How Do You Choose the Right Column Grid and Bay Spacing for a Steel Structure Building? A Complete Framing Layout and Procurement Guide

A complete engineering and procurement guide for overseas B2B buyers on choosing the right column grid and bay spacing for a steel structure building — spans, frame spacing, eave height, cranes, racking, foundations, worked cost example, and a pre-order checklist.

1. What Are Column Grid and Bay Spacing in a Steel Structure Building?

Before comparing quotations, a buyer needs to speak the same vocabulary as the supplier's design office. Three terms carry most of the weight.

Transverse span is the distance between the two column lines across the building's width. In a single-span portal frame, this is also the clear span—the unobstructed width the operation actually gets. When a supplier says "30-meter span warehouse," they mean the frames bridge 30 m from column to column.

Longitudinal bay spacing (also called frame spacing) is the center-to-center distance between adjacent frame lines along the building length. A 100 m long building with 8 m bay spacing has roughly 13 frame lines and 12 bays. Each frame line carries a primary frame; the space between frames is spanned by secondary members—roof purlins and wall girts.

Eave height is the height of the column from the base plate to the point where the roof slope begins at the low edge. It is the dimension that controls racking, cranes, doors, and cladding area, yet it is the one buyers most often copy from a neighboring project without checking.

The grid is the skeleton decision. Everything else hangs on it:

The number of primary frames equals the number of frame lines—set by bay spacing.
The number of column foundations and anchor bolts follows the frame count, plus any interior column lines.
Purlin and girt spans equal the bay spacing, so secondary steel sizing follows directly.
Cladding, roof panels, and skylight modules repeat on the purlin rhythm.
Internal clear space—aisles, racking runs, crane coverage—is either enabled or blocked by where columns land.

Picture the geometry this way: take one portal frame spanning the width of the building, and copy it along the length at a fixed interval. Connect the frames with purlins running lengthwise on the roof and girts running lengthwise on the walls. Add bracing in a few selected bays. That repeating interval is the bay spacing; the depth of each copied frame is the span; the height of each frame is the eave height.

Why does this matter for procurement? Because two quotes for an identical-looking 40 × 80 m building can differ substantially in steel tonnage, foundation count, and price simply because one supplier assumed a single 40 m clear span and the other assumed a two-bay layout with a center column line. The grid is not a detail to be negotiated after the fact. It is the first engineering decision, and it should be made deliberately before quotations are requested.

2. Why the Column Grid Decision Comes Before Everything Else

The grid is not one decision among many. It sits upstream of every quantity that appears in a quotation.

Steel tonnage split. The grid determines how steel divides between primary frames (columns and rafters) and secondary members (purlins, girts, bracing). A wide-span, wide-bay grid concentrates steel in the frames; a narrow-bay grid shifts weight into secondary steel. The two extremes can differ by tens of kilograms per square meter.

Foundation count. Every column needs a foundation and an anchor bolt set. A 100 m building at 6 m bays needs about 17 frame lines and roughly 34 column foundations; at 9 m bays, about 12 frame lines and 24 foundations. In ground with poor bearing capacity, foundation cost can be a meaningful share of the project, so fewer frames are not automatically cheaper once excavation and concrete are counted.

Crane economics. If the building carries overhead cranes, the runway beams span between frame lines. Wider bay spacing means heavier runway beams and stiffer frames to absorb crane surge loads. The grid decides whether a crane runway is economical at all.

Internal layout fit. Racking lines, aisles, production machines, and vehicle circulation all occupy rectangles. If the grid doesn't align with those rectangles, you either accept columns in awkward positions or pay for special framing to move them.

Future expansion. Portal frames extend naturally along their length—add bays at the end wall, remove the cladding, continue the purlins. But extension across the width means new frames at a deeper span. A grid chosen with future bays in mind saves real money later.

Cladding and lighting modules. Roof sheets, skylights, and wall panels cut and repeat on the purlin rhythm. A coherent grid keeps those modules uniform, reducing offcut waste and simplifying installation.

The procurement trap is straightforward: two suppliers quoting a "40 × 80 m warehouse" may price entirely different structures. One assumes a 40 m clear span; the other assumes two 20 m bays with a center column row and half the frame weight. Neither is wrong—the difference is an assumed grid that the buyer never specified.

Fix the grid, or at minimum the governing constraints, before requesting comparable quotations. State the span (or allowable internal columns), the bay spacing, the eave height, and the crane or racking requirements in the enquiry. Only then can competing offers be compared on like-for-like engineering rather than on hidden assumptions.

3. The Three Dimensions of the Grid: Span, Bay, and Height

Each of the three grid dimensions has its own cost driver, and each responds to different operational needs.

Transverse span

Cost rises disproportionately with span. The bending moment in a portal frame grows with the square of the span (roughly, for a given load), so rafter depth and weight climb quickly as the width increases. Clear span, however, has real operational value: logistics flow, aircraft maintenance, sports halls, and large machine layouts all benefit from an unobstructed width. The rule is to buy span only where the operation needs it.

