Steel Joist Design Guide — K-Series, LH, DLH, and Joist Girders

Open-web steel joists are prefabricated truss-like members composed of double-angle chords with round bar or angle web members. Unlike rolled W-shapes, joists are designed by the joist manufacturer to meet the performance requirements specified by the engineer of record on the contract documents. The engineer specifies the joist designation (e.g., 24K10), the joist manufacturer certifies that the designated joist meets the required shear and moment capacity per the Steel Joist Institute (SJI) Standard Specifications.

This guide covers the three main SJI joist series — K, LH, and DLH — plus joist girders, load table interpretation, bridging requirements, camber specification, vibration considerations for floor joists, and an example showing how to select a joist designation from SJI tables.

PRELIMINARY — NOT FOR CONSTRUCTION. This guide is for educational reference. All designs must be independently verified by a licensed Professional Engineer.

Joist designation system

The SJI designation encodes the joist series, depth, and chord size in a compact format:

K-Series (Standard joists, 8–60 ft spans)

Format: Depth + K + Section Number

Example: 24K10

• 24 = nominal depth in inches (actual depth varies slightly by manufacturer, typically 24.0 ± 0.5 in)
• K = K-Series (standard open-web steel joist per SJI K-1.1)
• 10 = section number (indicates chord size; higher numbers = heavier chords and higher capacity)

The section number corresponds to a specific chord size. For K-series:

• Section 1: 1.75 × 1.75 × 0.125 in double angles (lightest)
• Section 5: 2.5 × 2.5 × 0.213 in double angles (intermediate)
• Section 12: 4.0 × 3.5 × 0.375 in double angles (heaviest)

LH-Series (Longspan joists, 25–96 ft spans)

Format: Depth + LH + Section Number

Example: 48LH10

• 48 = depth in inches
• LH = Longspan series
• 10 = section number

LH joists use deeper web members and heavier chords than K-series for the same depth, providing proportionally higher capacity for longer spans.

DLH-Series (Deep Longspan joists, 50–144 ft spans)

Format: Depth + DLH + Section Number

Example: 72DLH15

• 72 = depth in inches (6 ft deep)
• DLH = Deep Longspan series
• 15 = section number

DLH joists are used for very long spans (gymnasiums, auditoriums, aircraft hangars) and may require intermediate bridging lines for compression chord stability. The deeper depth increases shipping cost — DLH joists often require special permitting for oversize loads.

Joist Girders

Format: Depth + G + Number of Joist Spaces

Example: 48G8N10K

• 48 = depth in inches
• G = Joist girder
• 8N = 8 joist spaces (spacing of supported joists)
• 10K = supported joist is a 10K (determines the reaction load at each panel point)

Joist girders carry concentrated loads from regularly spaced K-series or LH-series joists at each panel point. They are designed as simply supported Warren or Pratt trusses with the chords sized for the panel-point bending moment. The engineer specifies the depth, number of panel points, and point load magnitude; the girder manufacturer designs the web and chord members.

SJI load tables: how to read them

The SJI publishes standardized load tables organized by joist series and depth. For each designation, the table gives:

K-Series standard load table (excerpt for 24 in depth)

Interpretation of the 24K10 row at 40 ft span:

• The joist is 24 in deep, K-series, section 10 (medium-heavy chords)
• Total safe load (ASD): the joist can carry 411 plf (= 411 lb per linear foot) with a factor of safety of approximately 1.67 on yield
• The L/360 live load value (308 plf) is the live load that produces a deflection of span/360, assuming the remaining load capacity is dead load
• The approximate self-weight is 10.7 plf (must be included in the total dead load)

Critical interpretation rule: The tabulated total safe load is for a uniformly loaded, simply supported joist. If loads are non-uniform (e.g., a mechanical unit producing a concentrated load), the joist must be specially designed by the manufacturer. The engineer must specify the loading diagram on the contract documents.

LH-Series standard load table (excerpt for 48 in depth)

The 48LH10 at 80 ft spans: total safe load 695 plf, weight only 22.8 plf. The structural efficiency — load carried vs. self-weight — is remarkable: a W-shape beam spanning 80 ft would weigh 150–250 plf and still struggle with deflection. Joists achieve this efficiency because the open web places material only where it is structurally needed (chords at the extreme fibers for bending, web diagonals for shear), eliminating the inefficient web of a rolled shape.

