Why Bay Spacing Deserves a Side-by-Side Comparison
A buyer comparing 6m, 8m and 9m bay spacing is usually looking at the same workshop footprint and asking whether fewer frames will reduce the total project cost. Frame count is only one part of the answer. The comparison also involves the primary frames, secondary steel, bracing, connections, crane supports, foundations and the local construction scope outside the steel supplier’s quotation.
This article uses one hypothetical 72m-long workshop to show how to structure a like-for-like comparison. The geometry is fixed for the example, but the structural design, steel quantity and total cost still require project-specific engineering and commercial review.
The 72m Comparison Layouts
The example assumes the same building width, clear height, intended use, design loads and enclosure requirements for all three options. Only the longitudinal frame spacing changes.
| Layout | Longitudinal frame spacing | Number of bays | Number of frame lines | Nominal internal-frame roof-load tributary width (assumed geometry) | Nominal purlin support span between adjacent frames (assumed geometry) | End-wall note |
|---|---|---|---|---|---|---|
| Option A | 6 m | 12 bays | 13 frame lines | 6 m | 6 m | End-wall frames may differ from internal main frames |
| Option B | 8 m | 9 bays | 10 frame lines | 8 m | 8 m | End-wall frames may differ from internal main frames |
| Option C | 9 m | 8 bays | 9 frame lines | 9 m | 9 m | End-wall frames may differ from internal main frames |
The tributary width and purlin support span shown above are geometric assumptions for the example only. The nominal purlin support distances assume purlins are supported at adjacent frame lines without intermediate supports. These geometric distances are not final member design spans. They are not member designs, steel-weight calculations or economic rankings. End-wall frames, braced bays and crane columns may use different configurations from the internal portal frames. Not every frame line should be assumed to have the same foundation arrangement.
72m Workshop — Longitudinal Frame Layout Comparison
Scroll horizontally to compare all three layouts.
Terminology: Five Different Dimensions
Bay spacing discussions often mix up several dimensions. They should be kept separate.
- Longitudinal frame spacing (bay spacing) — the distance between adjacent main frames along the building length. This is the dimension that changes in the 6m, 8m and 9m comparison.
- Building transverse span — the horizontal distance between the main supporting column lines across the building width. It is not simply the same as a clear internal width, because column dimensions and cladding lines can affect usable space.
- Roof purlin spacing — the distance between adjacent roof purlins, measured along the roof slope or across the roof plane.
- Purlin support span — the distance a purlin spans between its supports, typically the spacing between adjacent frames or secondary supports.
- Internal column grid — the layout of columns inside the building, if any. This is a separate design decision from the longitudinal frame spacing.
The 6m, 8m and 9m options are comparison scenarios for this article. They are not fixed Bingfa supply specifications and not a universal requirement in any country. The appropriate bay spacing should be determined by the project engineer based on the building use, loads, crane requirements, cladding and applicable design code.
How Bay Spacing Changes Load Collection
Increasing the longitudinal frame spacing generally increases the roof area that each internal main frame collects. In the example, the nominal tributary width increases from 6m to 8m to 9m for an internal frame with equal spacing on both sides. The purlin support span between adjacent frames also increases in the same way.
That does not mean the frame member sizes simply scale with the spacing. The actual member forces, deflection, connection demand and foundation reactions must be calculated for each layout. The frame that works at 6m may not be the same section at 9m. The purlin and girt sections may also change. The bracing arrangement may need to be reviewed. The comparison should therefore separate the geometric parameters that can be checked directly from the engineering results that must be recalculated.
Why Fewer Frames Does Not Automatically Mean Lower Cost
Fewer frame lines can reduce the number of frames and connections. Whether this also reduces the number of crane-support locations depends on whether the runway beams are supported by the main-frame columns or by independent crane columns. It can also reduce some erection activities. However, the remaining frames may carry a larger tributary load and may need heavier sections, different haunches or revised connections. The secondary steel may need to span further or be supported differently. The foundation reactions may change, and the local foundation cost may increase or decrease depending on the actual reactions.
The net effect on total project cost depends on the balance between these factors. It cannot be determined from the frame count alone. A comparison that looks only at the number of frames is not a cost comparison. For broader cost factors, see the steel workshop building cost guide.
How Each Layout Affects the Building Systems
Primary steel frames
The frame spacing affects the tributary roof load and the frame design. The engineer should confirm the member sizes, haunch details and connection design for each option. Frame configuration is discussed in more detail on the steel frame workshop page.
Secondary steel and connections
Purlins and girts span between frames or secondary supports. A wider frame spacing increases the purlin support span and may change the purlin section, spacing or connection details. The buyer should ask the supplier to state the purlin and girt assumptions in each quotation.
Bracing and stability
Longitudinal bracing is typically located in specific bays. The number and position of braced bays may need to be reviewed when the frame spacing changes. The bracing arrangement affects the stability system and the load path to the foundations.
Crane runway beams and supports
If the workshop includes an overhead crane, the runway beams are supported by the frames or by separate columns. The frame spacing affects the runway beam span between supports and the load transferred to each support. The crane supplier’s data and the structural engineer’s review are both required.
Foundations and column bases
The foundation arrangement should be determined from the column-base reactions, end-wall structure, crane-support arrangement and local ground conditions, rather than assuming that every frame line requires a separate foundation. End-wall frames, braced bays and crane columns may differ. The foundation reactions should be checked for each layout. The foundation design is a local civil/structural scope and is not automatically included in the steel supplier’s EXW quotation.
