A simple beam is worst when it is fully loaded. A continuous beam, or a beam with a cantilever, is not. Take the live load off one span and the span next to it can bend more, deflect more, and pull up off its support. That is pattern live loading — "partial loading" in ASCE 7-22 §4.3.3 — and the code requires you to check it wherever it is worse than the fully loaded case.
This page explains what the section asks for, why it happens, which arrangements to run, and how much they change real designs. The three beams below were run through the calculation engine of StructSuite's free Wood & Steel Beam module, once with live load on every span and once with the partial arrangements, and every number on the page is pinned to that engine by a regression test.
What the partial arrangements changed on the three beams
Two equal 16-ft spans (glulam, D + L): span moment 10,801 → 13,335 lb-ft (+23%) and live-load deflection 0.055 → 0.092 in, with live load on one span only.
Steel W12X45, 24 ft + 6 ft overhang (LRFD): span moment 57.2 → 60.6 kip-ft (+6%) with the overhang unloaded, and overhang-tip deflection 0.104 → 0.148 in.
Glulam balcony beam, 16 ft + 6 ft: balcony-tip live-load deflection 0.066 → 0.291 in (4.4×) with the room behind it empty. A 5.125 × 12 that passes at D/C 0.236 fails at 1.035, so the beam becomes a 5.125 × 13.5.
#What ASCE 7-22 §4.3.3 requires
The rule is short. Section 4.3.3, Partial Loading:
The uniform live load applied only to a portion of a structure or member shall be accounted for if it produces a more unfavorable load effect than the uniform live load applied over the full structure or member. Uniform live loads applied to a portion of a structure or member are permitted to be reduced in accordance with Section 4.7.
And §4.3.3.1, for roofs:
Where uniform roof live loads are reduced to less than 20 psf (0.96 kN/m²) in accordance with Section 4.8.2 and are applied to the design of structural members arranged so as to create continuity, the reduced roof live load shall be applied to adjacent spans or to alternate spans, whichever produces the greatest unfavorable load effect.
Three things follow from the wording:
- It is about live load. The section sits in Chapter 4 and speaks of uniform live load and roof live load: L and Lr. Dead load is always there and is never patterned. Snow has its own partial-loading rule in Chapter 7 (see the Frequently Asked Questions).
- "Load effect" means every effect. Moment, shear, reaction and deflection. An arrangement that does not raise the moment can still raise the deflection or lower a reaction to uplift.
- It is conditional. You only have to account for the arrangement if it is more unfavorable. On a single span it never is, because there is only one way to load it.
#Why less live load can be worse
On a continuous beam the spans hold each other back. Load both spans of a two-span beam and each span is partly restrained by its loaded neighbour: the middle support carries a large negative moment, and the span moments stay modest. Take the live load off span 2 and span 2 stops pushing back. Span 1 rotates more freely over the middle support, so its positive moment and its sag both grow, while the support moment falls.
A cantilever works the same way, in reverse. Load on the overhang lifts the back span, like a see-saw. Remove the overhang's live load and that help disappears, so the back span sags further and its moment goes up. Load the overhang alone, with the back span empty, and nothing holds the back span down: the cantilever tip drops the furthest and the far support's reaction falls toward uplift.
So no single arrangement governs everything. Full loading usually governs the support moment and the reaction at an interior support. A partial arrangement governs the span moment, the deflection, and the end reactions, both the largest and the smallest.
#Which arrangements to check
For a uniform live load on a prismatic beam, the arrangements that can govern are the ones §4.3.3.1 names:
| Arrangement | What it maximises |
|---|---|
| All spans loaded | The negative moment over interior supports, in many layouts, and the largest interior-support reactions |
| Alternate spans (spans 1, 3, 5 … then 2, 4, 6 …) | The positive moment and the deflection in the loaded spans |
| Adjacent pairs (spans 1–2, 2–3, …) | The negative moment over the support between the pair, on beams of up to three spans |
A cantilever counts as a span. On a beam with one back span and one overhang, "alternate spans" means back span only and overhang only — exactly the two cases in the section above.