Bay spacing

Bay spacing is an economic trade-off. Wider bays mean fewer primary frames and fewer foundations—but deeper and heavier purlins and girts spanning those bays. Narrower bays reverse the balance: more frames and foundations, but lighter secondary steel. In most designs, the economic optimum for standard cold-formed purlin systems lands in the 6–9 m range.

Eave height

Height is driven from inside the building: pallet stacking, racking height, crane hook height, and mezzanine or machinery clearance. Each extra meter adds wall cladding and girt area on all sides, increases column weight and bracing demand, and enlarges the wind-exposed surface. Height is usually the cheapest dimension to increase slightly and the most expensive to increase a lot.

Dimension	Primary cost driver	What typically pushes it up
Transverse span	Frame weight grows steeply with span² behavior	Operations needing unobstructed width: logistics flow, aircraft, machinery
Bay spacing	Balance of frame count vs. purlin/girt weight; foundation count	Roof plant, service corridors, mezzanine modules
Eave height	Wall area, frame weight, bracing per extra meter	Storage height, crane hook height, doors, mezzanine headroom

A practical note: the three dimensions interact. A wide span plus a wide bay plus a tall eave is the most expensive corner of the design space, and it should be reached only when the operation genuinely requires all three. Most buildings do not. When a brief arrives specifying 36 m clear span, 9 m bays, and 12 m eave height, a competent supplier will ask which of those numbers came from an operational need and which from habit.

Set each dimension from the operations inside the building, not from a template. The next chapters give the typical ranges and the mechanics behind each one.

4. Typical Column Grid Ranges for Common Steel Building Types

The ranges below are common planning starting points drawn from typical practice, not limits. Local codes, wind and seismic conditions, crane loads, and the specific function of the building override them. Use them to sanity-check a quotation, not to dictate the design.

Building type	Typical transverse span	Typical bay spacing	Typical eave height	Note
Single-span warehouse	18–40 m	6–9 m	6–10 m	Clear span for free floor flow; cost climbs fast beyond ~30 m
Multi-bay workshop	15–36 m per bay, 2–5 bays	6–9 m	8–12 m	Cranes commonly 5–32 t per bay; interior column rows carry runway loads
Logistics / 3PL warehouse	24–36 m, or multi-bay	8–9 m	8–12 m	Bay spacing often aligned to mezzanine and rack modules
Aircraft hangar	30–60 m+ clear	8–10 m	10–15 m+	Tail-height doors dominate the elevation; special door framing
Agricultural building	12–24 m	4.5–6 m	4–7 m	Light loads allow economical narrow bays and low frames
Retail / commercial	16–24 m	8–9 m	5–8 m	Showroom clear space; often mixed with mezzanine office areas

A few patterns are worth noticing.

First, warehouse-type buildings cluster at 6–9 m bays because that is where standard purlin systems are most efficient. Second, crane workshops tend toward the lower end of bay spacing because runway beams and crane-loaded columns favor closer frame lines. Third, agricultural buildings are forgiving: light loads and wide tolerances let the designer use narrow bays and simple frames, which is why their steel weight per square meter is the lowest of any category.

For a deeper treatment of the most common case, see our portal frame warehouse guide, which compares portal framing with other warehouse structure types.

When a quote departs from these ranges, ask why. Sometimes there is a good reason—a 42 m span to clear an existing machine, a 10 m bay to match a future mezzanine module. Sometimes it is simply a supplier default that costs you steel. Either way, the number deserves a question before the contract is signed.

5. How Transverse Span Affects Steel Quantity and Cost

The mechanics are simple and unforgiving. A portal frame rafter works primarily in bending under roof load, snow, and wind uplift. Bending moment scales roughly with span times load, and for distributed loads the moment grows on the order of the span squared. Rafter depth, flange thickness, and weight all follow. Column weight rises too, because the columns must resist larger horizontal thrust from a wider frame.

At planning level, typical behavior looks like this: a 20 m portal frame may need on the order of 8–12 kg/m² of primary steel, while a 40 m clear span can climb well beyond that range depending on loads, frame geometry, and haunch detailing. The honest way to compare is supplier calculation packages—member schedules with weights—not rules of thumb. Two competent designers can differ by 15% or more on the same span through rafter taper, haunch depth, and frame spacing choices.

The buyer's decision rule is therefore an operational one, not a technical one:

Pay for clear span only when the operation needs unobstructed width. Forklift cross-flow, dock orientation, aircraft movement, and certain machine layouts genuinely need it. A general storage warehouse usually does not—a center row of columns at a 7–8 m module rarely disturbs racking runs, and back-to-back rack layouts can absorb interior columns almost invisibly.

Compare the two options for the same 40 × 80 m footprint:

Option	Structure	Relative steel behavior	Best when
Single 40 m clear span	Deep rafters, heavy columns	Highest primary steel per m²	Unobstructed width is operationally required
Two 20 m bays	Lighter frames plus one interior column line	Notably lower steel and foundation loads per m²	Racking, shelving, or machinery can accept interior columns

Detailed cost trade-offs sit in our steel structure estimation guide and the steel building cost breakdown.

