7.3 Structural Materials & Framing: Concrete, Masonry, Steel & Wood

Structural Materials & Framing: Concrete, Masonry, Steel & Wood

Quick Answer: The IBC regulates structural framing systems through referenced material standards: ACI 318 for structural concrete (IBC Ch 19), TMS 402/602 for masonry (IBC Ch 21), AISC 360 / AISI S100 for structural/cold-formed steel (IBC Ch 22), and AWC NDS for wood framing (IBC Ch 23). Concrete requires minimum rebar covers: 3 inches for cast against earth, 1.5"–2" for weather-exposed elements, and 0.75"–1.5" for interior members. Masonry requires mortar per ASTM C270 (Type M for high load/below grade, Type S for seismic/flexure) and grout per ASTM C476 (8"–11" slump) with cleanouts for pours $>5\text{ ft}$. Structural steel connections must be classified as snug-tight, pretensioned, or slip-critical. Wood framing enforces prescriptive limits under §2308 (max 3 stories) and ground clearance offsets (18" for joists, 12" for girders) to prevent decay.

---

1. Reinforced & Prestressed Concrete Construction (IBC Ch 19, ACI 318)

Structural concrete must be proportioned, mixed, placed, and cured in accordance with IBC Chapter 19 and ACI 318 (Building Code Requirements for Structural Concrete).

+-----------------------------------------------------------------------------------------+
|                        STRUCTURAL CONCRETE DESIGN OVERVIEW                              |
+-----------------------------------------------------------------------------------------+
                                                                                           
  [ COMPRESSIVE STRENGTH (f'_c) ] ===> [ REINFORCING REBAR ] ===> [ MIN CONCRETE COVER ]   
  - Minimum f'_c = 2,500 psi           - ASTM A615 (Billet)       - Cast vs Earth: 3"      
  - Seismic SDC D-F: min 3,000 psi     - ASTM A706 (Low-alloy     - Weather: 1.5" to 2"    
  - Special Moment Frames: 3k-5k+        weldable / seismic)      - Interior: 0.75" to 1.5"

Specified Compressive Strength ($f'_c$) Minimums:

• General Structural Concrete: Minimum $f'_c = 2,500\text{ psi} (17.2\text{ MPa})$.
• Seismic Design Categories D, E, F (Special Moment Frames & Shear Walls): Minimum $f'_c = 3,000\text{ psi} (20.7\text{ MPa})$.
• Prestressed Concrete: Minimum $f'_c = 4,000\text{ psi}$ for post-tensioned elements.

Minimum Specified Concrete Cover for Reinforcing Bars (ACI 318 Table 20.6.1.3.1 / IBC Ch 19)

Concrete cover protects reinforcing steel against corrosion from moisture/chlorides and provides thermal insulation during a fire event:

+-------------------------------------------------------------------------+
|                    CONCRETE REBAR COVER CROSS-SECTION                   |
+-------------------------------------------------------------------------+
                                                                           
       EXTERIOR (EARTH CONTACT)                                            
       ===========================================================         
       |                                                         |         
       |   <------------------ 3.0" COVER ------------------>    |         
       |                                                         |         
       |       +-----------------------------------------+       |         
       |       | (O)   (O)   (O)   (O)   (O)   (O)   (O) |       |         
       |       |           REINFORCING REBAR MAT         |       |         
       |       +-----------------------------------------+       |         
       |                                                         |         
       |   <------------------ 3.0" COVER ------------------>    |         
       |                                                         |         
       ===========================================================         
       EXTERIOR (EARTH CONTACT - FOUNDATION FOOTING SLAB)                  

Reinforcement Splicing & Development Length ($l_d$):

• Tension Lap Splices (ACI 318 Section 25.5):
  • Class A Splice: $1.0 \cdot l_d$ (Permitted where provided reinforcement area is at least twice the required area and splice is in low-stress zone).
  • Class B Splice: $1.3 \cdot l_d$ (Standard for most structural beams, slabs, and columns).
• Seismic Ductility (ASTM A706): In Seismic Design Categories D, E, and F, longitudinal rebar in special moment frames and special shear walls must conform to ASTM A706 (Grade 60 or 80) due to its controlled yield-to-tensile ratio and minimum elongation for weldability and plastic deformation.

---

2. Structural Masonry Construction (IBC Ch 21, TMS 402/602)

Masonry systems (concrete block / CMU and brick) are designed under TMS 402 (Building Code Requirements for Masonry Structures) and constructed under TMS 602 (Specification for Masonry Structures).

