What are the main foundation types for steel structures?

Five primary foundation types support steel columns, selected based on soil bearing capacity, column loads, and site constraints: (1) ISOLATED SPREAD FOOTING — single footing under each column. Most common and economical. Square for axial-only columns, rectangular for combined axial + moment. Size: B = sqrt(P_total / q_allowable). For 200 kip column on 4000 psf soil: B = sqrt(200,000/4,000) = sqrt(50) = 7.07 ft, use 7 ft x 7 ft. Depth: 18-36 in governed by punching shear and flexure. Cost: $300-800/footing (formed, reinforced, concrete). (2) COMBINED FOOTING — one footing supporting two columns. Used when columns are closely spaced (footings would overlap) or near property line (eccentric single footing would tilt). Trapezoidal shape so centroid of footing area coincides with resultant of column loads. More reinforcement and formwork than isolated. (3) MAT (RAFT) FOUNDATION — continuous slab under entire building footprint (2-5 ft thick). Used when: (a) soil bearing capacity is low (< 2000 psf), (b) column loads are heavy (high-rise), (c) water table is high (mat acts as waterproof barrier), (d) differential settlement must be controlled (sensitive equipment). Mat thickness governed by punching shear at each column location. Typical rebar density: 80-150 lbs of rebar per cubic yard. (4) PILE CAP — transfers column load to pile group. 2-4 piles per column typical. Depth: enough to develop pile reinforcement and resist punching shear. Strut-and-tie model for deep pile caps (depth > 2x pile spacing). Pile capacity: 50-200 tons per pile (driven steel H-pile or concrete-filled pipe). (5) DRILLED PIER (CAISSON) — single large-diameter concrete shaft (2-8 ft) under heavy column (500-5000 kips). Bell at bottom for end bearing in rock or dense soil. No pile cap needed — pier connects directly to column via base plate and anchor bolts.

How is a spread footing sized for a steel column?

Spread footing design follows ACI 318 Chapter 13 for geotechnical strength and Chapter 7/8 for structural concrete design: (1) SIZE FOR BEARING: Af = P_service / q_allowable. For axial + moment: q_max = P/Af + M/S = 0 (no uplift). For W14x90 column with P=300k DL+LL, M=50k-ft, q_a=4000 psf: Try B=8 ft, L=10 ft (Af=80 ft^2). S = L x B^2 / 6 = 10 x 64 / 6 = 106.7 ft^3. q_max = 300/80 + 50/106.7 = 3.75 + 0.47 = 4.22 ksf = 4,220 psf. Slightly over 4,000 — increase to 8.5 x 10.5 = 89.25 ft^2. (2) DEPTH FOR PUNCHING SHEAR: d >= Vu / (phi x 4 x bo x lambda x sqrt(f'c)). Critical section at d/2 from column face. bo = perimeter of critical section = 2 x (bf + d) + 2 x (d_col + d). For W14x90 (14.5 x 14.0 in) on 4 ksi concrete: Vu = factored column load - soil pressure within critical section. Iterate d until phi Vc >= Vu. (3) DEPTH FOR ONE-WAY SHEAR: d >= Vu / (phi x 2 x B x lambda x sqrt(f'c)). Critical section at d from column face. (4) FLEXURAL REINFORCEMENT: Mu = qu x B x (L/2 - c/2)^2 / 2 (factored soil pressure x cantilever length of footing). As = Mu / (phi x fy x 0.9d). Minimum: As_min = 0.0018 x B x h for Grade 60. (5) BASE PLATE TRANSFER: the steel base plate distributes column load to the footing. Bearing strength of concrete: phi Pn = 0.65 x 0.85 x f'c x A1 x sqrt(A2/A1), where A1 = base plate area, A2 = footing area (A2/A1 <= 4.0). The base plate MUST be smaller than the footing pier or additional confinement reinforcement is needed.

How do anchor bolts transfer column forces to the foundation?