When large span is genuinely required, modern fabrication makes it manageable. Our 48-meter clear-span warehouse solutions show what large-span practice looks like in production: deeper welded H-section rafters, controlled camber, and heavier base connections. The structure works well—it simply costs more per square meter than the same footprint with a center column row. Decide which you are buying.

6. How Bay Spacing (Frame Spacing) Affects Steel Quantity

Bay spacing works through the secondary framing. The bay dimension is simultaneously the purlin span on the roof and the girt span on the walls. Widen the bays and you remove frames and foundations—but every purlin and girt now spans farther, so they get deeper and heavier, and the connections anchoring girts against wind suction get heavier too.

Beyond a certain point the trade-off turns negative. Standard cold-formed Z and C purlins, including lapped continuous systems, reach their efficient depth limit at around 9 m spans. Past that, purlin weight grows quickly, lap detailing becomes awkward, and sag rods or sub-purlins may be needed. For most portal-frame buildings, economic bay spacing lands between 6 and 9 m.

Bay spacing	Frames per 100 m length	Purlin / girt demand	Foundation count	Best suited to
6 m	~17 lines	Lightest purlins, shortest girt spans	Highest	Crane workshops, heavy wind zones, tight member sizes
7.5 m	~14 lines	Moderate, efficient Z-purlin laps	Moderate	The general-purpose default for most warehouses
9 m	~12 lines	Deepest standard purlins; efficiency limit	Lowest	Mezzanine or rack module alignment; fewer frame erections

Note what moves in each row. Frame count per 100 m and foundation count move together—that is the primary-side saving of wider bays. Purlin demand moves the opposite way—that is the secondary-side cost. The optimum is where the sum of frame weight, purlin weight, foundation cost, and erection time is lowest, and that point shifts with steel prices, ground conditions, and wind zone.

Secondary member selection deserves its own attention; our purlins and girts selection guide covers sizing, lap systems, and anti-corrosion choices in detail.

A final point on erection: wider bays mean fewer frames to stand, which shortens erection time per ton of primary steel—but the heavier purlins and the longer spans make each secondary lift slightly more involved. In practice the erection effect is modest within the 6–9 m band. Do not stretch bays beyond 9 m to save frames unless a module below (racks, mezzanines, roof plant) demands it; the secondary steel usually takes back the savings.

7. Eave Height and Clearance: The Dimension Buyers Underestimate

Eave height is the grid dimension most often copied instead of calculated. A buyer sees a neighbor's 8 m building and specifies 8 m—then discovers the racking, the crane, or the fire truck clearance doesn't fit. Height errors are asymmetric: too low is a redesign, too high is money spent on cladding, girts, and steel you never needed.

Start from what the building must contain.

Pallet racking and sprinkler clearance. Tall racking plus the top-loading clearance plus sprinkler and lighting zones below the roof structure sets a minimum underside-of-structure height. A 10 m rack need does not mean a 10 m eave—it means roughly 10 m clear under the deepest roof member at the rack location, which translates differently depending on frame depth and roof slope.

Crane hook height. The chain most buyers miss: required hook height + trolley and hoist body + runway beam depth + connection and deflection allowance = minimum eave height at the crane column line. A 5 t crane with a 6 m hook height commonly lands around 8 m eave once the stack is added. Crane buildings are almost always taller than buyers first assume.

Mezzanine headroom. A two-level office block needs about 2.7–3 m headroom per level plus the floor structure depth. Under a roof, that pushes eave height past 7 m quickly.

Doors. Door head height is a hard constraint: a 4.5 m high dock door cannot pass under a 5 m eave once the lintel and frame depth are deducted.

Now the cost side. Every extra meter of eave height adds wall girt area on all four elevations, wall cladding on all four elevations, column weight on every frame line, and additional bracing demand because the wind-exposed surface grows. The increase is roughly linear per meter for cladding, but frame weight and bracing rise faster on tall, crane-loaded columns.

A short method that works:

Take the tallest stored unit, rack, machine, or vehicle.
Add handling clearance (forklift mast above load, crane lift paths, typically 0.5–1 m).
Add structure depth (rafter depth at that point, or runway and crane stack).
Round up to a sensible module—most fabricators work cleanly at 0.5 m increments.

Do not choose eave height because a neighboring building has it. Height is cheap to specify correctly on paper and expensive to fix in steel.

8. Functional Requirements That Reshape the Grid

The blank-canvas grid rarely survives contact with what the building must actually do. Four functional requirements most commonly reshape it.

8.1 Overhead Cranes

Crane columns carry vertical wheel loads, surge, and longitudinal braking forces on top of normal frame loads, so crane brackets load columns heavily and locally. Crane buildings usually favor bay spacing around 6–7.5 m, because runway beams span between frame lines—wider spacing means heavier runways and stiffer frames to control deflection under moving loads. State crane capacity, span, and duty class explicitly in the design brief so the frame is designed for the crane, not upgraded afterward. See our overhead crane workshop design guide.