+-------------------------------------------------------------------------+
|                    MASONRY MATERIAL SPECIFICATIONS                      |
+-------------------------------------------------------------------------+
|  1. Concrete Masonry Units (CMU): ASTM C90 loadbearing hollow/solid     |
|  2. Clay/Shale Brick: ASTM C62 (building) / ASTM C216 (facing)          |
|  3. Mortar: ASTM C270 (Types M, S, N, O)                                |
|  4. Grout: ASTM C476 (Fine or Coarse, 8" to 11" slump)                 |
|  5. Reinforcing Steel: ASTM A615 or ASTM A706 deformed bars             |
+-------------------------------------------------------------------------+

Mortar Selection Criteria (ASTM C270 / IBC Table 2103.2.1)

Mortar types are designated using alternating letters from the phrase "M-a-S-o-N w-O-r-K" in order of decreasing compressive strength:

Grouting Protocols & Cleanouts (TMS 602 / IBC §2104):

• Grout Slump: Structural grout must have a slump of 8 to 11 inches (203 to 279 mm) at time of placement per ASTM C476 (water is rapidly absorbed into porous CMU block walls, leaving dense consolidation around rebar).
• Cleanout Openings: Where grout pour height exceeds 5 feet (1,524 mm) (high-lift grouting), cleanout openings must be provided at the bottom of every vertical grout cell containing reinforcement to allow removal of mortar droppings and debris prior to grouting.

+-------------------------------------------------------------------------+
|                   HIGH-LIFT GROUTING & CLEANOUT DETAIL                  |
+-------------------------------------------------------------------------+
                                                                           
       +---------------------------------------+                           
       | [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]   | <--- Grout Pour > 5'-0"   
       |---------------------------------------|                           
       | [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]   |                           
       |---------------------------------------|                           
       | [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]   |                           
       |---------------------------------------|                           
       | [X]   [ ]   [X]   [ ]   [X]   [ ] [X] | <--- CLEANOUT OPENINGS    
       +=======================================+      AT BASE OF CELLS     
       |          CONCRETE FOUNDATION          |      WITH REBAR           
       +---------------------------------------+                           

---

3. Structural Steel & Cold-Formed Steel Framing (IBC Ch 22, AISC 360, AISI S100)

Structural steel framing members must comply with AISC 360 (Specification for Structural Steel Buildings), AISC 341 (Seismic Provisions), and AWS D1.1 (Structural Welding Code - Steel).

Material Specifications for Structural Steel:

• Wide-Flange W-Shapes: ASTM A992 ($F_y = 50\text{ ksi}, F_u = 65\text{ ksi}$). (Standard universal shape for columns and beams).
• Angles, Channels, and Plates: ASTM A36 ($F_y = 36\text{ ksi}$) or ASTM A572 Grade 50.
• Hollow Structural Sections (HSS): ASTM A500 Grade C ($F_y = 50\text{ ksi}$ round, $F_y = 50\text{ ksi}$ rect).

+-------------------------------------------------------------------------+
|                   STRUCTURAL STEEL BOLTED CONNECTIONS                   |
+-------------------------------------------------------------------------+
|  1. SNUG-TIGHT (IBC §1705.2.1 / RCSC Section 8.1):                      |
|     - Tightness attained by full effort of an ironworker with spud      |
|       wrench or pneumatic impact wrench (all plies in firm contact).    |
|     - Allowed in shear connections not subject to slip or tension.      |
+-------------------------------------------------------------------------+
|  2. PRETENSIONED (RCSC Section 8.2):                                    |
|     - High-strength bolts (ASTM F3125 Grade A325 or A490) tightened to  |
|       specified minimum bolt tension (Turn-of-nut, Calibrated wrench,   |
|       or Direct Tension Indicator / DTI washers).                       |
|     - Required for connections subject to tension load or fatigue.      |
+-------------------------------------------------------------------------+
|  3. SLIP-CRITICAL (RCSC Section 4.3):                                   |
|     - Clamping force creates frictional resistance between faying       |
|       surfaces (Class A or Class B blast-cleaned unpainted steel).      |
|     - MANDATORY in Seismic Force-Resisting Systems (SDC C-F), oversized |
|       holes, slotted holes, and structures subject to load reversal.    |
+-------------------------------------------------------------------------+

Structural Welding Classifications (AWS D1.1):

• Complete-Joint-Penetration (CJP) Groove Welds: Fusion extends through the entire thickness of the joint; develops full strength of base metal; continuous special inspection required.
• Partial-Joint-Penetration (PJP) Groove Welds: Fusion does not extend through full thickness; throat dimension determines capacity.
• Fillet Welds: Triangular cross-section weld connecting overlapping plates or perpendicular tees. Minimum and maximum weld sizes governed by base metal thickness.

