Date: 201811
Local time of day: 1201-1800
Location: D10.TRACON, TX
Flight conditions: VMC
Light: Daylight
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B DFW
ATC / advisory: TRACON D10
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Carrier
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude
Detector: Automation Aircraft TA; Automation Aircraft RA; Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Airspace Structure; Company Policy; Human Factors; Procedure
Primary problem: Airspace Structure

Narrative: While operating on an IFR flight plan in VMC conditions in the terminal area; we were above a scattered cloud layer and within Class Bravo airspace. We were being vectored at the very bottom of a Class B shelf. We were vectored toward a VFR aircraft operating just below the class B airspace; without enough vertical separation; causing us to initiate climb as well as receive a TCAS RA. This occurred with the traffic in sight; while maintaining visual separation in VMC conditions.After descending via arrival for a busy secondary commercial satellite airport within the primary Class B airspace; we were on ATC vectors. Our vectors began just outside the Class B airspace; 30 miles from the primary Class B airport; and we were initially given headings circumnavigating the airspace. We were cleared to descend to 4;000 feet which is the bottom of the outermost Class B shelf. At 4;000 feet we were above a scattered cloud layer and noticed on TCAS display several VFR traffic targets in the vicinity and only slightly below our altitude due to a scattered layer they were trying to remain clear of (above). As Pilot Flying; in my arrival briefing; I had anticipated and briefed the 'greatest threat' for this terminal arrival to be traffic; especially VFR aircraft operating just outside of or below Bravo airspace based on experience flying into this airport. We expanded our TCAS displays for greater situational awareness and monitored closely. We also used our rudimentary fix/radius feature to effectively 'draw' the airspace rings on the MFD for greater situational awareness.We passed by one aircraft indicating only about one or two hundred feet below us; but we did not get closer than about 2.5 miles. As we passed that traffic; the initial controller turned us and we entered the Class B airspace. I was reluctant to make the turn because I noticed another VFR aircraft on TCAS in that direction and approximately 3-4 miles. We complied with the instruction and turned; and I slowed to 200 knots. Before we could query; [we] were handed off to the final approach controller. The Pilot Monitoring and I visually acquired the traffic coming slightly toward us indicating 100 feet below and passing right to left as we received a TA. The Pilot Monitoring was quick to check in with the final controller; reporting 'level 4;000; heading XXX; traffic ahead in sight'. The controller promptly responded; acknowledging the VFR traffic; and told us 'you can climb if you need to'. We initiated a climb with autopilot still engaged; but as we did so; we received the RA and instruction 'CLIMB; CLIMB'. I disengaged the autopilot and promptly complied with the FD/RA guidance while we also kept visual contact with the general aviation aircraft. As we initiated the climb; the controller advised us of additional traffic ahead passing left to right and asked us to turn 15 degrees left to pass behind and we visually acquired that immediately. The TCAS also noticed the secondary threat and commanded a secondary RA 'LEVEL OFF' to avoid creating a conflict with that traffic above. I believe they were 2;000 feet above and we ended up climbing only 500 feet toward them; but rate of climb must have triggered the 'level off' command.There were scattered to broken clouds in places creating marginal VFR weather beneath the Class B airspace causing several VFR aircraft to be operating just beneath the Class B shelf; effectively 'sandwiched' above clouds and below airspace. Since we were operating to a satellite airport and not the primary airport; ATC leaves us at the very bottom of the airspace; creating a strong potential for conflict.Arrival procedures or controllers should slow aircraft to 210 knots earlier when operating just outside or within the B but at the bottom of a 'shelf'. Typically at this airport; pilots are seemingly expected to keep the speed up or to slow themselves as the last published speed on the arrival is 250 knots. Alternatively; as we did; pilots should slow on their own and advise ATC. Ideally; the local procedures wouldn't have airliners operating at the very bottom of the shelf; where aircraft can be operating 100 feet below and be 'clear of Bravo airspace'. I believe these VFR pilots to be within their rights to be operating just below the Bravo airspace; and the airspace or arrival altitudes should likely be adjusted to give the IFR traffic within the Bravo better separation from traffic underneath. Going into the busy secondary airport seems to provide a lower level of safety (due to the high density GA traffic) vs. flying into the primary airport.


Second narrative: [Report narrative contains no additional information.]


Synopsis: EMB-175 flight crew airspace reported taking evasive action and receiving an RA for an unidentified VFR target operating just below the base of the Class B airspace.