8.2 Pallet Racking and Automation

Rack modules repeat in multiples of pallet width and aisle width, and those multiples should align with column positions so no column lands in an aisle or mid-rack run. Back-to-back rack runs are forgiving of interior columns and can absorb a center grid cheaply. ASRS and automation systems are less forgiving: they demand tighter deflection tolerances on the rack-supporting structure and often impose their own grid logic, including dedicated column lines under high-bay equipment. See our ASRS warehouse design guide.

8.3 Mezzanines and Two-Story Office Blocks

A mezzanine grid almost never matches the roof grid naturally: office partitions want 4–6 m modules, while the roof frames sit at 7–9 m. Decide early whether mezzanine columns land on the main column grid—cheaper, because loads drop straight into frame foundations—or between frames, which requires local beams or transfer framing to collect the load and deliver it to frame lines. The second option works, but it adds steel and connection complexity that a slightly adjusted roof grid could have avoided. See our steel mezzanine engineering guide.

8.4 Doors, Openings, and Interior Walls

Main portalized door openings—dock portals, vehicle doors, hangar doors—interrupt the frame line they sit on and usually need reinforced jambs, lintel framing, and local bracing between frames. Where possible, position doors on frame lines or half-bay positions so the surrounding framing can be detailed as standard. Internal fire walls similarly want to sit on a frame line: a wall landing mid-bay needs its own spanning structure and connection back to the frames. Coordinating openings with the grid during layout costs nothing; coordinating them after fabrication costs steel and schedule. See our doors, windows and wall openings guide.

9. Climate and Site Conditions That Influence Grid Choices

The column grid is not chosen in a vacuum. Wind, snow, and seismic loads all scale with frame spacing, and site-specific climate can push the optimum bay module several meters away from where a "standard" calculation would land it. Buyers who hand a supplier only the building footprint, without climate data, invite designs that later fail a code review or need costly rework.

Wind uplift and coastal exposure

Roof uplift on a portal frame building concentrates at purlin clips, at the eave and ridge zones, and at the anchorage of each frame line. Wider bays mean longer purlin spans carrying more tributary area per fastener, so the demand on clips and screws climbs steeply. In typhoon and cyclone regions, it is common practice to tighten purlin spacing into the 1.0–1.3 m range rather than the 1.5 m common inland, and to specify higher-capacity clip systems in edge and corner zones. If your site has a basic wind speed above roughly 40 m/s, tell the supplier before the grid is fixed—a 9 m bay that is economical inland may become expensive once the secondary framing is upsized to survive storm events.

Snow drift and multi-bay valleys

Drift snow accumulates where geometry traps it: against the leeward side of a taller adjacent wall, and in the valley between adjacent bays of different heights in multi-span buildings. Valley drift can double or triple the local roof load over a strip several meters wide, and that load lands squarely on the purlins spanning between the frames bounding the valley. Where drift governs, wider bays suffer more, because the purlin span over the drift zone grows. Options to discuss with the engineer include stepping bay widths so the drift zone falls near a frame line, adding a parapet or raising one bay's eave to control the drift pocket, or locally thickening insulation to manage partial-melt refreezing. If an adjacent structure exists or is planned—yours or a neighbor's—flag it in the RFQ.

Seismic zones and grid regularity

Seismic design rewards regularity. A symmetric, repetitive grid with consistent frame stiffness distributes lateral load evenly and keeps the analysis simple. Irregular grids—mixed bay widths, offset bracing bays, a long open side with no cross frames—concentrate demand at a few locations and force heavier connections. In moderate-to-high seismic zones, plan the bracing layout together with the grid so that at least one braced bay per wall line falls at each building end, and confirm the brace configuration against door and window positions. For a deeper treatment of stability systems, see our bracing system design guide.

Site constraints that shape the grid

Soil bearing capacity: weak soils penalize dense grids because every frame line adds a foundation; see Chapter 11.
Slope and cut-fill: frame lines crossing a grade break need foundation step details, which cost more than flat-site foundations.
Existing underground services: foundation positions may need to dodge utilities, which argues for fewer, wider-spaced frame lines.
Crane or vehicle access corridors: a forklift or truck aisle may demand an unusually wide bay at one location, breaking grid repetition—identify this early so the engineer can design a special end bay rather than an entirely irregular grid.

Decision rule: provide wind speed, snow load, seismic zone, and a geotechnical summary in your initial inquiry. A supplier who quotes a grid without this data is pricing a guess.

10. How the Grid Cascades into Purlins, Girts, and Cladding

Frame spacing sets off a chain of dimensional decisions through the whole enclosure. Understanding the chain helps you read a quotation critically—many grid problems first show up as an oddly heavy purlin schedule.

The two-span rule

Two relationships govern everything:

Frame spacing = purlin span. Roof purlins run transversely across the frames. At 6 m frame spacing, purlins span 6 m; at 9 m, they span 9 m—and their required depth and weight grow faster than linearly because bending demand scales with span squared.
Purlin spacing = cladding span. Roof sheeting and wall cladding span between purlins and girts. Common purlin spacing falls in the 1.0–1.5 m range, chosen so the chosen sheet profile meets its load-span capability under local wind and snow loads. Tighter purlin spacing allows thinner sheets; wider spacing demands thicker or deeper profiles.

