames with overhead cranes, or buildings in high-wind or high-seismic regions where drift control governs.

Semi-Rigid Base

A semi-rigid base provides partial moment restraint (20 to 80 percent fixity). The analysis must account for the base rotational stiffness explicitly, which the pipeline handles through a rotational spring at the column base node. Semi-rigid bases are an intermediate option when a pinned base results in drift that slightly exceeds limits but a full fixed base is not justified.

The Designer Hub S0 geometry page allows you to select the base condition and see the effect on the frame model (moment diagram and deflection shape) before running the full analysis.

Building Length and Expansion Joints

For long buildings, thermal expansion and contraction must be considered. A 100-metre-long steel building experiences approximately 48 mm of length change between -20 degrees C and +40 degrees C (coefficient of thermal expansion for steel is approximately 12 x 10^-6 per degree C). This movement can overstress cladding fasteners and girt connections if not accommodated.

Expansion joints are typically provided at intervals of 60 to 80 metres for buildings without temperature control and 100 to 120 metres for heated buildings. The joint is formed by a double portal frame (two frames spaced 300-500 mm apart) with independent foundations and a sliding roof and wall cladding detail.

Practical Geometry Checklist for S0 Input

Before starting the Designer Hub pipeline, assemble the following geometric information:

Building length and number of bays (or bay widths)
Clear span width (inside face of column to inside face of column)
Required clear height (floor to underside of roof structure)
Roof pitch (degrees or ratio)
Frame type (single-span, multi-span, lean-to)
Column base condition (pinned, fixed, or semi-rigid)
Crane requirements (capacity, hook height, runway level) if applicable
Mezzanine or internal platform locations if applicable
Bracing locations (roof and wall bracing bays)

With this information in hand, S0 geometry input takes approximately 5 to 7 minutes. The Designer Hub geometry page provides input fields for each parameter with diagrams showing what each dimension means.

Frequently Asked Questions

What is the optimal bay spacing for a steel portal frame building?

The economic optimum for most industrial buildings is 6 to 8 metres. At 6-metre spacing, a Z150 purlin can span with single bridging. At 8 metres, a Z200 or Z250 purlin is needed, increasing purlin cost by about 25 percent but reducing the number of portal frames by 25 percent. The break-even point depends on local steel prices and erection costs. For buildings with overhead cranes, tighter spacing (6 metres) is preferred because the crane runway beam depth grows rapidly with span.

What is the difference between clear height and eave height?

Clear height is the vertical distance from finished floor to the underside of the lowest roof member (typically the haunch tip). Eave height is from the column base to the intersection of the column and rafter centrelines. Eave height equals clear height plus the rafter depth plus a small correction for roof slope. Clear height governs usable space; eave height is the geometric input for analysis.

When should I use a multi-span portal frame instead of a single-span?

Multi-span frames become economical above approximately 30 to 35 metres total width. A 48-metre-wide building using two 24-metre spans reduces rafter moments by a factor of approximately 5.5 compared to a single 48-metre span. The cost of the additional internal columns and foundations is typically less than the steel savings from the lighter rafters. Multi-span frames also reduce lateral drift.

How does roof pitch affect portal frame design?

Roof pitch affects structural action (steeper pitch shifts load from bending to axial), snow load (lower pitches are treated as "flat" roofs for snow accumulation, with no slope reduction factor), and drainage (minimum 5 degrees for reliable runoff with metal cladding). Most industrial portal frames use 5 to 10 degrees, balancing structural efficiency with metal building system conventions.

Related Resources

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.