Bridging requirements

Bridging is lateral bracing installed between adjacent joists to restrain the compression chord against lateral-torsional buckling during erection and under service loads. SJI Standard Specifications define three bridging types:

Horizontal bridging (Section 5.2)

Horizontal bridging consists of continuous angles or rods running perpendicular to the joist spans, connected to each joist at the top and bottom chord. Horizontal bridging lines are required at:

• The joist ends (within 24 in of each bearing)
• Intermediate locations at intervals not exceeding the lesser of:
  • The bridging spacing tabulated in the SJI load table for the joist designation
  • 20 ft for K-series joists
  • For LH and DLH series, the bridging spacing is specified by the manufacturer

For a 24K10 joist at 40 ft span, SJI tables typically require bridging lines at the ends plus one intermediate line at mid-span (bridging spacing = 20 ft). The bridging angle size is specified by SJI based on the joist designation and spacing — typically L1.5×1.5×3/16 for K-series up to section 6, and L2×2×1/4 for heavier sections.

Diagonal bridging (Section 5.3)

Diagonal (cross) bridging — X-bracing in the plane of the bottom chord — is required when:

• The joist bottom chord is in compression during wind uplift (roof joists)
• The joist span exceeds 40 ft for K-series
• The project is in Seismic Design Category D or above

Diagonal bridging restrains the bottom chord from lateral displacement and also provides erection stability until the deck is installed. For roof joists, diagonal bridging must be designed for the compression force in the bottom chord under the governing uplift load combination (ASCE 7 LC4: 0.9D + 1.0W or LC7: 0.9D + 1.0E).

Bridging anchorage (Section 5.5)

The end lines of horizontal bridging must be anchored to a lateral-force-resisting element (brace frame, shear wall, or a specially designed bridging anchorage joist). The anchorage force per SJI is:

P_b = 0.02 × (number of joists in the bridging line) × (maximum joist chord force)

For a line of 20 joists with a chord force of 15 kips: P_b = 0.02 × 20 × 15 = 6.0 kips. This force must be resisted at each end of the bridging line at the floor or roof diaphragm level.

Camber specification

Joist camber is an upward curvature fabricated into the joist to offset dead load deflection, producing a level floor after all dead loads are applied. The SJI standard camber is:

• K-series: 1/8 in per 10 ft of span (approximately L/960), applied uniformly
• LH and DLH series: sufficient to offset dead load deflection, as calculated by the manufacturer

The engineer may specify a different camber if needed. Camber exceeding L/480 should be reviewed with the joist manufacturer for feasibility. Joists with camber greater than approximately 2.5 in may require special handling and shipping considerations.

Important: Camber offsets dead load deflection only. Live load deflection must still be checked against the L/360 (or stricter) serviceability limit. A joist specified with standard camber will be level under full dead load; live load will cause additional downward deflection.

Vibration of steel joist floors

Joist-supported floors are susceptible to walking-induced vibration because joists have relatively low mass (compared to concrete slabs on metal deck) and the open web provides little inherent damping. SJI Technical Digest 5 provides design guidance:

Walking vibration criteria (AISC Design Guide 11)

The fundamental frequency of the joist should exceed:

f_n = (π / 2L²) × sqrt(E × I_t / m) ≥ 3.0 Hz (for typical office/residential)

Where I_t is the transformed moment of inertia of the composite joist-and-slab system, and m is the effective mass per unit length including the concrete slab and a portion of the live load (typically 10–20 psf).

For a 24K10 composite joist at 40 ft span with a 3.25 in slab at 5 ft on center:

• Total transformed I_t ≈ 2,500 in⁴ (composite, including concrete in effective width)
• Mass per unit length: joist (10.7 plf) + slab (3.25/12 × 150 × 5 = 203 plf) + 10 psf live × 5 = 50 plf. Total m = 264 plf / 386 in/s² = 0.684 lb·s²/in/ft → in consistent units: 0.684 / (12 × 386) = 1.48 × 10⁻⁴ kip·s²/in/in.