Openings, equipment and cladding
Door and window openings, equipment interfaces and cladding layouts must be coordinated with the frame positions. A change in bay spacing can move the frames relative to the openings. The buyer should confirm that the proposed spacing works with the actual opening schedule and cladding system.
Purchase Comparison Matrix
The matrix below separates what can be compared directly from what must be redesigned or recalculated.
| Comparison category | What can be compared directly | What requires redesign or recalculation | Information to request | How to keep EXW quotations comparable |
|---|---|---|---|---|
| Building geometry and frame count | Building length, width, clear height, bay count and frame-line count | Usable internal width after columns and cladding | Confirmed building dimensions and layout | Use the same building geometry and design basis for all options |
| Primary steel frame and connections | General frame arrangement and scope description | Member sizes, haunches, connections and steel quantity | Engineer’s frame design and connection details for each option | Confirm that each quote covers the same primary steel scope |
| Secondary steel (purlins, girts) and connections | Nominal purlin support span and general layout | Purlin and girt sections, spacing and connection details | Purlin and girt schedule for each option | Confirm that secondary steel is included on the same basis |
| Bracing and stability system | General bracing concept | Braced bay locations, bracing members and connections | Bracing layout and design for each option | Confirm that bracing scope is included and not excluded |
| Foundations and column reactions | General foundation scope boundary | Column base reactions, foundation sizes and local design | Reaction schedule from the engineer; local foundation design separately | Keep foundations outside the EXW steel quote and compare local costs separately |
| Crane runway beams and supports (if applicable) | General crane support concept | Runway beam span, supports, brackets and connections | Crane supplier data and runway support design | Confirm whether crane-support steel is included in each EXW quote |
| Openings, equipment and cladding interfaces | Opening schedule and general interface requirements | Framing around openings and cladding coordination | Door schedule, equipment layout and cladding details | Confirm that all options use the same opening and cladding assumptions |
| Fabrication scope and EXW supply boundary | Listed supply items and exclusions | Any scope item that differs between quotes | Written EXW scope, inclusions and exclusions | Normalize the EXW scope before comparing totals |
| Local civil, transport and erection costs | Local scope description | Local foundation, transport, unloading and erection costs | Separate local quotations | Compare local costs separately from the EXW steel quote |
| Commercial terms and exclusions | Currency, date, validity and stated exclusions | Any commercial condition that differs | Written commercial terms for each option | Confirm that all quotes use the same currency and validity basis |
Keeping Three Options Comparable
Before comparing totals, align the following items: the same building length, width and clear height; the same design basis and load inputs; the same roof and wall cladding specification; the same crane requirements if applicable; the same opening schedule; and the same EXW supply scope.
If one quotation assumes a different cladding system, a different crane requirement or a different scope, the comparison is not like-for-like. Ask the supplier to restate the quotation on the same basis, or identify the difference and compare it separately. For internal column-grid context, see the steel warehouse column spacing guide.
Discuss Your Workshop Frame Layout
What the Steel Supplier’s EXW Quote Covers
Bingfa’s steel structure quotation is based on the confirmed EXW supply scope. It may include the primary steel frame, secondary steel, bracing, connections and any agreed enclosure or accessory items. It does not automatically include local foundations, local civil works, transport, unloading, site erection, local permits or any other work outside the written scope.
The buyer should compare steel suppliers on the same EXW basis, and compare local foundation, transport and erection costs separately. A lower EXW steel price does not necessarily mean a lower total project cost if the local scope differs.
Questions to Ask the Engineer and Supplier
- What frame member sizes and connection details are assumed for each option?
- What purlin and girt sections and spacings are assumed for each option?
- How is the bracing system affected by the frame spacing?
- What column-base reactions apply to each option, and how do they affect the local foundation design?
- If a crane is required, how are the runway beams and supports affected?
- Do any openings, equipment interfaces or cladding details conflict with the frame positions?
- What is the exact EXW supply scope, and what is excluded?
- What are the quotation currency, date and validity?
These questions help compare the options on a consistent basis and expose hidden scope differences.
Frequently Asked Questions
Does a wider bay spacing always reduce the total steel weight?
No. A wider bay spacing reduces the number of frame lines, but each internal frame may collect more roof load and require heavier sections, revised connections or different secondary steel. The total steel weight depends on the engineer’s design for each option and cannot be determined from the frame count alone.
Can I compare the three options using the same foundation design?
Not without verification. The column-base reactions may differ between the three options, and end-wall frames, braced bays or crane columns may have different foundation requirements. Each layout’s reactions should be checked before the local foundation design is confirmed.
Is 6m, 8m or 9m a standard bay spacing?
None of these is a universal standard. They are comparison scenarios for this article. The appropriate bay spacing depends on the building use, loads, crane requirements, cladding system and applicable design code. The project engineer should determine the suitable spacing for the actual project.
What should I check in a steel supplier’s quotation for each option?
Check the EXW supply scope, the primary and secondary steel included, the bracing and connections, any accessories, the currency, the quotation date and validity, and the exclusions. Confirm that each quotation is based on the same building geometry, loads, cladding and crane requirements.
Request a Comparable Steel Workshop Quote
Technical References
- SteelConstruction.info — Portal frames — general engineering principles of portal frame action, restraint and load distribution.
- SteelConstruction.info — Single-storey industrial buildings — general context for industrial building design and bay spacing considerations.
These references support general engineering principles only. They are not Bingfa design standards or project-specific design results.