Anchor bolts transfer tension, shear, and moment from the steel column to the concrete foundation. Per ACI 318 Chapter 17 (Anchoring to Concrete): FAILURE MODES (all must be checked): (1) Steel strength of anchor in tension: phi Nsa = 0.75 x Ase x Futa (F1554 Gr 36: 58 ksi, Gr 55: 75 ksi, Gr 105: 125 ksi). For 3/4 in Gr 55: Ase = 0.334 in^2. phi Nsa = 0.75 x 0.334 x 75 = 18.8 kips per anchor. (2) Concrete breakout in tension: phi Ncb = 0.70 x ANc/ANco x psi factors x Nb, where Nb = kc x lambda x sqrt(f'c) x hef^1.5. The failure cone extends at ~35 degrees from the anchor head. For 4 ksi concrete, 3/4 in anchor with 12 in embedment (hef = 12 in): Nb = 24 x 1.0 x sqrt(4000) x 12^1.5 = 24 x 63.2 x 41.6 / 1000 = 63.2 kips. (3) Pullout: anchor head pulls through concrete. phi Npn = 0.70 x 8 x Abrg x f'c. For headed anchor with 3 in square plate: Npn = 8 x 9 x 4 = 288 kips — rarely controls. (4) Side-face blowout: thin concrete member, anchor near edge. Controls when edge distance c 2.5 in). ALL failure modes must have phi Rn >= factored load for each anchor. For bolt groups: the tension anchors and shear anchors can be assigned to different bolts (shear resisted by shear lugs or specific shear anchors). This is the most common mistake — assigning all shear to a bolt that also carries tension without checking combined tension-shear interaction.

When is a mat foundation needed instead of spread footings?

Mat foundations replace individual footings when: (1) SOIL BEARING CAPACITY < 2000 PSF — spread footings would cover more than 50% of the building footprint (they'd overlap). The mat uses the entire footprint area for bearing, distributing loads optimally. For a 10-story steel building on 1500 psf soil: column loads average 500 kips, spread footing would be 500/1500 = 333 ft^2 per column (18 x 18 ft). At 25 ft column spacing, footings would overlap — mat required. (2) HIGH WATER TABLE — the mat acts as a waterproof raft, eliminating individual footing waterproofing details. The mat is designed for buoyancy (uplift) when the water table is above the mat bottom. Factor of safety >= 1.2 against flotation (dead weight / buoyant force >= 1.2). (3) DIFFERENTIAL SETTLEMENT CONTROL — mat spreads loads across a larger area, reducing differential settlement. For settlement-sensitive equipment (MRI machines, precision manufacturing): differential settlement <= L/1000. Mat allows the building to settle uniformly (rigid body settlement) rather than differentially. (4) BASEMENT STRUCTURE — mat serves as the basement slab AND foundation. When a basement is required anyway (parking, mechanical), the mat kills two birds — saves cost vs separate footings + slab-on-grade. MAT THICKNESS: governed by punching shear at interior columns (most critical) and at edge/corner columns (reduced critical perimeter). For 24 in square column with 1000 kip factored load on 4 ksi concrete: bo = 4 x (24 + d). Vu = 1000k. Set phi Vc = 0.75 x 4 x lambda x sqrt(f'c) x bo x d = 1000k. For d = 30 in: bo = 4 x (24 + 30) = 216 in. phi Vc = 0.75 x 4 x 1 x sqrt(4000) x 216 x 30 / 1000 = 0.75 x 4 x 63.2 x 216 x 30 / 1000 = 1,228 kips > 1,000 OK. This typically results in mat thickness 2.5-5 ft for mid-rise buildings. High-rise mats can be 6-10 ft thick at the core.

What is the interface between steel column and concrete footing?

