How are open web steel joists designated and what do the numbers mean?

Open web steel joists use the designation format: Depth + Series + Load Number. Example 18K3: 18 = nominal depth in inches, K = K-series (standard joist), 3 = load capacity code. Higher load numbers indicate greater load-carrying capacity at the same depth. The series indicate joist type: K = standard (8-30 in depth, spans to 60 ft), most common for floors and roofs of typical buildings. KCS = K-series Constant Shear, designed for non-uniform or moving loads with uniform web member capacity along the entire length. LH = Longspan (18-48 in depth, spans 25-96 ft), used for warehouses, gymnasiums, and industrial buildings. DLH = Deep Longspan (52-72 in depth, spans 89-144 ft), used for large industrial buildings, arenas, and aircraft hangars. Joist girders are primary framing members that support steel joists at concentrated panel points rather than uniform loads. The designation is stamped on the joist tag at each bearing end per SJI requirements. Load tables published by the Steel Joist Institute (SJI) give the allowable total uniform load (psf) for each joist designation at various spans. Selection: determine span + total load (dead + live psf) + deflection requirement, then use SJI tables to find the shallowest depth joist that exceeds the required load at that span.

What are the span capabilities of K-series, LH, and DLH joists?

OWSJ span capabilities by series: K-series (standard, 8-30 in depth) — spans from 8 ft (8K1) to 60 ft (30K12). Floor loading typically 50-100 psf total, roof loading 30-50 psf. Weight range: 5-22 plf. Example: 18K3 at 24 ft span carries 231 psf; at 36 ft, it carries 74 psf. Doubling the span reduces capacity to approximately 32% (governed by bending at short spans, deflection at long spans). LH-series (Longspan, 18-48 in depth) — spans from 25 ft to 96 ft. Weight range: 9-38 plf. Used for roofs of warehouses, gymnasiums, and industrial buildings where W-shapes would be too heavy. At 60 ft span, a 32LH10 carries approximately 150-180 psf. DLH-series (Deep Longspan, 52-72 in depth) — spans from 89 ft to 144 ft. Used for large industrial buildings, arenas, and aircraft hangars. A 72DLH15 at 120 ft span carries approximately 80-100 psf. DLH joists are 5-7 ft deep and weigh 30-60 plf. Span-to-depth ratio: K-series L/d ≈ 20-24 (standard), LH-series L/d ≈ 20-25, DLH-series L/d ≈ 18-22. Joists are typically most economical when L/d is in the 18-24 range. Below L/d = 18, consider W-shapes; above L/d = 24, check vibration and ponding.

What is joist bridging and when is it required?

Bridging is horizontal and diagonal bracing between adjacent joists that prevents lateral buckling of the bottom chord during construction and service. SJI requirements: Horizontal bridging — continuous horizontal angles or rods connecting the bottom chords of adjacent joists. For K-series spans ≤ 30 ft: 1 row at midspan. For K-series spans > 30 ft: 2 rows at third points. For LH/DLH-series: per SJI tables (typically 2-3 rows). Maximum horizontal bridging spacing: 40 × r_bottom_chord (radius of gyration of bottom chord about vertical axis). Diagonal bridging — X-bracing between top chord of one joist and bottom chord of adjacent joist at each bearing end (end anchorage). These diagonals transfer lateral loads to the roof/floor diaphragm. Connection type: bolted bridging is required at bearing ends and for all rows on spans > 60 ft. Welded bridging is permitted for interior rows on spans ≤ 60 ft. Purpose: (1) during construction — bottom chord of joist is in compression from dead load before the deck is placed and before the diaphragm provides lateral support, (2) in service — prevents lateral-torsional buckling of the joist under full load, (3) load sharing — bridging helps distribute concentrated loads to adjacent joists. Failure to install bridging before construction loads are applied is a leading cause of joist collapse during erection. OSHA 1926.751 requires bridging to be installed before construction loads are placed on joists. The erector must bolt or weld bridging connections per the SJI bridging tables.

How do KCS joists differ from standard K-series joists?

KCS (K-series Constant Shear) joists differ from standard K-series in their web member design. Standard K-series joists have web members designed for the shear envelope, resulting in varying web member sizes along the length — heavier webs near supports (high shear) and lighter webs near midspan (low shear). This is efficient for uniform loads. KCS joists have web members sized for constant shear capacity along the entire joist length — all web members have the same capacity regardless of position. This makes them suitable for: (1) Non-uniform loading — point loads from equipment, monorails, or mezzanine posts that can occur anywhere along the span. (2) Moving loads — crane runway support or movable partitions where the high-shear zone moves. (3) Future flexibility — where loading may change or equipment may be relocated. Trade-off: KCS joists are approximately 5-10% heavier than equivalent K-series joists because the lighter web members near midspan are upsized to match the end web capacity. KCS depth range: 10-30 in, same as K-series. Span range: 10-60 ft. KCS designation: same format as K-series (e.g., 18KCS4 = 18 in depth, KCS series, load code 4). When to specify KCS: if the joist supports any concentrated load > 500 lb that is not at a panel point, if the load pattern is unknown or may change, or if moving loads are present. For standard uniform floor/roof loading, K-series is more economical.