Why lapped, continuous purlins make wider bays viable

Z-purlins are typically installed in lapped runs over the frames, making the purlin line structurally continuous rather than a series of simple spans. A continuous multi-span purlin over 6–9 m frame spacing behaves far more efficiently than simple spans of the same length, which is exactly why 6–9 m is the most common frame-spacing band in modern steel warehouses: the lap continuity absorbs the span penalty up to about 9 m, and beyond that the section weight climbs noticeably. When a quotation shows purlins noticeably deeper than 200–250 mm at a stated bay spacing, or heavier gauge than you expected, it is usually a signal that the bays are wide relative to the load—or that the load assumptions are conservative. Ask the supplier to state purlin spans, spacing, and section designations line by line; a transparent schedule is easy to verify, and the relationship between frame spacing and purlin size should be self-evident once you see it.

Cladding module rhythm

Corrugated and trapezoidal profiles have a fixed effective coverage width—commonly around 1.0 m per sheet after laps. Purlin and girt spacing should land on sensible multiples of that coverage so sheet joints fall on support lines, fasteners land in flats, and the visible rhythm of the elevations stays regular. Daylighting strips (translucent roof panels) and ridge vents are manufactured at these same module widths, so aligning purlin spacing with the cladding module keeps openings, light panels, and vents cutting cleanly into the framing without bespoke infill pieces. This coordination sounds trivial; in practice it is where material waste and awkward site cutting originate when it is ignored.

Girts follow the same logic

Wall girts span horizontally between columns, so wall cladding span depends on frame spacing too. Where frame spacing exceeds about 8 m and wind loads are significant, girts may need sag rods or intermediate struts to control deflection under wind suction. If the architectural elevation requires panels at a specific visual rhythm—rib spacing, window head lines—check that girt positions can accommodate them before the frame spacing is locked.

For a full comparison of enclosure options that ride on this framing, see our roofing and cladding system selection guide.

11. Foundations: Every Extra Frame Line Costs a Foundation

Foundations are the quiet cost driver of dense grids. They sit below the concrete slab and below the line of sight in most quotations, which is why they are so often underestimated by first-time buyers comparing steel tonnage only.

Count the columns

For a single-span portal building, every frame line has two columns, so:

> Number of foundations = 2 × number of frame lines.

A 40 × 80 m building at 6 m bays needs 14 frame lines and 28 independent foundations, each with its own anchor bolt group, pad or strip footing, and, on poor soils, possibly a pile. At 10 m bays the same building needs 18 foundations. That difference is not cosmetic—a pad footing with anchor cage commonly represents a meaningful share of total civil cost, and the ratio worsens on weak ground, where each footing grows in size or converts to a piled foundation. Two practical implications follow:

On poor soils, widen the grid. Larger pads or piles multiply the penalty of dense frame spacing faster than the heavier frames and purlins compensate.
Anchor bolts are designed with the grid. Bolt diameter, projection, and group layout depend on column base reactions, which scale with frame spacing. Coordinate the anchor bolt package with the same engineer who sets the grid—see our anchor bolt design and installation guide.

Where the cost actually goes

Widening bay spacing shifts money between buckets: fewer foundations and lighter total main steel on one side, heavier and deeper purlins and girts on the other. There is no universal winner. The cheapest grid is site-specific—it depends on your soil report, your local concrete and excavation prices, your wind and snow loads, and your internal layout. A grid that minimizes steel tonnage can maximize civil cost, and vice versa. Ask any serious supplier to quote steel and assumed foundation counts separately so you can substitute your own civil costs into the comparison. Chapter 12 works a full example of exactly this arithmetic.

12. Worked Example: Comparing Three Grid Options for a 40 × 80 m Warehouse

Important framing: this is a planning-level illustration using assumed unit figures to demonstrate the method. It is not a quotation, and the dollar figures should not be reused for budgeting. Recompute with your supplier's actual unit weights and your local civil costs.

The building

40 m transverse clear span, single portal frame
80 m longitudinal length, 8 m eave height
Floor area: 3,200 m²
Ground conditions: assume a uniform bearing soil so each column foundation (including anchor bolts) costs roughly USD 180—illustrative only

Assumptions (fixed across all options)

Main frame weight: approximately 3.2 t per frame line
Secondary steel (roof purlins + wall girts): approximately 8 kg/m² of floor area at 6 m bays, 10 kg/m² at 8 m bays, 12.5 kg/m² at 10 m bays
Fabricated and delivered steel: approximately USD 1,000 per tonne (illustrative)
Per-foundation cost including anchor bolts: approximately USD 180 (illustrative)

The three options

Option A: 6 m bays → 80/6 rounds to 13 bays, 14 frame lines, 28 foundations
Option B: 8 m bays → 10 bays, 11 frame lines, 22 foundations
Option C: 10 m bays → 8 bays, 9 frame lines, 18 foundations

The arithmetic

Main steel (frames × 3.2 t):

A: 14 × 3.2 = 44.8 t
B: 11 × 3.2 = 35.2 t
C: 9 × 3.2 = 28.8 t

Secondary steel (kg/m² × 3,200 m²):