The steel-to-concrete interface consists of: base plate, anchor bolts, grout, and leveling nuts. Each has a specific function: (1) BASE PLATE — steel plate welded to column bottom. Distributes column load to concrete. Thickness tp = l x sqrt(2 x Pu / (0.9 x Fy x B x N x phi_c x 0.85 x f'c)) per AISC DG1. For lightly loaded columns: tp = 1/2 to 3/4 in. Heavily loaded: 1.5-4 in. Base plate must be large enough that the concrete bearing stress phi x 0.85 x f'c x sqrt(A2/A1) is not exceeded. (2) ANCHOR BOLTS — typically 4 bolts (minimum) for pinned bases (axial + shear), 8-12 bolts for moment bases. Bolt pattern symmetrical about column centerlines. Bolt diameter: 3/4 in for light columns (W8-W10), 1-1.5 in for heavy columns (W14). Anchor bolt holes in base plate: 1/16 in oversize for 3/4 in bolts. (3) GROUT — non-shrink cementitious grout, 1-2 in thick, placed UNDER the base plate after the column is leveled. Functions: (a) full bearing contact — fills the gap between base plate and concrete (concrete top is rough, +/- 1/4 in). (b) Level transfer — column is leveled on shim packs or leveling nuts, then grout fills the remaining gap. (c) Corrosion protection — grout prevents water and debris accumulation under the base plate. Dry-pack grout (stiff consistency) or fluid grout (pourable). Minimum grout compressive strength: 5000 psi at 28 days, or match footing concrete strength. (4) LEVELING NUTS — nuts below the base plate on the anchor bolts. Column is positioned, nuts adjusted to level (surveyor checks plumb), then grout placed. After grout cures, top nuts are tightened (snug-tight for pinned base, fully pretensioned for moment base). The leveling nuts carry all construction loads before grouting and transfer shear after grouting through bolt bearing. The column base is the most unforgiving steel-concrete interface — base plate level +/- 1/16 in over the plate diagonal per AISC 303 tolerances.

Foundation Types for Steel Structures — Spread Footings, Piles & Mats

Q: How do anchor bolts transfer column forces to the foundation?

Anchor bolts transfer tension, shear, and moment from the steel column to the concrete foundation. Per ACI 318 Chapter 17 (Anchoring to Concrete): FAILURE MODES (all must be checked): (1) Steel strength of anchor in tension: phi Nsa = 0.75 x Ase x Futa (F1554 Gr 36: 58 ksi, Gr 55: 75 ksi, Gr 105: 125 ksi). For 3/4 in Gr 55: Ase = 0.334 in^2. phi Nsa = 0.75 x 0.334 x 75 = 18.8 kips per anchor. (2) Concrete breakout in tension: phi Ncb = 0.70 x ANc/ANco x psi factors x Nb, where Nb = kc x lambda x sqrt(f'c) x hef^1.5. The failure cone extends at ~35 degrees from the anchor head. For 4 ksi concrete, 3/4 in anchor with 12 in embedment (hef = 12 in): Nb = 24 x 1.0 x sqrt(4000) x 12^1.5 = 24 x 63.2 x 41.6 / 1000 = 63.2 kips. (3) Pullout: anchor head pulls through concrete. phi Npn = 0.70 x 8 x Abrg x f'c. For headed anchor with 3 in square plate: Npn = 8 x 9 x 4 = 288 kips — rarely controls. (4) Side-face blowout: thin concrete member, anchor near edge. Controls when edge distance c 2.5 in). ALL failure modes must have phi Rn >= factored load for each anchor. For bolt groups: the tension anchors and shear anchors can be assigned to different bolts (shear resisted by shear lugs or specific shear anchors). This is the most common mistake — assigning all shear to a bolt that also carries tension without checking combined tension-shear interaction.

Every steel structure transfers load to the ground through a foundation. The foundation type depends on the soil bearing capacity, the magnitude and type of loading (axial, moment, lateral), the depth to competent bearing stratum, and the tolerance for settlement. Steel structures impose concentrated column loads — typically 100 to 5,000 kips per column — which must be spread to a pressure the soil can safely carry.