Cold-Formed Steel Framing (IBC §2211, AISI S100 / AISI S240):

• Structural cold-formed steel loadbearing studs and joists must be stamped with minimum base metal thickness (gauge/mils), yield strength ($33\text{ ksi}$ or $50\text{ ksi}$), and galvanizing designation (minimum G60).
• Bridging/blocking is required at vertical intervals (typically 4 to 6 ft o.c.) to prevent lateral torsional buckling of studs under axial compression.
• Deflection tracks (slip tracks) at the top of non-loadbearing curtain wall studs must accommodate floor slab live load deflection without transferring axial gravity loads into non-bearing walls.

---

4. Wood Construction & Engineered Timber (IBC Ch 23, AWC NDS)

Wood structural systems are governed by IBC Chapter 23 and the AWC NDS (National Design Specification for Wood Construction).

+-------------------------------------------------------------------------+
|                        WOOD DESIGN METHODOLOGY                          |
+-------------------------------------------------------------------------+
                                                                           
  [ 1. REFERENCE DESIGN VALUES ]  ====>  [ 2. NDS ADJUSTMENT FACTORS ]     
  - Bending (F_b), Tension (F_t)         - C_D (Load Duration Factor)      
  - Shear (F_v), Compression (F_c)       - C_M (Wet Service Factor)        
  - Modulus of Elasticity (E)            - C_t (Temperature Factor)        
                                         - C_r (Repetitive Member = 1.15)  
                                         - C_F (Size Factor)               

NDS Load Duration Factors ($C_D$):

Because timber can support higher loads for short durations before wood fiber failure occurs, reference stress values are adjusted by $C_D$:

• Dead Load ($D$): $C_D = 0.90$ (Permanent duration $> 10\text{ years}$).
• Occupancy Live Load ($L$): $C_D = 1.00$ (Standard 10-year cumulative baseline).
• Snow Load ($S$): $C_D = 1.15$ (2-month duration).
• Construction / Roof Live Load ($L_r$): $C_D = 1.25$ (7-day duration).
• Wind & Seismic Loads ($W, E$): $C_D = 1.60$ (10-minute short-term gust duration).

Conventional Light-Frame Construction Limitations (IBC §2308):

Prescriptive wood framing (without professional engineer structural calculations) is permitted ONLY when the building satisfies all statutory geometric boundaries:

1. Building Height: Maximum 3 stories in height.
2. Story Height: Maximum bearing wall height of 11 feet 7 inches (10 ft nominal studs).
3. Dead Load: Maximum floor dead load of 15 psf and maximum roof/ceiling dead load of 15 psf.
4. Floor Live Load: Maximum floor live load of 40 psf (e.g., standard residential; commercial office 50 psf cannot use §2308).
5. Braced Wall Lines (IBC §2308.6): Braced wall panels (sheathed with OSB, plywood, or gypsum board) must be spaced not more than 25 feet on center (or 35 ft with specific aspect ratio adjustments).

+-------------------------------------------------------------------------+
|              PROTECTION OF WOOD AGAINST DECAY (IBC §2304.12)            |
+-------------------------------------------------------------------------+
                                                                           
       INTERIOR FINISHED FLOOR                                             
       =========================================================           
       | [ WOOD JOISTS ]                                                   
       | Minimum 18 inches clearance to exposed earth (IBC §2304.12.1.1)   
       |                                                                   
       | [ WOOD GIRDERS / BEAMS ]                                          
       | Minimum 12 inches clearance to exposed earth (IBC §2304.12.1.1)   
       v                                                                   
       ---------------------------------------------------------           
       EXPOSED CRAWL SPACE EARTH / SOIL                                    
                                                                           
  *Sill Plate Requirement (IBC §2304.12.1.4):*                             
  Wood sill plates resting directly on exterior concrete or masonry        
  foundations MUST be naturally durable wood or preservative-treated wood. 

Wood Shear Walls & Diaphragm Aspect Ratios (IBC Table 2305.3.4):

• Engineered Wood Shear Walls (Wood Structural Panels - OSB/Plywood): Maximum height-to-width aspect ratio is 2:1 (up to 3.5:1 where allowable shear capacity is reduced by the factor $2w/h$).
• Diaphragms (Roofs/Floors): Blocked wood structural panel diaphragms have a maximum span-to-width ratio of 4:1; unblocked diaphragms have a maximum ratio of 3:1.