A: 8 kg/m² = 25.6 t
B: 10 kg/m² = 32.0 t
C: 12.5 kg/m² = 40.0 t

Total steel:

A: 44.8 + 25.6 = 70.4 t
B: 35.2 + 32.0 = 67.2 t
C: 28.8 + 40.0 = 68.8 t

Steel cost at USD 1,000/t:

A: USD 70,400 B: USD 67,200 C: USD 68,800

Foundation cost at USD 180 each:

A: 28 × 180 = USD 5,040 B: 22 × 180 = USD 3,960 C: 18 × 180 = USD 3,240

Item	A: 6 m bays	B: 8 m bays	C: 10 m bays
Frame lines / foundations	14 / 28	11 / 22	9 / 18
Main steel (t)	44.8	35.2	28.8
Secondary steel (t)	25.6	32.0	40.0
Total steel (t)	70.4	67.2	68.8
Steel cost (USD)	70,400	67,200	68,800
Foundation cost (USD)	5,040	3,960	3,240
Combined (USD)	75,440	71,160	72,040

What the numbers say

Option B (8 m bays) comes out lowest on combined cost. Option C nearly ties: the heavier purlins required at 10 m spans almost exactly offset the savings from six fewer foundations. Option A loses on cost but wins only when purlin-span-driven loads—heavy cladding, high snow, Crane Class loads, roof-mounted equipment—or a rack module of 6 m dictates it. Notice also that the three totals sit within about 6% of each other; in real projects, the foundation figure is the most volatile, so a site with weak soil would push the optimum decisively toward wider bays, while heavy snow would push it back toward tighter spacing.

The takeaway is the method, not the dollars: build this same table with your supplier's real unit weights and your local foundation costs before committing to a grid. Any competent manufacturer can produce it in a day.

13. Grid Planning for Future Expansion

Expansion is cheap to plan and expensive to retrofit. Three decisions made at design stage determine whether Phase 2 is a bolt-on operation or a demolition project.

Fix a repeatable bay module now

Choose a bay module the next phase can repeat exactly—same width, same frame section logic, same eave height. A Phase 1 building on a 9 m module expands cleanly with 9 m bays; a building with mixed 7 m and 9.5 m bays expands awkwardly, forcing special frames at the junction. If expansion is plausible even at 30–40% probability, standardize the module. The cost of doing so at design stage is close to zero.

Design a removable end wall

The end wall where growth will occur should be a "soft" end: light gable framing—posts, girts, and diagonal rod bracing—rather than a rigid portal frame, so it can be unbolted, the frame line extended, and the panels relocated to the new end wall. If Phase 1 already has a full rigid portal at that end, it still works, but the dismantled frame may not match the new one. Specify the soft end explicitly in the RFQ; it is a small steel-tonnage difference that saves real cost later.

Plan for thermal movement on long buildings

If Phase 2 will push total length past roughly 100–150 m of uninterrupted building—depending on restraint conditions, climate range, and cladding system—thermal expansion joints become a live issue. Ideally, locate the joint at a doubled frame line: two frame lines placed back to back at the joint location, each carrying half the tributary load, which cleanly separates the two structural segments. Doubling a frame line is far easier to arrange when it is anticipated in the original grid rather than improvised later. For joint spacing logic, detailing, and the conditions that trigger them, see our expansion joints guide.

Decision rule: if expansion is likely, buy the grid that Phase 2 can inherit. If expansion is truly remote, say so explicitly and let the engineer optimize for Phase 1 alone.

14. How the Grid Interacts with Roofing, Walls, and Openings

The grid and the envelope negotiate constantly. Openings, sheet modules, and drainage details all want to land on framing lines, and the cheapest resolutions come from coordination before fabrication, not after.

Sheet coverage versus purlin spacing

Corrugated and trapezoidal roof sheets commonly provide about 1.0 m of effective coverage per sheet after side laps. Purlin spacing should be set so that sheet end laps and seams fall on or near support lines and that daylighting panels—which come in the same module widths—drop in without cutting. When purlin spacing ignores the coverage module, the roof accumulates off-cuts, fasteners land in ribs instead of flats, and the visible rhythm of the roof degrades. This is a five-minute check on the grid plan that prevents months of site complaints.

Repeating eave details

Gutters, downpipes, eave flashings, and trim repeat per bay. Regular bay spacing means every eave segment is identical—efficient to fabricate and fast to install. Irregular spacing forces a mix of gutter lengths and trim transitions at each change of module, adding labor and leak risk at every transition. If the grid must vary somewhere, confine the variation to one end bay and keep the balance uniform.

Daylighting strips, vents, and local framing

Translucent daylighting strips and roof vents interrupt standard purlin runs. Each needs local framing: trimmer purlins framing the opening edge, sometimes a doubled purlin carrying the interrupted load, and closure flashings matched to the sheet profile. Ridge ventilators and smoke extraction vents span across multiple purlin spaces and need heavier local support. These elements are individually small but collectively add up; count them on the layout and have the engineer detail the trimmer framing rather than improvising on site.