Foundation type selection

Foundation type	Typical column load	Soil requirement	Depth	Cost ranking
Isolated spread footing	50–800 kips	qa ≥ 2,000 psf, firm soil at shallow depth	3–6 ft	1 (lowest)
Combined footing	200–1500 kips (2 columns)	qa ≥ 2,000 psf	3–6 ft	2
Strip/continuous footing	Bearing wall, 5–20 klf	qa ≥ 1,500 psf	3–5 ft	1
Mat (raft) foundation	> 2,000 kips total, close columns	Weak soil, qa ≥ 1,000 psf	4–8 ft	3
Driven steel piles	50–500 kips per pile	Soft soil overlying hard stratum	30–100 ft	4
Drilled shafts (caissons)	200–5,000 kips per shaft	Rock or dense soil at depth	20–80 ft	5 (highest)

Spread footing design per ACI 318-19

For a steel column on an isolated spread footing, the design process involves four checks:

1. Bearing pressure: The footing area must spread the column load to a pressure below the allowable soil bearing capacity. For a square footing under service loads:

B_required = sqrt(P_service / qa)

For a W14 column carrying Pservice = 400 kips on soil with qa = 4,000 psf: B = sqrt(400,000/4,000) = 10.0 ft. Use 10'-0" x 10'-0" footing.

2. Punching shear (two-way): The critical section is at d/2 from the base plate edge. Per ACI 318-19 Section 22.6.5.2:

phi × Vc = phi × 4 × lambda × sqrt(f'c) × bo × d

Where bo = perimeter of the critical section, d = effective depth, phi = 0.75, lambda = 1.0 for normal weight concrete.

3. One-way shear: Critical section at d from the base plate face. Must resist the soil pressure acting on the footing area beyond d from the column.

4. Flexure: The footing cantilevers from the column face. The critical section for moment is at the face of the base plate (not the column flange). Moment per unit width:

Mu = qu × l² / 2    where l = (B - base plate width) / 2

Worked example — isolated spread footing

Given: W14x90 column, Pu = 600 kips (factored), Pservice = 420 kips, base plate 16" x 16", qa = 4,000 psf, f'c = 4,000 psi, rebar Fy = 60 ksi.

Step 1 — Footing size: B = sqrt(420,000/4,000) = 10.25 ft. Use 10'-6" x 10'-6" (B = 10.5 ft). A = 110.25 sf.

Step 2 — Factored soil pressure: qu = 600 / 110.25 = 5.44 ksf.

Step 3 — Punching shear: Try d = 24 in. Critical perimeter at d/2 from base plate: bo = 4 × (16 + 24) = 160 in. phi × Vc = 0.75 × 4 × 1.0 × sqrt(4000) × 160 × 24 / 1000 = 729 kips. Vu,punch = 600 - 5.44 × ((16+24)/12)² = 600 - 5.44 × 11.11 = 600 - 60.4 = 540 kips. 729 > 540 — OK.

Step 4 — Flexure: Cantilever l = (10.5 × 12 - 16) / 2 = 55 in = 4.58 ft. Mu = 5.44 × 4.58² / 2 = 57.1 kip-ft per foot width. Required As = Mu / (phi × Fy × (d - a/2)). With phi = 0.90 and assuming a = 1.2 in: As = 57.1 × 12 / (0.90 × 60 × (24 - 0.6)) = 685 / 1264 = 0.54 in²/ft. Use #6 at 10" o.c. (As = 0.53 in²/ft).

Result: 10'-6" x 10'-6" x 28" footing with #6 at 10" o.c. each way, bottom.

Deep foundations for steel structures

When soil bearing capacity is insufficient for spread footings, deep foundations transfer load to competent strata at depth.

Driven steel H-piles (HP10x42, HP12x53, HP14x73) are common under steel structures. Capacity is determined by driving resistance (blow count) correlated to static load tests. Typical design capacities: HP10x42 = 80–120 tons, HP12x53 = 100–160 tons, HP14x73 = 140–200 tons (depending on soil conditions and driving criteria). Pile caps (reinforced concrete blocks connecting piles to the column base plate) are designed per ACI 318 Chapter 13.

Drilled shafts are preferred when loads exceed pile capacity or when vibration/noise from driving is unacceptable. A single drilled shaft can carry 500–5,000 kips depending on diameter (30" to 96") and bearing stratum.

Code comparison

ACI 318-19 (USA): Chapter 13 covers footing design. Punching shear at d/2 from loaded area. phi = 0.75 for shear, phi = 0.90 for flexure. Minimum footing reinforcement per Section 7.6.1 (temperature/shrinkage steel).