The count stays small: 3 arrangements for two spans, 5 for three, 8 for six. Checking every on/off combination of n spans (2n − 1 of them) grows to 255 on an eight-span beam before it is multiplied by every load combination. On two and three spans the named arrangements include the worst positive and negative moments for a uniform load. On four or more spans, the worst negative moment over a support needs the adjacent pair loaded plus every other span beyond it — spans 1, 2 and 4 on a four-span beam — an arrangement the named families do not include (see How to run it in StructSuite for what the module generates).
#Example 1 — Two equal spans
A glulam 5.125 × 18 floor beam, 24F-1.8E, continuous over two equal 16-ft spans on three pinned supports. It carries D = 200 plf and L = 400 plf on both spans, with the member's self-weight left out (untick Include member self-weight in Step 2 to reproduce the numbers). The ASD combination D + L is run twice: once with live load on both spans and once with live load on span 1 only. Dead load stays on both spans in both runs.
| D + L | Live load on both spans | Live load on span 1 only |
|---|---|---|
| Positive moment in span 1 | 10,801 lb-ft | 13,335 lb-ft |
| Negative moment over the middle support | 19,199 lb-ft | 12,800 lb-ft |
| Left reaction | 3,600 lb | 4,000 lb |
| Middle reaction | 12,000 lb | 8,000 lb |
| Right reaction (smaller is closer to uplift) | 3,600 lb | 800 lb |
| Live-load deflection (L/360 = 0.533 in) | 0.055 in | 0.092 in |
The bolded value is the worse one in each row. The partial arrangement raises the span moment by 23% and the live-load deflection by about 1.7 times. Full loading still governs the support moment, and with it the flexure check here (19,199 lb-ft, D/C 0.623). That is the usual pattern: you need both, one for the top of the beam over the support and one for the bottom at midspan.
On live load alone the textbook coefficients show the same thing more starkly. One span loaded gives 0.0957wL² in that span against 0.0703wL² with both loaded, 36% more, and a deflection coefficient of 0.0092 against 0.0054. Both loaded gives 0.125wL² over the support. Dead load is not patterned, so on D + L it dilutes the difference to the 23% above.
#Example 2 — A steel beam with an overhang
A W12X45 (A992), simply supported over a 24-ft span with a 6-ft overhang past the right support. It carries D = 420 plf and L = 250 plf along the whole length and is checked by LRFD, with self-weight left out, the compression flange braced continuously (Lb = 0) and Cb = 1.0. The figure shows combination 1.2D + 1.6L under the three arrangements.
| 1.2D + 1.6L | Both spans | Span 1 only (overhang empty) | Span 2 only (overhang loaded) |
|---|---|---|---|
| Positive moment in the 24-ft span | 57.2 kip-ft | 60.6 kip-ft | 28.6 kip-ft |
| Negative moment at the right support | 16.3 kip-ft | 9.1 kip-ft | 16.3 kip-ft |
| Left reaction | 10,170 lb | 10,470 lb | 5,370 lb |
With the overhang's live load removed, the back-span moment rises from 57.2 to 60.6 kip-ft (+6%), which you can check by hand. The overhang carries 1.2 × 420 = 504 plf, so the support moment is 504 × 6² / 2 = 9,072 lb-ft. The left reaction is 904 × 12 − 9,072 / 24 = 10,470 lb with 904 plf on the span, and M+ = 10,470² / (2 × 904) = 60,630 lb-ft. The W12X45 has plenty of strength here (D/C 0.252 against 0.238), so the number does not change the section. On a beam sized close to its limit, 6% does.
The deflection moves more. The overhang-tip live-load deflection goes from 0.104 in with live load everywhere to 0.148 in with the back span loaded alone, against a 0.400 in limit (twice the 6-ft projection over 360). Both are upward: the back span's live load rotates the beam over the right support and lifts the tip, and with the overhang's own live load removed nothing holds it down. So the overhang's own arrangement is not the only one to check — on this beam the largest tip movement comes from loading the other span.