Door and window openings between frames

Large doors—roller shutter openings, dock doors, vehicle entrances—frequently fall between frame lines. The gable or sidewall framing then needs jamb posts at each side of the opening and a lintel beam spanning between the frame line and the jamb posts, with bracing locally rearranged if it crossed the opening zone. Every oversized opening adds steel that never appears in the "standard" tonnage estimate. Mark every planned opening—position, width, height, and headroom clearance—on the grid plan before finalizing frame spacing. An opening schedule drafted after the frames are ordered is how buyers end up paying for reinforcement kits and site welding.

For types, sizing, and placement strategy of doors and openings, see our doors and wall openings guide.

15. MEP, Ventilation, and Lighting Coordination

A steel building's mechanical, electrical, and plumbing systems live off the frame. Coordinating them with the grid before fabrication is one of the highest-leverage, lowest-cost activities in the whole procurement sequence.

Routing between frames

Ducts, cable trays, and process piping route longitudinally between frame lines, hanging from purlins or from brackets welded or bolted to the frames. Two spacing facts matter: wider bays give longer unsupported tray spans (heavier tray or more hangers), and each frame line is a natural bracket point. If you know where major duct headers and cable trunking will run, mark them on the grid plan—the engineer can pre-position framing or confirm that purlin capacity covers the suspended load at those locations.

Sprinkler and fire zones

Sprinkler zoning and branch line layout follow the bay rhythm. Branch lines run between purlins or frames, and drops land near lights and rack aisles. In warehouses with in-rack sprinklers, the rack layout drives drop positions—another reason rack modules and the column grid should be designed together (Chapter 5 of the full guide). Confirm the local fire authority's density and zone requirements early; retrofitting zoning changes onto a fabricated structure is painful.

Lighting alignment

High-bay lighting rows typically align with frame lines or mid-bay lines so fixtures hang from a consistent support point and produce uniform illumination at a predictable spacing-to-mounting-height ratio. If lighting rows will sit mid-bay, verify the purlin can carry the fixture load or provide a local framing detail. Aisle-based lighting in racked warehouses should be laid out against the rack module and the grid simultaneously.

Vents and smoke extraction

Ridge ventilators run along the ridge line between purlins and need continuous local framing and flashing. Smoke vents and roof access hatches are point openings requiring trimmer purlins on all four edges. Coordinate their count and position with the ventilation calculation—see our ventilation design guide—and lock positions before purlin fabrication.

The coordination drawing

Issue an MEP coordination drawing—framing plan overlaid with duct routes, trays, lighting, sprinkler drops, and vent openings—before fabrication starts. Any bracket, trimmer, or local reinforcement identified at this stage is fabricated into the primary and secondary steel in the shop, with the dimensional control that implies. Brackets added later on site mean field welding or bolting onto finished members, higher labor cost, and often a coating repair at each touch point. For a full sequencing framework, see our MEP planning guide.

16. Common Column Grid Mistakes Overseas Buyers Make

These eight mistakes appear repeatedly in cross-border steel building projects. Each one is avoidable with a single deliberate step early in procurement.

Defaulting to clear span without an operational reason. Clear span is a premium product. If the interior will never need unobstructed width beyond, say, 30 m—for wide-machine aisles, sports use, or large equipment—interior columns on a multi-span grid save steel and money. Choose clear span only when the operation justifies it.
Ignoring rack module alignment. Pallet racking comes in fixed module widths. If the grid lands between rack modules, every aisle layout fights the columns. Align rack module, column positions, and bay spacing at the same time, or accept wasted floor area for the life of the building.
Fixing eave height before the process layout. Eave height follows from the tallest stored item, the highest rack beam, the crane hook requirement, and the clearance under mezzanines. Locking a number before the internal layout is set guarantees either obstruction or paying for unused height.
Forgetting crane data in the RFQ. Crane capacity, span, hook height, and duty class change column section, base reactions, corbel details, and often the entire grid economics. A crane added after the frame is designed is a redesign, not an addition.
Mixing metric and imperial units in briefs. A footprint given as "200 ft × 100 ft" with rack modules in meters produces conversion errors that surface on site. Pick one unit system, state it at the top of the brief, and convert once.
Comparing quotes built on different assumed grids. If you ask three suppliers for a price without fixing the grid, one may quote 6 m bays, another 8 m, another a multi-span layout—and the totals are not comparable. Fix the grid (or require each bidder to state theirs) before comparing totals.
Ignoring the soil report's implications. Dense grids are foundation-hungry. On weak soils, the civil cost of extra frame lines can dwarf the steel savings. Give the geotechnical report to whoever is optimizing the grid.
Not reserving a repeatable bay module for expansion. A Phase 1 grid of assorted bay widths makes Phase 2 structurally and commercially awkward. Fix a module now—even if expansion is only a possibility.

17. The Pre-Order Checklist: 14 Things to Confirm About Your Grid

Before you release a purchase order or approve fabrication drawings, confirm each of the following in writing. This list is deliberately short enough to use as a meeting agenda.