AS 2159-2009 / AS 3600-2018 (Australia): Piling per AS 2159, footing design per AS 3600 Section 12. Punching shear check uses dom (mean effective depth) and a critical perimeter at dom/2. Capacity reduction phi = 0.70 for shear without shear reinforcement. Australian practice uses geotechnical strength reduction factors that vary by pile testing level (0.40–0.65 for driven piles, higher with more testing).

EN 1997-1 / EN 1992-1-1 (Eurocode): Foundation design per Eurocode 7 (geotechnical) and Eurocode 2 (concrete). Three design approaches (DA1, DA2, DA3) apply different partial factors to actions and resistance. Punching shear per EN 1992-1-1 Section 6.4 uses a critical perimeter at 2d from the loaded area (not d/2 as in ACI) and does not include a strength reduction factor — instead, partial factors on loads provide the safety margin.

Common mistakes engineers make

Designing for factored loads when checking soil bearing. Soil bearing capacity (qa) from the geotechnical report is a service-level (unfactored) allowable pressure. Compare it with service-level column loads, not LRFD factored loads. Using factored loads against qa produces a footing that is too small.
Ignoring overturning moment from lateral loads. Moment frames and braced frames deliver significant overturning moment to the foundation. The footing must resist both axial load and moment, which produces trapezoidal or triangular bearing pressure. Check that the maximum edge pressure does not exceed qa and that the resultant falls within the middle third (for non-uplift condition).
Mislocating the flexure critical section. ACI 318-19 Section 13.2.7.1 defines the critical section for moment at the face of the column or base plate, not the centerline. For a 16" base plate on a 10.5 ft footing, the cantilever is 4.58 ft, not 5.25 ft. The difference is significant.
Undersizing pile caps for punching. Pile caps must resist punching shear from both the column bearing down and individual pile reactions punching up. Both checks are required per ACI 318 Section 13.2.7. Thin pile caps often fail the pile reaction punching check.

Foundation types for steel buildings: detailed comparison

Steel buildings impose unique demands on foundations: concentrated column base reactions, significant overturning from lateral loads, and the need for precise anchor bolt placement. The choice of foundation type depends on the soil conditions, column loads, lateral system type, and economic considerations.

Spread footings (isolated and combined)

Isolated spread footings are the most common foundation type for steel buildings on competent soil. Each column sits on its own reinforced concrete pad that spreads the concentrated load to the soil at an allowable bearing pressure. Combined footings support two or more columns when isolated footings would overlap or when property line constraints prevent eccentric loading. For steel moment frames, spread footings must resist overturning moment in addition to axial load, which significantly increases footing size.

Typical size: 6 ft x 6 ft to 14 ft x 14 ft, 18 in to 36 in thick
Soil requirement: Allowable bearing pressure qa >= 2,000 psf (stiff clay, dense sand, compacted fill)
Advantages: Simple construction, lowest cost, easy to inspect
Limitations: Not suitable for soft clay, loose sand, or high water table

Mat (raft) foundations

A mat foundation is a single thick reinforced concrete slab supporting all columns of the building. It is used when spread footings would overlap due to close column spacing, when soil bearing capacity is too low for individual footings, or when differential settlement must be minimized. Mat foundations are common for steel frame buildings with basements where the mat also serves as the basement floor slab.

Typical thickness: 24 in to 72 in, depending on column spacing and loading
Soil requirement: qa >= 1,000 psf (soft clay, loose sand, controlled fill)
Advantages: Reduces differential settlement, provides basement waterproofing slab, distributes heavy column loads
Limitations: High concrete volume, requires detailed structural analysis (beam-on-elastic-foundation or finite element)

Driven piles

Driven piles are installed by hammering pre-formed piles into the ground using impact hammers. They are used when competent bearing soil is too deep for spread footings. Steel piles are preferred under steel structures because they can be directly welded to pile caps and provide high capacity per pile.