---

5. Realistic Plan Review Scenario: Commercial Multi-Material Structural Review

Project Submittal Overview

A plans examiner is conducting a structural plan review for a 2-story mixed-use building consisting of a reinforced concrete podium at Level 1 with Type V-A light-frame wood residential units above (Level 2). Structural steel framing supports the open commercial retail spaces on the ground floor.

+-------------------------------------------------------------------------+
|                    MIXED-MATERIAL REVIEW CHECKLIST                      |
+-------------------------------------------------------------------------+
|  1. Ground Floor Columns: HSS 6x6x3/8 columns (ASTM A500 Gr C)          |
|  2. Podium Slab: 10" thick reinforced concrete slab cast over unheated  |
|     crawlspace with ground clearance of 24 inches.                      |
|  3. Level 2 Floor Joists: 2x10 untreated Douglas Fir-Larch joists       |
|     bearing on Level 1 concrete stem walls via wood sill plates.         |
|  4. Exterior CMU Fire Wall: 8" CMU with cleanouts at 8-ft grout lifts.   |
+-------------------------------------------------------------------------+

Step-by-Step Plans Examiner Code Verification

Check 1: Concrete Slab Cover & Reinforcement

• The podium slab is interior on top, but the underside is exposed to an unheated crawlspace in contact with earth.
• Under ACI 318 / IBC Table 20.6.1.3.1, concrete exposed to weather or earth on the bottom face requires minimum 1.5 inches of cover for #5 bars and smaller, or 2.0 inches for #6 bars and larger.
• Drawing Detail S-501 shows 3/4" cover on bottom bars. Result: NONCOMPLIANT. Rebar cover on bottom must be revised to minimum 1.5 inches.

Check 2: Wood Sill Plate & Joist Clearances

• Untreated 2x10 wood floor joists have 24 inches of clearance to exposed crawlspace soil ($24\text{ in} \ge 18\text{ in required per IBC §2304.12.1.1}$) $\rightarrow$ COMPLIANT.
• Wood sill plate resting directly on the concrete stem wall is specified as standard untreated Hem-Fir lumber.
• Under IBC Section 2304.12.1.4, wood sill plates resting on concrete or masonry foundations in contact with the ground must be preservative-treated wood or naturally durable decay-resistant wood. Result: NONCOMPLIANT. Revision required to specify pressure-preservative treated lumber with hot-dip galvanized fasteners (ASTM A153).

Check 3: Masonry Grouting & Cleanout Verification

• The 8-inch CMU fire wall is detailed for an 8-foot grout pour height.
• Because the grout pour exceeds 5 feet, IBC Section 2104 / TMS 602 requires cleanout openings at the base of every vertical cell containing reinforcement. Drawing Detail S-301 indicates cleanout locations at the base course. $\rightarrow$ COMPLIANT.

Check 4: Structural Steel Bolted Connections

• Steel moment connections in the lateral force-resisting system (SDC C) are detailed with ASTM F3125 Grade A325 bolts in standard round holes specified as "Snug-Tight".
• Under AISC 360 / AISC 341 and IBC Section 1705.2.1, bolted connections in designated Seismic Force-Resisting Systems must be detailed as pretensioned or slip-critical connections. Result: NONCOMPLIANT. Connections must be revised to slip-critical with specified surface preparation (Class A) and tensioning verification.

---

6. Common Plan Review & Exam Traps

• Trap 1: Confusing Concrete Cover for Earth Cast vs. Earth Contact. Concrete cast directly against excavation soil (such as footing bottom and sides without forms) requires 3 inches of cover. Formed concrete surfaces subsequently backfilled require 2 inches (or 1.5" for $\le #5$).
• Trap 2: Specifying Type N Mortar for Seismic Shear Walls. Type N mortar is prohibited for engineered lateral force-resisting masonry in SDC C, D, E, and F; Type S or Type M mortar is mandatory per TMS 402.
• Trap 3: Omitting High-Lift Cleanout Details. High-lift grouting pours exceeding 5 feet without cleanouts at the bottom of grouted cells violate TMS 602 and prevent inspection of debris prior to pouring.
• Trap 4: Missing Preservative Treatment on Sill Plates. Untreated wood sill plates resting on concrete/masonry foundation walls are a frequent code violation under IBC §2304.12.1.4.
• Trap 5: Exceeding Conventional Light-Frame Floor Live Load Limits. Using prescriptive framing under IBC Section 2308 for buildings with floor live loads exceeding 40 psf (such as commercial offices @ 50 psf or retail @ 100 psf) is illegal; complete engineered calculations are mandatory.