#Example 3 — The balcony beam whose size is set by one arrangement
This is the beam from the glulam cantilever balcony example: a 24F-1.8E glulam with a 16-ft back span inside the building and a 6-ft balcony cantilever. The room carries 40 psf live and the balcony 60 psf, both on a 6-ft tributary width, plus a 1,050 lb-ft guardrail moment at the tip.
| Live-load deflection at the balcony tip (limit 0.400 in) | Live load on both spans | Live load on the balcony only |
|---|---|---|
| 5.125 × 13.5 | 0.066 in, D/C 0.165 | 0.291 in, D/C 0.727 |
| 5.125 × 12 | 0.094 in, D/C 0.236 | 0.414 in, D/C 1.035 — fails |
With live load on both spans, the room's load rotates the beam over the exterior wall in the direction that lifts the balcony, and that hides most of the balcony's own sag. Empty the room and the help is gone: the tip drops 4.4 times as far. This is the most ordinary arrangement there is — people on a balcony, nobody standing in the room behind it.
Check only the fully loaded case and a 5.125 × 12 passes every check. Check the partial arrangement and it fails deflection while every strength check still passes, so the beam becomes a 5.125 × 13.5. The same arrangements also show:
- the back-span moment rising from 6,887 to 10,170 lb-ft with the balcony empty;
- the interior end reaction falling from 2,187 to 267 lb with the balcony loaded alone — 267 lb from lifting off its support.
#When §4.3.3 does not apply
- A single span. There is one arrangement, so there is nothing to check.
- Loads other than live load. D is always present, and W and E are not uniform gravity live loads. §4.3.3 governs L and Lr. Snow is not exempt from partial loading: it has its own rule, §7.5, which the beam module does not run.
- A combination without L or Lr. D alone, D + S and 0.6D are not patterned by §4.3.3 and are run once.
- A load that is really permanent. A "live" load that is always there belongs in D, and then it is correctly never patterned.
#How to run it in StructSuite
Partial live loading is a choice in the Wood & Steel Beam module. It works the same way for wood (ASD) and steel (LRFD):
- Step 1 — Geometry. Give the beam two or more spans. A free end support makes the end span a cantilever, and a cantilever counts as a span.
- Step 2 — Load Definition. Enter an L or Lr load. The box Include partial live loading (ASCE 7-22 §4.3.3) then appears under Load options.
- Unticked, the default, live load goes on every span: the simple analysis.
- Ticked, the arrangements are listed right under the box, for example "span 1 only; span 2 only". The book icon beside it opens the §4.3.3 text.
- Step 6 — Design Verification. Each arrangement is its own row, named after the load and where it sits: 2a. D + L — L on span 1. Its flexure and shear calculations state the arrangement. The live-load deflection checks solve every arrangement, keep the worst for each span, and say which one governed: Governing live load arrangement: L on span 2 only (ASCE 7-22 §4.3.3 partial loading).
- Step 7 — Visualization. Each arrangement is its own entry in the load-case list, and the figure draws the live load only on the spans it loads. A caption under the beam model says where the live load is, where it is not, and why that arrangement can govern. The caption prints with the figure.
The Step 2 summary, which is what prints in the report, records the choice either way: Partial live loading — Included, with the arrangements, or Not included — live load on all spans only. A plan checker reading the package can see which analysis you ran.
When to tick the box
Tick it for any beam with a cantilever or balcony, and for continuous floor beams where the live load is a large share of the total or the design is near a limit. Leaving it off is a simplification you are choosing to make. On the beams above it would have missed 6–48% of span moment and up to 4.4 times the deflection.
The arrangements StructSuite generates are the ones in the table above: all spans, alternate spans, and each adjacent pair. Two cases are not among them:
- On a beam with cantilevers at both ends, loading one cantilever alone, the case that lifts the opposite end hardest.
- On a beam of four or more spans, an adjacent pair loaded together with every other span beyond it, the case that maximises the negative moment over the support between the pair.
To check either, duplicate the module (the Duplicate module button in its header) and, in the copy, enter the live load only on the spans that arrangement loads.
#Mistakes to avoid
- Checking only the fully loaded case on a cantilever. It is the arrangement most likely to hide a deflection failure, as Example 3 shows.
- Patterning the dead load. D is on every span in every arrangement. Removing it too overstates the effect.
- Stopping at moment. The governing partial arrangement is often a deflection or a reaction, not the strength check.
- Forgetting uplift. The smallest reaction comes from a partial arrangement. If it approaches zero, the connection needs a tie-down. In Seismic Design Category D–F, ASCE 7-22 §12.4.4 adds its own net-upward check for horizontal cantilevers, a separate box in the module's Step 2.