HP-section piles (H-piles): Standard wide-flange sections (HP10x42, HP12x53, HP14x73, HP14x117) driven point-down. H-piles displace minimal soil and are effective in dense sand, hard clay, and weathered rock. The flanges provide surface area for skin friction, and the tip bears on hard stratum.

H-Pile Size	Area (in^2)	Typical Capacity (tons)	Best Soil Condition
HP10x42	12.4	60-100	Medium dense sand, stiff clay
HP12x53	15.5	80-140	Dense sand, hard clay
HP14x73	21.4	100-180	Bearing on rock
HP14x117	34.4	150-250	Bearing on competent rock

Pipe piles: Steel pipe sections (12 in to 24 in diameter) driven open-ended or closed-ended. Open-ended pipe piles penetrate dense soil layers more easily and are common in marine and coastal applications. The pipe is filled with concrete after driving for corrosion protection and additional capacity.

Concrete piles: Precast concrete piles (12 in to 20 in square or octagonal) are driven in regions where steel piles are cost-prohibitive. They are heavier and more susceptible to damage during driving but provide good corrosion resistance.

Drilled shafts (caissons)

Drilled shafts are constructed by drilling a cylindrical hole, placing a reinforcing cage, and filling with concrete. They range from 30 in to 96 in in diameter and can carry 500 to 5,000+ kips per shaft. Drilled shafts are preferred when pile driving vibrations would damage adjacent structures, when very high single-element capacity is needed, or when the bearing stratum is rock at moderate depth (20-80 ft).

Advantages: Very high capacity, minimal vibration, can socket into rock
Limitations: Requires specialized drilling equipment, groundwater management, concrete quality control in deep shafts

Micropiles

Micropiles are small-diameter (5 in to 12 in) drilled and grouted piles used for underpinning existing foundations, supporting steel structures in restricted access areas, and providing foundation capacity in soil conditions where conventional piles cannot be installed. They are installed by drilling a small-diameter hole, placing a central steel reinforcing bar or pipe, and pressure-grouting the annulus.

Typical capacity: 20 to 200 kips per micropile
Advantages: Can be installed in any soil type, minimal vibration, small equipment fits inside buildings
Limitations: Low individual capacity (requires many piles), high cost per kip of capacity

Foundation selection table by soil and load

Soil Condition	Column Load (kips)	Recommended Foundation	Key Design Consideration
Dense sand/gravel, qa > 4 ksf	50-500	Isolated spread footing	Bearing capacity check, frost depth
Stiff clay, qa 2-4 ksf	100-800	Spread footing, wide base	Consolidation settlement, bearing check
Soft clay, qa < 2 ksf	100-500	Combined footing or mat	Total and differential settlement
Loose sand over dense	200-1,000	Driven H-piles	Pile capacity from load test
Soft soil over rock	500-5,000	Drilled shafts	Socket into rock, groundwater control
Variable fill	50-200	Micropiles	Grouting procedure, load testing
High water table	100-1,000	Driven pipe piles	Corrosion protection, uplift resistance
Karst (limestone)	Any	Drilled shafts to rock	Irregular rock surface, voids

Steel-specific foundation considerations

Steel buildings require special attention to several foundation design aspects that differ from concrete buildings.

Base plate anchorage: Steel columns transfer load to the foundation through base plates connected by anchor bolts. The base plate is designed per AISC 360-22 Chapter J and AISC Design Guide 1 (Base Plate and Anchor Rod Design). For columns with moment (moment frame bases), the base plate and anchor bolts must resist both axial load and bending moment. The anchor bolts are typically headed studs embedded in the concrete with sufficient development length. For fix-based moment frames, 4 to 12 anchor bolts may be required, with diameters from 3/4 in to 2-1/2 in.

Tie rods and hairpin reinforcement: Steel portal frames with pinned bases generate large horizontal thrust at the base (the "kick" or thrust). For a portal frame with 30 ft span, 20 ft height, and 15 psf wind load, the horizontal base reaction can exceed 20 kips. Tie rods connecting opposite column bases across the building width resist this thrust directly, preventing lateral spread of the footings. Hairpin reinforcement (U-shaped bars wrapping around the anchor bolts) transfers the horizontal shear from the base plate into the footing concrete.