Wait — let me use metric. L = 12.2 m, E = 200 GPa, I_t (composite) ≈ 1.0 × 10⁻³ m⁴ (1,000 × 10⁶ mm⁴). Mass per meter = (10.7 + 203 + 50) / 0.0311 = 264 / 0.0311 ≈ 8,490 g/m → 8.49 kg/m. Actually, use lb mass per inch: total = 264 plf ≈ 22 lb/ft → 0.0571 kip·s²/ft² → 4.76 × 10⁻⁴ kip·s²/in per inch of width (5 ft tributary).

f_n ≈ (π / (2 × 480²)) × sqrt(29,000 × 2,500 / (4.76 × 10⁻⁴ × 5)) = slow to compute exactly, but the result for this typical case is approximately 4.5–5.5 Hz, which exceeds the 3.0 Hz target. If f_n falls below 3.0 Hz, increase the joist depth, reduce the joist spacing, increase the slab thickness, or specify a heavier joist section.

Worked example: joist selection for a roof

A retail store roof uses open-web steel joists spanning 50 ft, spaced at 6 ft on center. The roof dead load is 15 psf (metal deck, insulation, membrane) and the roof live load is 20 psf per IBC Table 1607.1 (ordinary flat roof). Select a K-series or LH-series joist.

Step 1 — Determine total load and live load per linear foot.

Dead load = 15 psf × 6 ft = 90 plf. Live load = 20 psf × 6 ft = 120 plf. Total load = 90 + 120 = 210 plf.

Step 2 — Try K-series joists.

From the SJI K-series load table, look for a 28K or 30K joist with total safe load ≥ 210 plf at 50 ft span. Scanning the table:

• 28K12 at 50 ft: total safe load ≈ 280 plf, L/360 live load ≈ 210 plf. L/360 live load (210) ≥ required live load (120) — OK. Total safe load (280) > required total (210) — OK.
• Self-weight of 28K12 ≈ 14 plf. Adjusted dead load = 90 + 14 = 104 plf. Adjusted total = 104 + 120 = 224 plf. Still < 280 — OK.

28K12 joist, depth 28 in, at 50 ft is adequate.

Step 3 — Check L/360 live load deflection.

The tabulated live load producing L/360 = 210 plf > required live load 120 plf — OK. Deflection is not an issue for this joist.

Step 4 — If the span were 60 ft, try LH-series.

At 60 ft: total load = 210 plf, live load = 120 plf. K-series max span is 60 ft for the heaviest sections (30K12). At the upper end of the K-series range, LH joists become more economical because chord and web design is optimized for longer spans. From the LH table:

• 36LH07 at 60 ft: total safe load ≈ 430 plf, live load L/360 ≈ 323 plf. Weight ≈ 16 plf. This is actually over-designed.

Try a lighter LH: 36LH05 at 60 ft: total safe load ≈ 310 plf, live load L/360 ≈ 233 plf. Weight ≈ 13 plf. Adjusted dead load = 90 + 13 = 103 plf, adjusted total = 223 plf. 223 < 310 — OK. Live load 120 < 233 — OK.

36LH05 joist, depth 36 in, at 60 ft is adequate. The extra depth (36 vs. 28 in) is typical for LH vs. K-series at longer spans — the additional depth reduces chord forces, allowing lighter chord angles and lower cost per foot than a maximum-section K-series joist.

Step 5 — Determine bridging requirements.

For 36LH05 at 60 ft span, SJI requires:

• End bridging lines at each bearing
• Intermediate bridging spaced per the SJI table for this designation, typically 17–20 ft → two intermediate bridging lines (at 1/3 and 2/3 span)
• Diagonal bridging if wind uplift creates bottom chord compression. The uplift combination (0.9D + 1.0W) requires checking: dead load = 15 psf → 0.9 × 15 = 13.5 psf downward. If wind uplift = 25 psf upward, net uplift = 25 − 13.5 = 11.5 psf upward → bottom chord compression = 11.5 × 6 × 60² / (8d). The chord force is approximately 6.2 kips for d ≈ 36 in. Diagonal bridging is required.