What are joist girders and how are they designed?

Joist girders are primary framing members (truss girders) that support open web steel joists at concentrated panel points. They span between columns and carry the concentrated reactions from the joists they support — typically 5-50 kips per panel point. Joist girders are designated by depth and span, with panel point spacing matching the supported joist spacing (typically 4-6 ft). Standard depth range: 24-48 in, with spans 20-96 ft. Unlike standard joists that carry uniform loads, joist girders are designed for concentrated loads at panel points. The joist girder chord members are typically double angles; web members are single or double angles. Design is per SJI specifications: the joist manufacturer designs the girder for the specified panel point loads, not the specifying engineer. The specifying engineer provides: girder depth, span, panel point spacing, and magnitude of each panel point load on the structural drawings. The manufacturer sizes chords and webs accordingly. Key requirements: (1) Joist girders must be laterally braced by the joists they support — joist bottom chord extension must connect to the girder top chord, (2) The joist-to-girder connection typically uses a joist seat (angle) welded or bolted to the girder top chord, (3) Joist girder bearing at columns: minimum 4 in bearing on steel, 6 in on masonry, (4) Joist girders are designed as simply-supported trusses — moment connections to columns are not standard and require special detailing. For spans > 60 ft, camber is typically specified: 1.5× dead load deflection for roof, 1.0× dead load for floor.

Steel Joist Sizes — K, KCS, LH & DLH Span Tables

Open web steel joists (OWSJ), also called bar joists, are prefabricated steel trusses used as floor and roof framing members. They are lighter and more economical than W-shape beams for medium to long spans. This page covers joist types, size designations, span capabilities, and load tables per the Steel Joist Institute (SJI).

What Are Open Web Steel Joists?

Steel joists are parallel-chord trusses with:

Top chord: Typically double angles or a single angle
Bottom chord: Typically double angles (for K-series) or rods
Web members: Round bars, angles, or formed shapes
End connections: Welded or bolted to bearing seats

Advantages

Advantage	Detail
Light weight	30-50% lighter than W-shape beams
Long spans	K-series to 60 ft, LH to 96 ft, DLH to 144 ft
MEP integration	Open web allows ductwork, piping, and conduit to pass through
Fast erection	Lightweight, simple connections, rapid installation
Economy	Lower material cost per square foot than W-shapes

Disadvantages

Disadvantage	Detail
Limited moment capacity	Not suitable for moment frames
No moment connections	Simple supports only
Vibration	Can be more susceptible to floor vibration
Fireproofing	Difficult to spray on small web members
Modification	Cannot be cut, drilled, or field-modified without engineer approval

Joist Series Designations

K-Series (Standard)

Format: Depth + K + Number

Example: 18K3 = 18 inch depth, K-series, load capacity code 3

Depth (in)	Designations	Span Range (ft)	Weight Range (plf)
8	8K1	8-16	5.1-6.2
10	10K1, 10K2	10-20	5.0-7.8
12	12K1, 12K3, 12K5	12-24	5.0-9.8
14	14K1, 14K3, 14K4, 14K6	14-28	5.2-11.3
16	16K2, 16K4, 16K5, 16K7	16-32	5.5-12.7
18	18K3, 18K4, 18K5, 18K7, 18K9	18-36	6.0-14.5
20	20K3, 20K4, 20K5, 20K7, 20K9, 20K10	20-40	6.4-16.0
22	22K4, 22K5, 22K6, 22K7, 22K9, 22K11	22-44	7.0-17.5
24	24K5, 24K6, 24K7, 24K8, 24K9, 24K10, 24K12	24-48	7.4-19.0
26	26K5, 26K6, 26K7, 26K8, 26K9, 26K10	26-52	8.0-19.5
28	28K6, 28K7, 28K8, 28K9, 28K10, 28K12	28-56	8.5-20.5
30	30K7, 30K8, 30K9, 30K10, 30K11, 30K12	30-60	9.0-22.0

KCS-Series (Constant Shear)

Designed for non-uniform or moving loads. "KCS" stands for K-series Constant Shear. They have uniform web member capacity along the entire length.

Depth (in)	Designations	Span Range (ft)
10	10KCS1, 10KCS2	10-20
12	12KCS1, 12KCS2, 12KCS3	12-24
14	14KCS1 to 14KCS4	14-28
16	16KCS2 to 16KCS5	16-32
18	18KCS2 to 18KCS6	18-36
20	20KCS3 to 20KCS7	20-40
22	22KCS3 to 22KCS8	22-44
24	24KCS4 to 24KCS9	24-48
26	26KCS5 to 26KCS10	26-52
28	28KCS6 to 28KCS11	28-56
30	30KCS7 to 30KCS12	30-60

LH-Series (Longspan)

Deeper joists for spans from 25 to 96 feet. Used for roofs of warehouses, gymnasiums, and industrial buildings.