Tie rod force = M_base / span_width (for portal frame thrust)
Required tie rod area = F_thrust / (0.9 x Fy_rod)

Grade beams: Grade beams are reinforced concrete beams spanning between column footings at or below grade level. They serve multiple purposes in steel buildings: (1) tie footings together to prevent differential movement, (2) provide resistance to lateral soil pressure, (3) support exterior wall cladding, and (4) provide a load path for horizontal base reactions. Grade beams are typically 18 in to 36 in deep and 12 in to 24 in wide, reinforced with continuous top and bottom bars.

Anchor bolt detailing: Proper anchor bolt placement is critical for steel erection. Bolts must be positioned within AISC erection tolerances (typically +/- 1/4 in from theoretical position). Common detailing practices include using template plates during concrete placement and specifying bolt sleeves for minor adjustment. Anchor bolt materials per ASTM F1554 (Grades 36, 55, and 105) cover most building applications.

Anchor bolt design overview (ACI 318 Chapter 17)

ACI 318-19 Chapter 17 provides the design provisions for anchors (anchor bolts, headed studs, and post-installed anchors) in concrete. The design methodology uses the Concrete Capacity Design (CCD) approach, which is a strength design method similar in concept to the LRFD approach used for steel design.

Anchor bolt limit states

Limit State	Description	ACI 17 Section
Steel strength (tension)	Anchor rod yields in tension	17.6.1
Steel strength (shear)	Anchor rod yields in shear	17.7.1
Concrete breakout (tension)	Concrete cone failure around anchor group	17.6.2
Concrete breakout (shear)	Concrete edge breakout	17.7.2
Concrete pullout	Anchor head pulls through concrete	17.6.3
Concrete side-face blowout	Lateral blowout for anchors near an edge in tension	17.6.4
Bond strength (adhesive)	Adhesive bond failure for post-installed anchors	17.6.5
Pryout	Short anchors in shear rotate and pull out	17.7.3

The design strength of an anchor or anchor group is the minimum of the steel strength and the concrete breakout strength, with appropriate strength reduction factors.

Steel strength in tension

phi x N_sa = phi x n x A_se,N x f_uta

where phi = 0.75 (ductile steel), A_se,N = effective cross-sectional area of the anchor in tension, f_uta = specified tensile strength of the anchor steel (limited to 1.9 x f_ya or 125 ksi), and n = number of anchors in the group.

Concrete breakout in tension

The concrete breakout capacity is calculated using a projected area method. The projected area is the area of the failure cone projected onto the concrete surface:

phi x N_cb = phi x (A_Nc / A_Nco) x psi_ed,N x psi_cp,N x N_b

where A_Nc is the actual projected area (limited by edges and adjacent anchors), A_Nco = 9 x h_ef^2 is the reference projected area for a single anchor far from edges, and N_b is the basic concrete breakout strength of a single anchor:

N_b = k_c x lambda x sqrt(f'c) x h_ef^1.5   (lb)

where k_c = 24 for cast-in anchors, h_ef is the effective embedment depth (in), and lambda = 1.0 for normal weight concrete.

Interaction for combined tension and shear

When anchors are subject to simultaneous tension and shear (common at column bases with lateral loads), the interaction equation is:

(N_ua / phi x N_n)^5/3 + (V_ua / phi x V_n)^5/3 <= 1.0

For typical steel column bases, the governing load combination for anchor design is 0.9D + 1.0W (uplift plus lateral) or 1.2D + 1.0E + 0.5L (seismic), both of which can produce significant tension and shear on the anchor bolts simultaneously.

Minimum embedment and edge distance

ACI 318-19 requires minimum edge distances for anchors based on both structural and constructability considerations. For untorqued cast-in anchors, the minimum edge distance is the greater of 6d_a (where d_a is the anchor diameter) or 2.5 in. For torqued anchors (pretensioned base plates), the minimum edge distance is the greater of 8d_a or 4 in. Reduced edge distances trigger a reduction factor psi_ed,N that decreases the breakout capacity.

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This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.