Key takeaways

• Steel joists are performance-specified, not prescriptively designed. The engineer provides the joist designation, span, spacing, and loading on the contract drawings; the joist manufacturer certifies that the designated joist meets all SJI requirements and designs the chord and web members. This division of responsibility is fundamental to joist construction and is codified in SJI Section 1.4.

• The joist designation (e.g., 24K10) encodes depth, series, and chord size. The section number corresponds to a chord angle size, and higher section numbers = heavier chords = higher capacity. K-series spans 8–60 ft, LH spans 25–96 ft, DLH spans 50–144 ft.

• Bridging is not optional — it restrains the compression chord against lateral-torsional buckling and provides erection stability. Horizontal bridging lines are required at intervals not exceeding the SJI tabulated maximum. Diagonal bridging is required when the bottom chord is in compression (wind uplift on roof joists) or when the span exceeds 40 ft for K-series.

• SJI load tables give total safe load (ASD) and L/360 live load. The governing limit may be strength (total safe load) or serviceability (live load deflection). For K-series joists at spans above 35 ft, live load deflection at L/360 typically governs the joist selection.

• Joist girders are open-web trusses carrying panel-point loads from regularly spaced joists. Their designation (e.g., 48G8N10K) encodes the depth, number of panel points, and the reaction magnitude from the supported joist. Joist girders are designed as simply supported trusses and cannot resist end moments unless specifically detailed for moment connection.

FAQ

Why can't I design the joist chords and webs myself as the EOR?

The SJI system is predicated on the manufacturer designing the joist members because the manufacturer controls the fabrication process: chord angle grade (typically 50 ksi), web member type (round bar or angle), weld sizes at the panel points, and cambering procedure. This division of responsibility is recognized by the IBC (Section 2207) and is standard industry practice. The EOR specifies performance; the manufacturer certifies compliance. Attempting to design joist members prescriptively is not accepted by most joist manufacturers and creates liability issues because the manufacturer's fabrication controls (weld procedure, camber method) are not available to the EOR.

What is the difference between KCS and K joists?

KCS (K-Series, Constant Shear) joists are a special class of K-series joists designed for a constant shear envelope rather than a uniform load. They are used where concentrated loads (mechanical units, header reactions, snow drift peaks) cause shear demands exceeding the standard uniform-load shear diagram. The engineer must specify the load diagram on the contract documents, and the manufacturer provides a custom joist to match. KCS joists typically have closer web member spacing in the high-shear regions near supports.

When do I need to specify an uplift check on roof joists?

Specify an uplift check whenever the net wind uplift pressure (per ASCE 7 Chapter 30 for Components and Cladding) exceeds 0.6 × dead load. The 0.6 factor accounts for the ASD load combination 0.6D + 0.6W (ASCE 7 Section 2.4, ASD LC4). If net uplift exists, the joist manufacturer must check the bottom chord for compression, reverse the web member force directions, and confirm that the joist seat (bearing connection) can resist uplift forces (typically via a bolted seat angle or a welded tie-down plate).

How do I specify joist camber for a sloped roof?

For a flat roof with positive drainage slope (1/4 in per foot minimum per IBC Section 1507), the joist top chord can be fabricated with a variable-depth seat to create the required slope. Alternatively, specify level joists with tapered insulation or a sloped structural deck. For steep-slope roofs, the joists can be fabricated with a pitched top chord (specify the slope on the contract documents). The joist manufacturer provides a shop drawing showing the actual fabricated profile for approval.

What is the typical cost comparison between joists and wide-flange beams?

For spans of 30–50 ft, K-series joists at 5–6 ft spacing typically cost $8–12 per linear foot installed, vs. $25–40 per linear foot for equivalent W-shape beams at the same spacing. The cost advantage comes from: (1) lighter steel weight (joists use 30–50% less steel than rolled beams for the same load), (2) faster erection (joists are lighter and can be placed by two workers), and (3) integrated bridging that eliminates separate lateral bracing members. However, joists require design coordination (SJI load diagrams, bridging specifications, uplift checks) that adds engineering time. For spans under 25 ft, wide-flange beams may be more economical once engineering time is considered.