Building function statement—one paragraph describing what the building must do operationally.
Internal footprint layout—rack positions, machinery footprints, aisles, and circulation drawn to scale on the plan.
Required clear width versus the width acceptable with interior columns, stated explicitly.
Eave height derivation—the tallest stored item, equipment, or clearance that sets it.
Crane data—capacity, span, hook height, and duty class per bay, if any.
Rack module dimensions—frame centers, depth, and aisle widths from your racking supplier.
Door and opening schedule—every opening with width, height, and position referenced to the grid.
Mezzanine locations and loads—area, use, and imposed load, since mezzanines interact with frames and bracing.
Future expansion direction and phase width—and whether the end wall should be designed as removable.
Soil report or bearing assumption—actual geotechnical data where available.
Local code and deflection requirements—the governing standard and any project-specific deflection limits.
Preferred bay module—your target spacing, or agreement to accept the engineer's optimized module.
Cladding module preferences—daylighting strip locations, vent positions, and any elevation rhythm requirements.
Grid ownership—who provides the framing layout and calculation package, and what deliverables (framing plan, member schedule, connection summary) you receive before fabrication.

Item 14 matters more than buyers expect. A supplier who volunteers a grid proposal with transparent member weights and foundation counts is far easier to verify than one who quotes only a tonnage total. Jidian's engineering team, for example, issues the grid proposal and calculation package as a standard pre-order deliverable, with main frames in Q345B/Q355B welded H-sections fabricated to roughly ±2 mm tolerance.

Frequently Asked Questions

FAQ 1: What is the most common bay spacing for steel structure buildings?

In most modern portal frame warehouses, longitudinal bay spacing falls between 6 and 9 m, with 7.5–8 m the most frequent optimum. Below 6 m, the added frame lines and foundations usually outweigh the savings in secondary steel; above 9 m, purlin and girt spans grow quickly and lap-continuity efficiency fades. Transverse spans run 15–36 m per bay, with multi-span layouts using interior columns where the operation tolerates them. Treat 7.5–8 m as a sensible starting point, then verify against your loads, rack modules, and foundation costs.

FAQ 2: Does a wider transverse span always cost more per square meter?

Generally yes, but not linearly. Steel weight per square meter rises with span because frame bending demand scales with span squared, and connections and base fixings grow with it. However, the increase is gradual across the common 18–30 m range, and wider spans reduce interior columns, which can save racking layout space and simplify traffic flow. Beyond roughly 36 m per bay, cost per square meter climbs noticeably and special framing (trusses, tapered webs) is usually required. Judge the premium against what clear width is operationally worth to you.

FAQ 3: Can bay spacing be changed after the design is quoted?

Yes, but the later you change it, the more it costs. Before fabrication drawings are released, adjusting bay spacing means revising the frame analysis, member schedule, purlin design, and foundation reactions—a manageable engineering exercise, though anchor bolt drawings already issued to the civil contractor may need reissuing. After fabrication begins, changes mean scrapping or reworking cut steel. Treat bay spacing as a frozen decision once you approve fabrication drawings; raise any doubt about the grid during the drawing-approval stage.

FAQ 4: How does bay spacing affect crane buildings?

Cranes load the frames directly through corbels or crane beams spanning between columns, so crane beams span across the longitudinal bays. Wider bays mean longer, heavier crane beams and stiffer columns to control deflection and fatigue, especially at higher duty classes. Common practice in multi-bay crane workshops keeps bay spacing moderate—often 6–7.5 m—because crane beam weight dominates the secondary steel budget. Crane capacity, span, hook height, and duty class all change the optimum grid, so always include full crane data in the RFQ rather than bolting it on later.

FAQ 5: What bay spacing works best with pallet racking?

Match the rack module to a multiple of the bay spacing, or vice versa. Typical pallet rack modules (including aisle) run around 2.7–3.5 m, so a 6 m bay fits two modules cleanly, and an 8 m bay fits two wider modules or two-and-a-bit narrower ones. The important rule is that columns should not land inside racks or aisles—they should coincide with rack flue spaces or fall outside the racking footprint entirely. Give your racking supplier's module dimensions to the steel engineer before the grid is fixed, not after.

FAQ 6: How do I know if my building needs interior columns (multi-bay) instead of clear span?

Ask what the interior width is actually used for. If the operation can tolerate a column line—racked storage, manufacturing with fixed equipment positions, general warehousing—a multi-span grid saves meaningful steel and money, commonly 10–20% on frame tonnage compared with clear span of the same total width. If you need genuinely unobstructed floor—large machinery moves, aircraft or vehicle handling, sports use, dense narrow-aisle racking with specific aisle geometry—clear span is justified. A practical threshold: for total widths above roughly 30–36 m, always price both options before deciding.

FAQ 7: Should I set the grid myself or let the supplier's engineer propose it?

Let the supplier's engineer propose it, but constrain and verify it. You own the operational inputs: function, rack modules, crane data, opening schedule, expansion intent, and soil report. The engineer owns the optimization: frame spacing versus purlin weight versus foundation count under your local loads. Request the grid proposal with transparent unit weights and assumed foundation counts, then sanity-check it against a simple comparison like the worked table in Chapter 12—recomputed with your own civil costs. Avoid the opposite failure too: issuing a self-designed grid that the engineer must simply replicate without review.