- Using reduced roof live load without the roof rule. §4.3.3.1 applies where the roof live load has been reduced below 20 psf.
#Summary of results
| Beam | Effect | Live load on all spans | Worst partial arrangement |
|---|---|---|---|
| Two 16-ft spans, glulam 5.125 × 18, D + L | Span moment | 10,801 lb-ft | 13,335 lb-ft (span 1 only) |
| Support moment | 19,199 lb-ft | 12,800 lb-ft | |
| Live-load deflection | 0.055 in | 0.092 in (span 1 only) | |
| W12X45, 24 ft + 6 ft overhang, 1.2D + 1.6L | Span moment | 57.2 kip-ft | 60.6 kip-ft (overhang empty) |
| Overhang-tip live-load deflection | 0.104 in | 0.148 in (back span only) | |
| Glulam balcony, 16 ft + 6 ft | Tip live-load deflection, 5.125 × 13.5 | 0.066 in | 0.291 in (balcony only) |
| Tip live-load deflection, 5.125 × 12 | 0.094 in (passes) | 0.414 in (fails, limit 0.400 in) | |
| Interior end reaction, D + L | 2,187 lb | 267 lb (balcony only) |
#Frequently Asked Questions
#What is pattern live loading?
Pattern live loading means placing the live load on only some spans of a continuous beam or a beam with a cantilever, because that can produce a larger moment, deflection or uplift than loading every span. ASCE 7-22 calls it partial loading, in §4.3.3.
#Does pattern loading apply to a simple-span beam?
No. A single span can only be loaded one way, so the fully loaded case is the worst case. Pattern loading matters only when a beam has two or more spans, and a cantilever counts as a span.
#Is a cantilever a span for pattern loading?
Yes. The overhang and the back span are loaded and unloaded independently. Back span loaded alone gives the largest back-span moment, and can lift the tip further than any other arrangement, as in Example 2. Overhang loaded alone gives the largest downward tip deflection, as in Example 3, and the smallest reaction at the far support.
#Does pattern loading apply to snow, wind or dead load?
§4.3.3 governs live load (L) and roof live load (Lr). Dead load is always on every span. Snow has its own provision, ASCE 7-22 §7.5 Partial Loading: selected spans with the balanced snow load and the remaining spans with half of it. StructSuite's beam module does not generate the §7.5 snow arrangements. Wind and seismic loads are not patterned by §4.3.3.
#Can the partial live load be reduced?
Yes. The last sentence of §4.3.3 permits the live load on a portion of the member to be reduced in accordance with §4.7. StructSuite does not apply the §4.7 reduction itself; if your member qualifies, enter the reduced L.
#Why not check every possible combination of loaded spans?
The full set of on/off combinations grows very quickly: 255 arrangements on an eight-span beam, each run through every load combination. The alternate-span and adjacent-pair arrangements that §4.3.3.1 names cover the worst moments on two- and three-span beams. On four or more spans the worst support moment also needs the spans beyond the pair loaded alternately, which StructSuite does not generate; check that case in a copy of the module, as described in How to run it in StructSuite.
#Does StructSuite pattern point loads too?
Yes. A point load or applied moment of type L or Lr is kept in an arrangement when it sits within a loaded span and removed when it does not. A load exactly on an interior support belongs to the span on its left. The guardrail moment in Example 3 is an L moment at the balcony tip, so it moves with the balcony's live load.
#Why is the partial-loading box unticked by default?
Many beams are checked with the simple analysis — live load on every span — and on a beam where the partial arrangements cannot govern, they only add rows. The choice is yours and it prints in the Step 2 summary either way. Beams with a cantilever, and continuous beams near a limit, are the ones to tick. A wood beam saved before the box existed (November 7, 2026) opens with it ticked, because that is how it was calculated.
#Where to go next
- The full design of the balcony beam in Example 3, every check: Glulam Beam Design Example — Cantilever Balcony (NDS 2024)
- Shear, moment and deflection formulas for single and multi-span beams: Beam Design Formulas with Shear and Moment Diagrams
- Every assumption the beam module makes: How StructSuite Works — Engineering Assumptions
- Run your own beam, wood or steel: StructSuite's free Wood & Steel Beam module