Pipe Culverts: Elliptical and Arch Pipe Class

Table IIB covers reinforced concrete horizontal elliptical pipe under ASTM C 507 and reinforced concrete arch pipe under ASTM C 506. Both exist to solve one problem: delivering the waterway area of a large circular pipe within substantially less vertical rise. Where a road profile, a shallow crossing or an existing utility limits the height available, these are the sections that make the crossing possible at all.

End view of a horizontal elliptical concrete pipe section showing its flattened profile.

Equivalent round size

The tables are entered by equivalent round size — the diameter of the circular pipe whose waterway area the section matches. This is the key idea and the most common source of confusion. A 36 in. equivalent round elliptical section is not 36 in. in either dimension; it is 45 in. across the span and 29 in. in rise, and it carries approximately the waterway area of a 36 in. circular pipe. The hydraulic design produces a required area, that area is expressed as an equivalent round size, and the table converts it into the physical dimensions and the class.

The span and rise columns give those physical dimensions separately for elliptical and for arch sections, because the two shapes reach the same area by different geometry. Arch sections are generally shallower in rise but wider than elliptical sections at the same equivalent round size, which is why the choice between them is usually settled by the actual clearance envelope rather than by preference.

Class notation

Classes here are not the Roman numerals used for circular pipe. Horizontal elliptical classes are prefixed HE and arch classes A, each followed by a class number defined in the relevant product standard. HE-I through HE-IV and A-II through A-IV appear in ordinary Illinois practice. As with circular pipe the class corresponds to a defined supporting strength, expressed for these shapes in the terms the product standard uses rather than as a simple circular D-load.

Minimum fill height

The minimum fill height column is the one to read first, and it is the reason many designers arrive at this table. It gives the least cover over the crown at which the section may be used, generally one foot, with a somewhat greater requirement at the smallest equivalent round size. Below that minimum the live load reaching the crown is no longer within the range the class system covers and the installation needs direct design. Construction traffic before final grade is reached is the usual way this limit is exceeded in practice, not the design traffic.

English units

Pipe culverts, Table IIB — class of elliptical and arch pipe by equivalent round size and fill height (English units)
Equivalent round size, in	Elliptical span, in	Elliptical rise, in	Arch span, in	Arch rise, in	Minimum fill height (RCCP, HE & A)	Type 1 — HE	Type 1 — arch	Type 2 — HE	Type 2 — arch	Type 3 — HE	Type 3 — arch
18	23	14	22	13.50	1' - 3"	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
21	30	19	26	15.50	1' - 0"	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
24	30	19	28.50	18	1' - 0"	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
27	34	22	36.25	22.50	1' - 0"	HE-III	A-III	HE-I	A-II	HE-III	A-III
30	38	24	36.25	22.50	1' - 0"	HE-III	A-III	HE-I	A-II	HE-III	A-III
36	45	29	43.75	26.625	1' - 0"	HE-III	A-III	HE-I	A-II	HE-III	A-III
42	53	34	51.125	31.3125	1' - 0"	HE-II	A-II	HE-I	A-II	HE-III	A-III
48	60	38	58.50	36	1' - 0"	HE-II	A-II	HE-I	A-II	HE-III	A-III
54	68	43	65	40	1' - 0"	HE-I	A-II	HE-I	A-II	HE-III	A-III
60	76	48	73	45	1' - 0"	HE-I	A-II	HE-I	A-II	HE-III	A-III
66	83	53	88	54	1' - 0"	HE-I	A-II	HE-I	A-II	HE-III	A-III
72	91	58	88	54	1' - 0"	HE-I	A-II	HE-I	A-II	HE-III	A-III

Metric units

Pipe culverts — class of elliptical and arch pipe by equivalent round size and fill height (metric units)
Equivalent round size, mm	Elliptical span, mm	Elliptical rise, mm	Arch span, mm	Arch rise, mm	Minimum fill height (RCCP, HE & A)	Type 1 — HE	Type 1 — arch	Type 2 — HE	Type 2 — arch	Type 3 — HE	Type 3 — arch
450	584	356	559	343	0.4 m	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
525	762	483	660	394	0.3 m	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
600	762	483	724	457	0.3 m	HE-IV	A-IV	HE-I	A-II	HE-III	A-III
686	864	559	921	572	0.3 m	HE-III	A-III	HE-I	A-II	HE-III	A-III
750	965	610	921	572	0.3 m	HE-III	A-III	HE-I	A-II	HE-III	A-III
900	1143	737	1111	676	0.3 m	HE-III	A-III	HE-I	A-II	HE-III	A-III
1050	1346	864	1299	795	0.3 m	HE-II	A-II	HE-I	A-II	HE-III	A-III
1200	1524	965	1486	914	0.3 m	HE-II	A-II	HE-I	A-II	HE-III	A-III
1350	1727	1092	1651	1016	0.3 m	HE-I	A-II	HE-I	A-II	HE-III	A-III
1500	1930	1219	1854	1143	0.3 m	HE-I	A-II	HE-I	A-II	HE-III	A-III
1676	2108	1346	2235	1372	0.3 m	HE-I	A-II	HE-I	A-II	HE-III	A-III
1800	2311	1473	2235	1372	0.3 m	HE-I	A-II	HE-I	A-II	HE-III	A-III

Practical notes

Availability is a real constraint. Not every plant produces both shapes in every class. Where arch pipe is not produced locally, elliptical pipe of the same equivalent round size substitutes; the converse also holds. Confirm before the design is fixed.
Orientation is part of the specification. Horizontal elliptical pipe is installed with the major axis horizontal. C 507 also defines vertical elliptical classes for the opposite constraint. Marking the axis orientation on the plans is not optional.
End treatment differs. Precast flared end sections are made for many arch, elliptical and box sizes as well as for circular pipe; see flared end sections.
Hydraulics do not transfer directly. Non-circular sections have their own performance curves and partial-flow relationships; the FHWA hydraulics library and the design data sheets listed under design aids treat each shape separately.

Dimensions, waterway areas and approximate weights for elliptical sections are tabulated under elliptical pipe dimensions and weights.