Depth (in)	Designations	Span Range (ft)	Weight Range (plf)
18	18LH02 to 18LH06	25-36	9.0-16.0
20	20LH02 to 20LH08	25-40	9.5-18.0
22	22LH02 to 22LH09	25-44	10.0-19.5
24	24LH03 to 24LH10	25-48	10.5-21.0
26	26LH03 to 26LH11	25-52	11.0-22.5
28	28LH04 to 28LH12	25-56	11.5-24.0
30	30LH05 to 30LH13	25-60	12.0-25.5
32	32LH05 to 32LH14	25-64	12.5-27.0
36	36LH06 to 36LH16	25-72	13.5-30.0
40	40LH07 to 40LH18	25-80	15.0-33.0
48	48LH10 to 48LH20	25-96	18.0-38.0

DLH-Series (Deep Longspan)

Very deep joists for spans from 89 to 144 feet. Used for large industrial buildings, arenas, and aircraft hangars.

Depth (in)	Designations	Span Range (ft)
52	52DLH10 to 52DLH22	89-104
56	56DLH11 to 56DLH24	89-112
60	60DLH12 to 60DLH25	89-120
64	64DLH13 to 64DLH27	89-128
68	68DLH14 to 68DLH28	89-136
72	72DLH15 to 72DLH30	89-144

Load Tables (Selected K-Series)

Allowable Total Load (psf) — K-Series at Various Spans

Joist	20 ft	24 ft	28 ft	32 ft	36 ft	40 ft	44 ft	48 ft
14K1	288	167	107	—	—	—	—	—
16K2	366	214	137	94	—	—	—	—
18K3	386	231	149	102	74	—	—	—
20K4	420	253	165	114	83	63	—	—
22K5	438	267	176	122	89	68	53	—
24K6	461	282	186	130	95	73	57	46
26K7	458	292	198	139	102	78	62	50
28K8	464	302	206	146	108	83	66	54
30K10	538	350	240	170	125	97	77	63

Values are total safe uniformly distributed load in psf for simply supported joists at the span shown. Verify with SJI tables for actual design.

Joist Girders

Joist girders are primary framing members that support steel joists at panel points. They are designed for concentrated loads from the joists rather than uniform loads.

Depth (in)	Span Range (ft)	Panel Point Loads (kips)
24	20-48	5-25
30	20-60	5-30
36	20-72	5-35
40	20-80	5-40
44	20-88	5-45
48	20-96	5-50

Bridging Requirements

Bridging prevents lateral buckling of the bottom chord during construction and provides stability.

Joist Type	Horizontal Bridging	Diagonal Bridging
K-series (≤ 30 ft)	1 row at midspan	At each bearing
K-series (> 30 ft)	2 rows at thirds	At each bearing
LH-series	Per SJI tables	At each bearing
All joists > 60 ft	Per SJI requirements	Per SJI requirements

Bridging spacing: Maximum spacing is typically 40 × r (radius of gyration of the bottom chord), but not more than the SJI table value.

Bolted bridging: Required at bearing ends and at all rows for spans > 60 ft. Welded bridging: Permitted for interior rows on spans ≤ 60 ft.

Bearing Requirements

Condition	Minimum Bearing Length
Joist on steel beam	2-1/2 inches
Joist on masonry wall	4 inches
Joist on concrete wall	4 inches
Joist girder on column	Per connection design

End bearing seats are typically 2-1/2 inches standard. Larger seats available for deeper bearing requirements.

Frequently Asked Questions

What is the difference between K-series and LH-series joists? K-series joists are standard depth (8-30 inches) for spans up to 60 feet, used for roofs and floors of typical buildings. LH-series (Longspan) are deeper (18-48 inches) for spans up to 96 feet, used for warehouses, gymnasiums, and industrial buildings.

How do I select the right steel joist size? Determine the span, total load (dead + live in psf), and deflection requirement. Use SJI load tables to find a joist designation where the allowable load exceeds your required load at the given span. Select the shallowest depth that works for economy.

Can steel joists be used for floor framing? Yes. K-series joists are commonly used for floor framing with steel deck and concrete fill. Use KCS-series for floor loads that are non-uniform or where point loads occur. Design for vibration per AISC Design Guide 11.

What is joist bridging? Bridging is horizontal and diagonal bracing between joists that prevents lateral buckling of the bottom chord. It is required by SJI at specific intervals based on joist span and depth. Bolted bridging is preferred for spans over 60 ft.

Can I cut or modify a steel joist? No. Steel joists are prefabricated trusses designed for specific loading. Cutting, drilling, or modifying any member requires written approval from the joist manufacturer and the structural engineer of record. Unapproved modifications can cause collapse.

What is a joist girder? A joist girder is a primary framing member (like a truss beam) that supports open web steel joists at concentrated panel points. Joist girders span between columns and carry the concentrated reactions from the joists they support.

Steel Beam Sizes — W-shape section properties
Beam Deflection Calculator — Deflection analysis
Beam Formulas — Beam formula reference
Steel Deck Types — Floor and roof deck profiles
Beam Calculator — Full beam analysis tool

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.